ELECTRONICS EVERYDAY

50 Watts Inverter Circuit

noreply@blogger.com (Sabarish) — Wed, 03 Jul 2013 17:57:00 +0000

A 50 watt inverter might look quite trivial, but it can serve some useful purposes to you. When outdoors, this small power house can be used for operating small electronic gadgets, soldering iron, table top radios, incandescent lights, fans etc.

Let’s learn how to build this homemade 50 watt inverter unit, beginning with a brief description regarding the circuit diagram and its functioning:

Circuit Description

The circuit may be understood with the following points:

Referring to the figure, transistors T1 and T2 along with the other R1, R2, R3 R4, C1 and C2 together form a simple astable multivibrator (AMV) circuit. A multivibrator circuit basically is composed of two symmetrical half stages, here its formed by the left and the right hand side transistor stages which conduct in tandem or in simple words the left and the right stages conduct alternately in a kind of a perpetual “motion”, generating a continuous flip flop action.

The above action is responsible of creating the required oscillations for our inverter circuit. The frequency of the oscillation is directly proportional to the values of the capacitors or/and the resistors at the base of each transistor.

Lowering the values of the capacitors increases the frequency while increasing the values of the resistors decreases the frequency and vice versa. Here the values are chosen so as to produce a stable frequency of 50 Hz.

Readers, who wish to alter the frequency to 60 Hz, may easily do it by just changing the capacitor values appropriately.

Transistors T2 and T3 are placed at the two output arms of the AMV circuit. These are high gain; high current Darlington paired transistors, used as the output devices for the present configuration.

The frequency from the AMV is fed to the base of T2 and T3 alternately which in turn switch the transformer secondary winding, dumping the entire battery power in the transformer winding.

This results in a fast magnetic induction switching across the transformer windings, resulting the required the mains voltage at the output of the transformer.

Parts Required

You will require the following components for making this 50 watt homemade inverter circuit:

R1, R2 = 100K,

R3, R4 = 330 Ohms,

R5, R6 = 470 Ohms, 2 Watt,
R7, R8 = 22 Ohms, 5 Watt

C1, C2 = 0.22 uF, Ceramic Disc,
D1, D2 = 1N5402 or 1N5408

T1, T2 = 8050,

T3, T4 = BC316,
T5, T6 = 2N3055 (TO-220)

General purpose PCB = cut into the desired size, approximately 5 by 4 inches should suffice.

Battery: 12 volts, Current not less than 10 AH.

Transformer = 9 – 0 – 9 volts, 5 Amps, Output winding may be 220 V or 120 volts as per your country specifications

Sundries: Metallic box, fuse holder, connecting cords, sockets etc

Testing and Setting Up the Circuit

After you finish making the above explained inverter circuit, you may do the testing of the unit in the following manner:

Initially do not connect the transformer or battery to the circuit.

Using a small DC power supply power the circuit.

If everything is done rightly, the circuit should start oscillating at the rated frequency of 50 Hz.

You can check this by connecting the prods of a frequency meter across T3’s or T4’s collector and the ground. The positive of the prod should go to the collector of the transistor.

If you don’t own a frequency meter, never mind, you do a rough checking by connecting a headphone pin across the above explained terminals of the circuit. If you hear a loud humming sound, will prove that your circuit is generating the required frequency output.

Now it’s time to integrate the battery and the transformer to the above circuit.

Connect everything as shown in the figure.

Connect a 40 watt incandescent lamp at the output of the transformer. And switch ON the battery to the circuit.

The bulb will immediately come ON brightly…..your homemade 50 watt inverrer is ready and may be used as desired by for powering many small appliances whenever required.

Working of Camera

noreply@blogger.com (Sabarish) — Fri, 25 Nov 2011 18:22:00 +0000

The work of a camera – photography is considered to be one of the greatest inventions of mankind. It has not only helped us see the entire world through a click, but has also transformed how people conceive the world. They can also be kept as a remembrance for the rest of our life.

Camera can be defined as a device that is used to capture and record photos or videos.

After years of work by many prominent people the first colour photo was invented by the famous physicist James Clark Maxwell along with Thomas Sutton. Then came the invention of the video made in cameras during the early 1920s. This technology has eventually grown to such heights that in this 21st century, these ordinary film cameras have been replaced by digital cameras.

Parts of a camera

A camera has mainly three parts. They are

Mechanical part or the camera body

Optical part or the lens section

The chemical part or the film

The way in which these three parts are connected represents the different types of cameras. Thus by combining these three parts and using them under the correct calibration produces a correct picture. They are capable of working in both the visible spectrum as well as in other portions of the electromagnetic spectrum. The basic shape of a camera needs an enclosed hollow chamber with an opening at one end. This opening, also called aperture helps in the entrance of light. This light is the actual image that has to be captured. So a capturing mechanism is set at the other end. All cameras have the lens assembled in the front. This lens helps in capturing the light, which is in turn captured and stored by the recording surface. Most ordinary cameras can take one image at a time. Most video cameras can take a maximum of 24 film frames/sec.

Mechanism of a camera

To know the complete mechanism of the camera, it is better to know each and every parameter of the camera.

1. Focus

A camera’s focus greatly depends on the clarity of the picture taken. But the focus can be limited only to a certain distance. This range is limited to the range of the lens. This range when adjusted to get a perfect image is called the focus of the camera. For accurate focussing of cameras, the device is comprised of a fixed focus and also consists of a wide-angle lens and a small aperture in front of the camera. The range of focus will be clearly indicated in the camera with symbols like two people standing upright, mountains and so on. For a simple camera, a reasonable focus of about 3 meters to infinity is available. The focus available on each camera is different. Single-lens reflex (SLR) cameras have a focus that can be changed according to our like. This is done by providing a objective lens and a moving mirror so as to projecting the image to a ground glass or plastic micro-prism screen. Similarly each camera has different settings which will be explained briefly later.

The focus of a camera depends on two main features. They are

The structure and position of the lens.

The angle in which the light beams enter into the lens.

Consider a pencil kept at a short distance from the lens. When the distance is altered, that is kept near and then farther away from the lens, the angle of entry of the light changes accordingly. This light is hit on the film surface kept inside the camera. The angle becomes sharper when the image is close to the lens and will become narrower when the image is kept far away. Thus when the lens is focused farther and then nearer from the pencil, the image is actually moving closer or farther away from the film surface. The correct image will be obtained when the focus is adjusted in such a way that you can line up the focused real image of an object so it falls directly on the film surface.

2. Camera Lens

The quality of the photograph taken largely depends on the type of lens used. The precision of a lens depends on a factor called “bending angle”. This in turn, depends on the structure of the lens. If the lens has a flat shape, the bending angle is less. Thus the light beams will converge a little distance farther away from the lens. Thus the image is also formed farther away. Thus when the distance increases, the size of the image also increases, though the size of the film is constant. If the lens has a round shape, the bending angle will be high. Thus the image will be formed a lot more nearer to the lens.

Costly cameras have a lot of lenses, which are replaced or combined according to the magnification required. This magnification power of a lens is called the focal length. Greater the focal length, greater the magnification.

3. Camera Film

For an image to be recorded and viewed it must be stored in a film. When an image is captured, it is actually being “chemically” recorded onto a film. The film mainly consists of millions of light-sensitive grains, which are suspended on a plastic strip. These grains chemically react, when exposed to light. This reaction causes the image to be recorded on the film. This film is then developed by reacting it with other chemicals. For black and white films, the chemicals cause the grains to appear darker when exposed to light. Thus, the darker areas appear lighter and the lighter areas appear darker. This is reversed while printing out the photos.

For producing colour films, the film consists of light sensitive materials that respond to colours red, green and blue. When they are washed and chemically reacted, you get a negative of a colour photo.

Different camera designs

There are a lot of types of cameras like Plate camera, large format camera, medium format camera, folding camera, rangefinder camera and so on. Out of these the most used ones are the single-lens reflex camera (SLR) and the point and shoot camera. The difference comes in the manner in which the photographer visualizes the scene. In a point and shoot camera, you do not see the real image through the camera lens. Instead, you get to see only a blurred vision of the image.

In an SLR camera, you can see the real image of the scee you are about to capture. It has the same configuration as that of a periscope. When the image is seen from the lens, it hits the lower mirror and bounces from there. It then hits the prism. This prism flips the image to form the original image. The mirror and translucent screen help in providing the exact image to the photographer. Thus, you can focus and compose the image so as to get the exact picture you have in mind.

: SLR Camera

With upcoming technology, the point and shoot cameras are nowadays fully automatic. SLR is built with both manual and automatic controls. The only difference between the manual and automatic cameras is that the former will be controlled by a central processor, instead of the photographer.

The focus system and the light meter transmit the signals to the microprocessor and thus activate all the motors accordingly. These motors control the adjusting lens and also open and close the aperture.

Metal Detector

noreply@blogger.com (Sabarish) — Fri, 25 Nov 2011 18:15:00 +0000

It's a simple metal detector design that has the quite good characteristics. the principle of operation which one differs from the classic schemes (BFO, transmit-receive known as "two-boxes" metal detector, inductive).

The dynamic mode is used to find targets in interference environment. There is known from theory of signal filtration that if signal shape is determined we can construct optimal filter - the best one for extracting the signal with maximum signal/noise ratio. This filter is known as optimal matched filter. In our device we realized digital optimal matched filter as part of microcontroller software. The filter parameters are optimized for effective ferro- and non-ferro targets detection on 0.5-1.0 m/s velocity of sensor.

Features of the Metal Detector:
Power supply .............................4.5-6V;
DC consumption .......................15 mA;
Indication ...................................sound + 8 LEDs;
Modes ........................................static or dynamic;
Discrimination.............................ferro/non-ferro.

Metal Detector Schematic

Switches controlled (versions V1.9 and V2.0 of firmware):
S0: reset device;
S1: reserved;
S2: on - threshold high, off - threshold low;
S3: measuring time on - 30ms, off - 120ms;
S4: self tuning on/off (in dynamic mode only);
S5: mode on - static, off - dynamic.

Metal detector PCB Layout

Metal Detector Coil Design

Approx. 100 curls 200 mm in diameter. Copper wire in isolation 0,35 mm diameter

Mobile phone call indicator

noreply@blogger.com (Sabarish) — Fri, 25 Nov 2011 17:08:00 +0000

Purpose
This circuit can be used to escape from the nuisance of mobile phone rings when you are at home. This circuit will give a visual indication if placed near a mobile phone even if the ringer is deactivated. This circuit was designed to detect when a call is incoming in a cellular phone (even when the calling tone of the device is switched-off) by means of a flashing LED.

The device must be placed a few centimeters from the cellular phone, so its sensor coil L1 can detect the field emitted by the phone receiver during an incoming call.

Device operation

When a call is coming to the mobile phone, the transmitter inside it becomes activated. The frequency of the transmitter is around 900MHz.The coil L1 picks up these oscillations by induction and feds it to the base of Q1. This makes the transistor Q1 activated.Since the Collector of Q1 is connected to the pin 2 of IC1 (NE555) , the IC1 is triggered to make the LED connected at its output pin (pin 3) to blink. The blinking of the LED is the indication of incoming call.

The signal detected by the sensor coil is amplified by transistor Q1 and drives the monostable input pin of IC1. The IC's output voltage is doubled by C2 & D2 in order to drive the high-efficiency ultra-bright LED at a suitable peak-voltage.

Note:

Stand-by current drawing is less than 200µA, therefore a power on/off switch is unnecessary.
Sensitivity of this circuit depends on the sensor coil type.
L1 can be made by winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can). Remove the coil from the former and wind it with insulating tape, thus obtaining a stand-alone coil.
A commercial 10mH miniature inductor, usually sold in the form of a tiny rectangular plastic box, can be used satisfactorily but with lower sensitivity.
IC1 must be a CMos type: only these devices can safely operate at 1.5V supply or less.
Any Schottky-barrier type diode can be used in place of the 1N5819: the BAT46 type is a very good choice.

HOW SOLAR CELLS WORK

noreply@blogger.com (Sabarish) — Sat, 19 Nov 2011 14:41:00 +0000

SOLAR CELLS, How They Work

The solar cell offers a limitless and environmentally friendly source of electricity. The solar cell, is able to create electricity directly from photons. A photon can be thought of as a packet of light and the amount of energy in a photon is proportional to the wavelength of light.

Solar Cell Structure:

A. Encapsulate - The encapsulate, made of glass or other clear material such clear plastic, seals the cell from the external environment.

B. Contact Grid- The contact grid is made of a good conductor, such as a metal, and it serves as a collector of electrons.

C. The Anti reflective Coating (AR Coating)- Through a combination of a favorable refractive index, and thickness, this layer serves to guide light into the solar cell. Without this layer, much of the light would simply bounce off the surface.

D. N-Type Silicon - N-type silicon is created by doping (contaminating) the Si with compounds that contain one more valence electrons* than Si does, such as with either Phosphorus or Arsenic. Since only four electrons are required to bond with the four adjacent silicon atoms, the fifth valence electron is available for conduction.

E. P-Type Silicon- P-type silicon is created by doping with compounds containing one less valence electrons* than Si does, such as with Boron. When silicon (four valence electrons) is doped with atoms that have one less valence electrons (three valence electrons), only three electrons are available for bonding with four adjacent silicon atoms, therefore an incomplete bond (hole) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different Si atom. This movement of holes is available for conduction.

F. Back Contact - The back contact, made out of a metal, covers the entire back surface of the solar cell and acts as a conductor.

*[ A valence electron is an electron found in the outermost electron shell. An element containing more valence electrons will try to donate valence electrons to an element containing fewer valence electrons.] *

A photon's path through the solar cell

Once the photon passes the anti reflective layer, it will either hit the silicon surface of the solar cell or the contact grid metallization. The metallization, being opaque, lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet cover as little of the solar cell's surface, allowing more photons to penetrate.

A Photon causes the Photoelectric Effect*.

The photon's energy transfers to the valence electron of an atom in the n-type Si layer. That energy allows the valence electron to escape its orbit leaving behind a hole. In the n-type silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As the term "carrier" implies, both are able to move throughout the silicon layer of the solar cell, and so are said to be mobile. Inversely, in the p-type silicon layer, electrons are termed minority carriers and holes are termed majority carriers, and of course are also mobile.

*[ The photoelectric effect is simply defined as an experimentally measurable effect where a metal emits electrons when hit by photons..] *

The p-n junction.

The region in the solar cell where the n-type and p-type Si layers meet is called the p-n junction. As you may have already guessed, the p-type silicon layer contains more positive charges, called holes, and the n-type silicon layer contains more negative charges, or electrons. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not in the other (reverse biased).

An interesting interaction occurs at the p-n junction of a darkened solar cell. Extra valence electrons in the n-type layer move into the p-type layer filling the holes in the p-type layer forming what is called a depletion zone. The depletion zone does not contain any mobile positive or negative charges. Moreover, this zone keeps other charges from the p and n-type layers from moving across it.

So, to recap, a region depleted of carriers is left around the junction, and a small electrical imbalance exists inside the solar cell. This electrical imbalance amounts to about 0.6 to 0.7 volts. So due to the p-n junction, a built in electric field is always present across the solar cell.

P = V × I

When photons hit the solar cell, freed electrons (-) attempt to unite with holes on the p-type layer. The p-n junction, a one-way road, only allows the electrons to move in one direction. If we provide an external conductive path, electrons will flow through this path to their original (p-type) side to unite with holes.

The electron flow provides the current ( I ), and the cell's electric field causes a voltage ( V ). With both current and voltage, we have power ( P ), which is just the product of the two. Therefore, when an external load (such as an electric bulb) is connected between the front and back contacts, electricity flows in the cell, working for us along the way.

Operational Amplifier

noreply@blogger.com (Sabarish) — Fri, 17 Jun 2011 18:15:00 +0000

OPAMP

The operational amplifier is arguably the most useful single device in analog electronic circuitry. With only a handful of external components, it can be made to perform a wide variety of analog signal processing tasks. It is also quite affordable, most general-purpose amplifiers selling for under a dollar apiece. Modern designs have been engineered with durability in mind as well: several "op-amps" are manufactured that can sustain direct short-circuits on their outputs without damage.

The Operational Amplifier is probably the most versatile Integrated Circuit available. It is very cheap especially keeping in mind the fact that it contains several hundred components. The most common Op-Amp is the 741 and it is used in many circuits.

The OP AMP is a ‘Linear Amplifier’ with an amazing variety of uses. Its main purpose is to amplify (increase) a weak signal - a little like a Darlington Pair.

The OP-AMP has two inputs, INVERTING ( - ) and NON-INVERTING (+), and one output.

Working in 2 ways

1. An inverting amplifier. Leg two is the input and the output is always reversed.

In an inverting amplifier the voltage enters the 741 chip through leg two and comes out of the 741 chip at leg six.If the polarity is positive going into the chip, it negative by the time it comes out through leg six.The polarity has been ‘inverted’.

GAIN (AV) = -R2 / R1

2. A non-inverting amplifier. Leg three is the input and the output is not reversed.

In a non-inverting amplifier the voltage enters the 741 chip through leg three and leaves the 741 chip through leg six. This time if it is positive going into the 741 then it is still positive coming out. Polarity remains the same.

GAIN (AV) = 1+(R2 / R1)

OPAMP AS COMPARATOR

However, this time the 741 is used as a comparator and not an amplifier. The difference between the two is small but significant. Even if used as a comparator the 741 still detects weak signals so that they can be recognised more easily. It is important to understand these circuits as they very regularly appear in examinations.

A ‘comparator’ is an circuit that compares two input voltages. One voltage is called the reference voltage (Vref) and the other is called the input voltage (Vin).When Vin rises above or falls below Vref the output changes polarity (+ becomes -).

Positive is sometimes called HIGH. Negative is sometimes called LOW

Thin Film Transistor

noreply@blogger.com (Sabarish) — Mon, 13 Jun 2011 17:47:00 +0000

TFT

The TFTs in active-matrix LCD act as simple ON/OFF switches, at different speeds which depend on the refresh rate of the LCD, for example 60Hz. Figure 1 shows a simple structure of TFT, it consists of three terminals: the gate, the source and the drain.

Figure 1. A simple Thin-Film-Transistor (TFT) structure

Why TFT

In TFT technology, a separate miniscule transistor works for each pixel on the display. As the transistors are so tiny, the charge required to operate it is very small too. This way, the display gets refreshed several times per second, ensuring great visual clarity.

In Passive Matrix LCD monitors that were in use before TFT, fast moving images could not be represented with adequate clarity. For instance, a body in motion from point A to point B would disappear between the two rest points. TFT could meet this challenge as each pixel is backed up with a transistor, and thus track the body throughout the screen. Thus TFT monitors are ideal for games, video displays and everything involving multimedia.

Working

In a simple TFT, for example N-channel TFT, a positive voltage is applied on the gate in order to switch it ON; the insulation layer can be considered as the dielectric layer in a capacitor, hence negative charges are induced on the semiconductor channel, which is the region between source and drain; these negative charges create a electrons flow from source to drain to make the channel conductive. When a negative voltage is applied on the gate, electrons are depleted in the channel, hence almost no current is present. The ON current depends on different parameters, for example channel width, channel length, gate voltage and the threshold voltage of the TFT.

When the TFT is switched ON, a data voltage is applied on the source, the drain with the LC load capacitance will charge up to the voltage with same amplitude, i.e. transferring the data voltage from the data line to the pixel electrode. When switched OFF, no current in the channel, and data voltage cannot be transferred.

The first TFT for LCD was made of Cd-Se semiconductor thin films, however this is not compatible with normal process. In spite AMLCD with Cd-Se has a better performance, its commercialization is still not success.

While a P-channel TFT can be switched ON by applying a negative voltage on the gate, and can be switched OFF by a positive voltage on the gate.

TFTs can be formed by three different silicons, they are: crystalline silicon, poly-silicon and amorphous silicon; and in practical manufacturing, poly-silicon can also be processed under low and high temperature, i.e. Low-Temperature Poly-Silicon (LTPS) which can be built on common low-cost glasses, and High Temperature Poly-Silicon (HTPS) which needs quartz plate.

Since the crystalline Si owns higher mobility, it could integrate more peripheral electronics, hence higher pixel-density-required devices, like projection light valves, usually use crystalline Si.

Amorphous silicon is widely used in LCD monitor and TV because of its easy manufacturing on large glass substrates, but it has a lower mobility; however during manufacturing, the a-Si is formed by using SiH4, the hydrogen enters into the silicon film, and can improve the loosen Si-lattice in a-Si, thus enhance its performance. The a-Si can therefore be also referred as a-Si:H. The normal electron mobility of a-Si:H is ~0.3-1 cm2/Vsec, compared with c-Si’s >500 cm2/Vsec, it is quite small. But for AMLCD’s TFT’s switch, it is enough. On the other hand, its hole mobility is very low, therefore only N-channel TFT can be practically used. Another drawback of a-Si is its high photoconductivity, which cause the undesirable photo-leakage current in the OFF state. To avoid it, a cover layer is used to shield it from ambient and backlight.

Poly-Si can be used to make both P-channel and N-channel TFTs. Because of its relatively high mobility, both row and column drivers can be integrated on the glass, even D/A converters, DC/DC converters and (micro)processors can be integrated too, which significantly cut the cost from external driver and other devices’ chips. However the off current of Poly-Si is much higher than a-Si, i.e. the OFF state is not stable because of the charge on the pixel capacitor cannot be maintained. In order to decrease the OFF current, a dual gate structure and a lightly doped drain (LDD) were proposed. Both methods can effectively lower the OFF current.

Figure 2 shows a typical pixel structure of AMLCD. It is worth to note that a storage capacitor is connected in parallel to pixel capacitor in order to retain the charge at OFF state, and decrease the voltage dependence and leakage current in the LC capacitance, hence the control of RMS voltage on the pixel is easier.

Figure 2. A TFT AMLCD's pixel layout (color-filter substrate is not shown).

RF based REMOTE CONTROL

noreply@blogger.com (Sabarish) — Tue, 07 Jun 2011 08:54:00 +0000

Introduction:-
The rf transmitter a gadget that disseminates radio waves with the help of an antenna. A rf transmitter consists of an oscillator that changes electrical power in to a reasonable frequency. There is also an amplifier for audio frequency (AF) and radio frequency (RF). A modulator regulates the signal information on to the transmitter for dispersion. The rf transmitter has an important component that makes its working possible; this is the antenna that transmits an electromagnetic signal for many types of communication. rf transmitter works together with rf receivers. There must be rf receivers on the other end for transmitters to play their role. Communication by use of these systems is possible for radios, cell phones, televisions, walkie –talkies and other electronics used for the purpose of broadcasting. It is possible to find both rf receivers and transmitters in one device. A good example of such technology is used in mobile phones where one can receive and call using one gadget. Currently communication does not rely on analogue mode but uses digital technology thus eliminating the need for complex wiring system to do the connections that carry information.

For an example: We will be using ASK (Amplitude Shift Keying) based Tx/Rx(transmitter/receiver) pair operating at 433 MHz. The transmitter module accepts serial data at a maximum of XX baud rate. It can be directly interfaced with a microcontroller or can be used in remote control applications with the help of encoder/decoder ICs.

RF transmitter module:-

RF transmitter module

RF receiver module:-

RF Receiver module

Working:-

The encoder IC takes in parallel data which is to be transmitted, packages it into serial format and then transmits it with the help of the RF transmitter module. At the receiver end the decoder IC receives the signal via the RF receiver module, decodes the serial data and reproduces the original data in the parallel format.In order to control say a dc motor, we require 2 bits of information (switching it on/off) while we need 4 bits of information to control 2 motors. HT12E and HT12D are 4 channel encoder/decoder ICs directly compatible with the specified RF module.In order to drive motors, we would need to connect a suitable motor driver at the output of the decoder IC. The motor driver circuit can consist of a relay, transistorized H-Bridge or motor driver ICs like the L293D, L298 etc.

Block Diagram:-

Application:-

Rf transmitter enables you to make use of your television, radio, phone and other communication hand held devices. Rf receivers are installed in these devices in factory to make them respond to any signal send. A rf transmitter may interfere with other communication services. This means that you should take precaution by seeking from the relevant authorities about the mode of frequency to use when installing your communication system. Rf receivers are capable of receiving broadcast information from a rf transmitter in different patterns like a single beam or broadcast system. You cannot however tell the difference when you are listening from your radio or watching TV. Rf receivers function hand by hand with transmitters so none can be on its own. Communication is hence a two-way traffic and some communication models are accommodating the receiving and transmitting factors in one equipment. These functions perform well by the energy from electricity or solar. The major important thing in a communication system is to send and receive feedback. Modern times have included the aspect of time in the instruments of passing information no matter how far you are located. This would however be impossible if it was not for transmitters and receivers.

noreply@blogger.com (Sabarish) — Sun, 05 Jun 2011 19:36:00 +0000

What is RFID

RFID stands for Radio-Frequency IDentification. The acronym refers to small electronic devices that consist of a small chip and an antenna. The chip typically is capable of carrying 2,000 bytes of data or less.
The RFID device serves the same purpose as a bar code or a magnetic strip on the back of a credit card or ATM card; it provides a unique identifier for that object. And, just as a bar code or magnetic strip must be scanned to get the information, the RFID device must be scanned to retrieve the identifying information.

Working

A Radio-Frequency IDentification system has three parts:

A scanning antenna

A transceiver with a decoder to interpret the data

A transponder - the RFID tag - that has been programmed with information.

The scanning antenna puts out radio-frequency signals in a relatively short range. The RF radiation does two things:

It provides a means of communicating with the transponder (the RFID tag) AND

It provides the RFID tag with the energy to communicate (in the case of passive RFID tags).

This is an absolutely key part of the technology; RFID tags do not need to contain batteries, and can therefore remain usable for very long periods of time (maybe decades). The scanning antennas can be permanently affixed to a surface; handheld antennas are also available. They can take whatever shape you need; for example, you could build them into a door frame to accept data from persons or objects passing through.
When an RFID tag passes through the field of the scanning antenna, it detects the activation signal from the antenna. That "wakes up" the RFID chip, and it transmits the information on its microchip to be picked up by the scanning antenna.

In addition, the RFID tag may be of one of two types. Active RFID tags have their own power source; the advantage of these tags is that the reader can be much farther away and still get the signal. Even though some of these devices are built to have up to a 10 year life span, they have limited life spans. Passive RFID tags, however, do not require batteries, and can be much smaller and have a virtually unlimited life span.
RFID tags can be read in a wide variety of circumstances, where barcodes or other optically read technologies are useless.

The tag need not be on the surface of the object (and is therefore not subject to wear)
The read time is typically less than 100 milliseconds
Large numbers of tags can be read at once rather than item by item.

Passive RFID vs. Active RFID

Passive RFID tags operate using power from the RFID transceiver. Passive tags are small and inexpensive, but do not have good range.
Active RFID tags are powered, usually by a battery. Active tags are larger and more expensive, but offer a much better identification range.
RFID tags store data, which is typically used for authentication. Passive tags typically store between 32 and 128 bits of data; Active tags can store up to 1MB of data.
Passive tags are Read-Only; Active tags are typically rewritable.

Applications

Please click at one of the applications below to read how Rotil Communications B.V. applied these techniques successfully at previous projects.

Vehicle start interruption
Access control and identification
Petrol stations
Working hours registration system

Other possibilities to apply the RFID technique are for example:

Vehicle identification

Every vehicle will carry a transponder. Through the unique id number in the transponder it will be easy to ‘recognise’ the vehicle. This can expanded with the registration of technical vehicle information such as temperature, GPS location.

Logistic processes

Every container, box or crate has a transponder fitted to it. With each step in the process, the transponder is being read and the operation is saved in the central database. By this is, it will be very easy to find out which steps in the production process has already been made.

Service and maintenance

All products, devices or vehicles will get a transponder. The service or maintenance mechanic scans this transponder for each maintenance. The software on a computer, which is connected to a central database, enables him to register for example the product details, customer number, maintenance tasks, date/time. Now it will be much easier to find out which maintenance or service has been provided to a certain product/vehicle with just a click. This will be very (cost) efficient.

Countdown Timer using 555 Timer

noreply@blogger.com (Sabarish) — Sat, 10 Oct 2009 17:13:00 +0000

Countdown Timer:-
In this Countdown Timer project, a 555 IC, a counter IC and a transistor switch to activate a relay either ON/OFF (mode selected by a jumper) as soon as the counting period is over. The circuit consists of an oscillator, a ripple and two switching transistors.

Oscillator:-
The 555 is configured in the standard astable oscillator circuit designed to give a square wave cycle at a period of around 1 cycle/sec. A potentiometer is included in the design so the period can be set to exactly 1 second by timing the LED flashes. A jumper connection is provided so the LED can be turned off. As soon as power is applied to the circuit counting begins. The output pulse from pin 3 of the 555 is fed to a the clock input pin 10 of the 14-stage binary ripple counter, the 4020 (or 14020.).

parts required:-

Circuitdiagram:-

Ripple Counter:-

The counter output wanted is set by a jumper. Ten counter outputs are available: 8/16/32/64/128/256/512/1024/4096 and 8192 counts. If the 555 is set to oscillate at exactly 1.0Hz by the on-board trimpot then the maximum timer interval which can be set is 8192 seconds (just over 2 hours.) At the end of the counting of the countdown timer period a pulse is output on the pin with the jumper on it. The 14020 ripple counter advances its count on each negative transistion of the clock pulse from the 555. So for each output cycle of low-high-low-high the count is advanced by two. It can be set to an zero state (all outputs low) by a logic high applied to pin 11.

In this circuit C3, R4 and D1 are arranged as a power-on reset. When power is applied to the circuit C3 is in a discharged state so pin 11 will be pulled high. C3 will quickly charge via R4 and the level at pin 11 falls thus enabling the counter. The 14020 then counts clock pulses until the selected counter output goes high. D1 provides a discharge path for C3 when the power is disconnected.

You can change the components values of R1 and C1 to set the 555 count frequency to more than 1.0 Hz. If you change the count to 10 seconds then a maximum timer delay of 81920 seconds, or 22.7 hours, can be obtained.

Transistors:-

The output from the 4020 goes to a transistor switch arrangement. Two BC547 are connected so that either switching option for the relay is available. A jumper sets the option. The relay can turn ON when power and counting start then turn OFF after the count period, or it can do the opposite. The relay will turn ON after the end of the count period and stay on so long as power is supplied to the circuit. Note that the reset pin of the 555 is connected to the collector of Q1. This enables the 555 during the counting as the collector of Q1 is pulled low.

MOSFET

noreply@blogger.com (Sabarish) — Sun, 27 Sep 2009 12:48:00 +0000

Introduction:-

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925. The MOSFET includes a channel of n-type or p-type semiconductor material (see article on semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistorwas at one time much more common.

Diagramatic representation...

Circuit symbol:-

MOSFET operation:-

A traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO₂) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is commonly used). As the silicon dioxide is a dielectric material, its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a semiconductor.

When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with $N A$ the density of acceptors, p the density of holes; p = N_A in neutral bulk), a positive voltage, $V G B$ , from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If $V G B$ is high enough, a high concentration of negative charge carriers forms in an inversion layer located in a thin layer next to the interface between the semiconductor and the insulator. Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region. Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage.

This structure with P-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regions.

Modes of operation

The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. In the following discussion, a simplified algebraic model is used that is accurate only for old technology. Modern MOSFET characteristics require computer models that have rather more complex behavior. For example, see Liu and the device modeling list at Designers-guide.org.

For an enhancement-mode, n-channel MOSFET, the three operational modes are:

Cutoff, subthreshold, or weak-inversion mode When V_GS <>_th: where $V t h$ is the threshold voltage of the device. According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain, resulting in a subthreshold current that is an exponential function of gate–source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage.In weak inversion the current varies exponentially with gate-to-source bias $V G S$ as given approximately by:

, where $I D 0$ = current at $V G S = V t h$ and the slope factor n is given by $n = 1 + C D / C O X$ , with $C D$ = capacitance of the depletion layer and $C O X$ = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once $V D S > > V T$ , but as channel length is reduced drain-induced barrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on). Frequently, threshold voltage V_th for this mode is defined as the gate voltage at which a selected value of current I_D0 occurs, for example, I_D0 = 1 μA, which may not be the same V_th-value used in the equations for the following modes. Some micropower analog circuits are designed to take advantage of subthreshold conduction. By working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible transconductance-to-current ratio, namely: $g m / I D = 1 / (n V T)$ , almost that of a bipolar transistor. The subthreshold I-V relation depends exponentially upon threshold voltage, introducing a strong dependence on any manufacturing variation that affects threshold voltage; for example: variations in oxide thickness, junction depth, or body doping that change the degree of drain-induced barrier lowering. The resulting sensitivity to fabricational variations complicates optimization of circuits operating in the subthreshold mode.

Triode mode or linear region (also known as the ohmic mode)

When V_GS > V_th and V_DS < ( V_GS - V_th )

The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:

where $μ n$ is the charge-carrier effective mobility, $W$ is the gate width, $L$ is the gate length and $C o x$ is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.
Saturation or active mode

When V_GS > V_th and V_DS > ( V_GS - V_th )The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate–source voltage, and modeled very approximately as:

The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is:
, where the combination V_ov = V_GS - V_th is called the overdrive voltage. Another key design parameter is the MOSFET output resistance $r O$ given by:
. If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.As the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in V_GS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage.

JFET

noreply@blogger.com (Sabarish) — Mon, 21 Sep 2009 12:13:00 +0000
INTRODUCTION:-

The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field effect transister. It can be used as an electrically-controlled switch or as a voltage-controlled resistance . electric charge flowflows through a semiconducting channel between "source" and "drain" terminals. By applying a bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.

Structure:-

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end form the source and drain. The gate (control) terminal has doping opposite to that of the channel, which it surrounds, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic.

Function:-

JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current depends also on the electric field between source and drain.

Symbols:-

The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable (which is not true of all JFETs).
Officially, the style of the symbol should show the component inside a circle (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices.

COMMON COLLECTOR CONFIGURATION

noreply@blogger.com (Sabarish) — Mon, 21 Sep 2009 12:03:00 +0000
INTRODUCTION:-

The COMMON-COLLECTOR CONFIGURATION (CC) is used as a current driver for
impedance matching and is particularly useful in switching circuits. The CC is also referred to as an
emitter-follower and is equivalent to the electron-tube cathode follower. Both have high input impedance
and low output impedance.In the CC, the input is applied to the base, the output is taken from the emitter, and the collector is
the element common to both input and output.

Circuit diagram:-

Gain:-

GAIN is a term used to describe the amplification capabilities of an amplifier. It is basically a ratio
of output to input. The current gain for the three transistor configurations (CB, CE, and CC) are ALPHA
(a), BETA (b), and GAMMA (g), respectively

formula:-

Purpose:-
The TRANSISTOR CONFIGURATION COMPARISON CHART gives a rundown of the
different properties of the three configurations.

COMMON BASE CONFIGURATION

noreply@blogger.com (Sabarish) — Mon, 21 Sep 2009 11:58:00 +0000
Introduction:-

The COMMON-EMITTER CONFIGURATION (CE) is the most frequently used configuration
in practical amplifier circuits, since it provides good voltage, current, and power gain. The input to the CE
is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, making the
emitter the element "common" to both input and output. The CE is set apart from the other configurations,
because it is the only configuration that provides a phase reversal between input and output signals.

Circuit diagram:-

Purpose:-

The COMMON-BASE CONFIGURATION (CB) is mainly used for impedance matching, since it
has a low input resistance and a high output resistance. It also has a current gain of less than
In the CB, the input is applied to the emitter, the output is taken from the collector, and the base is
the element common to both input and output.

Silicon-controlled rectifier

noreply@blogger.com (Sabarish) — Mon, 21 Sep 2009 08:06:00 +0000
INTRODUCTION:-

A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is Generel electronics's trade name for a type of thyrister. The SCR was developed by a team of power engineers led by Gordon Hall and commercialised by Frank W. "Bill" Gutzwiller in 1957.

SCR...

SYMBOLS:-

OPERATION:-

An SCR is a type of rectifier, controlled by a logic gate signal. It is a four-layer, three-terminal device. A p-type layer acts as an anode and an n-type layer as a cathode; the p-type layer closer to the n-type (cathode) acts as a gate. It is unidirectional in nature.

CONSTRUCTION OF SCR:-

It consists of a four layers pellet of P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor to which the proper impurities are added. The junctions are either diffused or alloyed. The Planar construction is used for low power SCR's, here all the junctions are diffused. The Mesa type construction is used for high power SCR's. In this case junction J2 is obtained by diffusion method and then the outer two layers are alloyed to it because the PNPN pellet is required to handle large currents. It is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink. The doping of PNPN will depend on the application of SCR.

MODES OF OPERATION:-

In the normal "off" state, the device restricts current to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current. The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding coupling. Once current falls below the holding current for an appropriate period of time, the device will switch "off". If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state.
If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using asnubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device.
SCRs can also be triggered by increasing the forward voltage beyond their rated break down voltage (also called as break ver voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device.
SCRs are made with voltage ratings of up to 7,500 V, and with current ratings up to 3,000 RMS amperes per device. Some of the larger ones can take over 50 kA in single-pulse operation. SCRs are used in power switching, phase control, chopper, battery charger, and inverter circuits. Industrially they are applied to produce variable DC voltages for moters (from a few to several thousand HP) from AC line voltage. They control the bulk of the dimmers used in stage lighted, and can also be used in some electric vehicles to modulate the working voltage in a jacabson circuit. Another common application is phase control circuits used with inductive loads. SCRs can also be found in welding power suplies where they are used to maintain a constant output current or voltage. Large silicon-controlled rectifier assemblies with many individual devices connected in series are used in high voltage DC converter stations.
Two SCRs in "inverse parallel" are often used in place of a TRIAC for switching inductive loads on AC circuits. Because each SCR only conducts for half of the power cycle and is reverse-biased for the other half-cycle, turn-off of the SCRs is assured. By comparison, the TRIAC is capable of conducting current in both directions and assuring that it switches "off" during the brief zero-crossing of current can be difficult.
Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a parasitic SCR. This SCR is undesired; if it is triggered by accident, the IC can go into latch up and potentially be destroyed.

Transistor codes..

noreply@blogger.com (Sabarish) — Sun, 20 Sep 2009 17:08:00 +0000

NPN transistors
Code Structure Case
style I_C
max. V_CE
max. h_FE
min. P_tot
max. Category
(typical use) Possible
substitutes
BC107 NPN TO18 100mA 45V 110 300mW Audio, low power BC182 BC547
BC108 NPN TO18 100mA 20V 110 300mW General purpose, low power BC108C BC183 BC548
BC108C NPN TO18 100mA 20V 420 600mW General purpose, low power
BC109 NPN TO18 200mA 20V 200 300mW Audio (low noise), low power BC184 BC549
BC182 NPN TO92C 100mA 50V 100 350mW General purpose, low power BC107 BC182L
BC182L NPN TO92A 100mA 50V 100 350mW General purpose, low power BC107 BC182
BC547B NPN TO92C 100mA 45V 200 500mW Audio, low power BC107B
BC548B NPN TO92C 100mA 30V 220 500mW General purpose, low power BC108B
BC549B NPN TO92C 100mA 30V 240 625mW Audio (low noise), low power BC109
2N3053 NPN TO39 700mA 40V 50 500mW General purpose, low power BFY51
BFY51 NPN TO39 1A 30V 40 800mW General purpose, medium power BC639
BC639 NPN TO92A 1A 80V 40 800mW General purpose, medium power BFY51
TIP29A NPN TO220 1A 60V 40 30W General purpose, high power
TIP31A NPN TO220 3A 60V 10 40W General purpose, high power TIP31C TIP41A
TIP31C NPN TO220 3A 100V 10 40W General purpose, high power TIP31A TIP41A
TIP41A NPN TO220 6A 60V 15 65W General purpose, high power
2N3055 NPN TO3 15A 60V 20 117W General purpose, high power
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors
Code Structure Case
style I_C
max. V_CE
max. h_FE
min. P_tot
max. Category
(typical use) Possible
substitutes
BC177 PNP TO18 100mA 45V 125 300mW Audio, low power BC477
BC178 PNP TO18 200mA 25V 120 600mW General purpose, low power BC478
BC179 PNP TO18 200mA 20V 180 600mW Audio (low noise), low power
BC477 PNP TO18 150mA 80V 125 360mW Audio, low power BC177
BC478 PNP TO18 150mA 40V 125 360mW General purpose, low power BC178
TIP32A PNP TO220 3A 60V 25 40W General purpose, high power TIP32C
TIP32C PNP TO220 3A 100V 10 40W General purpose, high power TIP32A
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.

Field Effect transister (FET)

noreply@blogger.com (Sabarish) — Sun, 20 Sep 2009 07:55:00 +0000
Introduction:-
A field-effect transistor (FET) is a type of transister commonly used for weak-signal amplification (for example, for amplifying wireless signals).The device can amplify analog or digital signals.It can also switch DC or function as an oscillator.Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips.A single IC can contain many thousands of FETs, along with other components such as resistors, capacitors, and diodes.

circuit symbol:-

in real....

In the FET, current flows along a semiconductor path called the channel. At one end of the channel, there is an electrode called the source. At the other end of the channel, there is an electrode called the drain. The physical diameter of the channel is fixed, but its effective electrical diameter can be varied by the application of a voltage to a control electrode called the gate.The conductivity of the FET depends, at any given instant in time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in the current from the source to the drain. This is how the FET amplifies signals.

Channel..
The junction FET has a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is made of the opposite semiconductor type. In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons.In a JFET, the junction is the boundary between the channel and the gate.Normally, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate.However, under some conditions there is a small current through the junction during part of the input signal cycle.

Classification:-

Field-effect transistors exist in two major classifications.These are known as the junction FET (JFET) and the metal-oxide- semiconductor FET (MOSFET).

Advantages & disadvantantages:-

The FET has some advantages and some disadvantages relative to the bipolar transister.Field-effect transistors are preferred for weak-signal work, for example in wireless communications and broadcast receivers.They are also preferred in circuits and systems requiring high impedance.The FET is not, in general, used for high-power amplification, such as is required in large wireless communications and broadcast transmitters.

Bipolar Junction Transistor

noreply@blogger.com (Sabarish) — Sat, 19 Sep 2009 18:34:00 +0000
INTRODUCTION:-
simple diodes are made up from two pieces of semiconductor material, either Silicon or Geranium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual diodes end to end giving two PN-junctions connected together in series, we now have a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short. This type of transistor is generally known as a Bipolar Transistor, because its basic construction consists of two PN-junctions with each terminal or connection being given a name to identify it and these are known as the Emitter, Base and Collector respectively.

The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development. There are two basic types of bipolar transistor construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made. Bipolar Transistors are "CURRENT" Amplifying or current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing current applied to their base terminal. The principle of operation of the two transistor types NPN and PNP, is exactly the same the only difference being in the biasing (base current) and the polarity of the power supply for each type.

Bipolar Transistor Construction:-

The construction and circuit symbols for both the NPN and PNP bipolar transistor are shown above with the arrow in the circuit symbol always showing the direction of conventional current flow between the base terminal and its emitter terminal, with the direction of the arrow pointing from the positive P-type region to the negative N-type region, exactly the same as for the standard diode symbol.

There are basically three possible ways to connect a Bipolar Transistor within an electronic circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.

1. Common Base Configuration - has Voltage Gain but no Current Gain.

2. Common Emitter Configuration - has both Current and Voltage Gain.

3. Common Collector Configuration - has Current Gain but no Voltage Gain.

The Common Base Configuration.

As its name suggests, in the Common Base or Grounded Base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a Current Gain for this type of circuit of less than "1", or in other words it "Attenuates" the signal.

The Common Base Amplifier Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are In-Phase. This type of arrangement is not very common due to its unusually high voltage gain characteristics. Its Output characteristics represent that of a forward biased diode while the Input characteristics represent that of an illuminated photo-diode. Also this type of configuration has a high ratio of Output to Input resistance or more importantly "Load" resistance (RL) to "Input" resistance (Rin) giving it a value of "Resistance Gain". Then the Voltage Gain for a common base can therefore be given as:

Common Base Voltage Gain:-

The Common Base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or RF radio amplifiers due to its very good high frequency response.

The Common Emitter Configuration:-
In the Common Emitter or Grounded Emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of connection. The common emitter amplifier configuration produces the highest voltage, current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased junction, while the output impedance is HIGH as it is taken from a reverse-biased junction.
The Common Emitter Amplifier Circuit:-

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the Current gain of the Common Emitter Transistor Amplifier is quite large as it is the ratio of Ic/Ib and is given the symbol of Beta, (β). Since the relationship between these three currents is determined by the transistor itself, any small change in the base current will result in a large change in the collector current. Then, small changes in base current will thus control the current in the Emitter/Collector circuit.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the amplifier can be given as:

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.

Then to summarise, this type of bipolar transistor configuration has a greater input impedance, Current gain and Power gain than that of the common base configuration but its Voltage gain is much lower. The common emitter is an inverting amplifier circuit resulting in the output signal being 180^o out of phase with the input voltage signal.
The Common Collector Configuration:-
In the Common Collector or Grounded Collector configuration, the collector is now common and the input signal is connected to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms, and it has relatively low output impedance.

The Common Collector Amplifier Circuit:-

The common emitter configuration has a current gain equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector and base currents combined, the load resistance in this type of amplifier configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:

This type of bipolar transistor configuration is a non-inverting amplifier circuit in that the signal voltages of Vin and Vout are "In-Phase". It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector amplifier configuration receives both the base and collector currents giving a large current gain (as with the Common Emitter configuration) therefore, providing good current amplification with very little voltage gain.

Bipolar Transistor Summary:-
The behaviour of the bipolar transistor in each one of the above circuit configurations is very different and produces different circuit characteristics with regards to Input impedance, Output impedance and Gain and this is summarised in the table below.

Transistor Characteristics:-

The static characteristics for Bipolar Transistor amplifiers can be divided into the following main groups.

Input Characteristics:- Common Base - I_E ÷ V_EB

Common Emitter - I_B ÷ V_BE

Output Characteristics:- Common Base - I_C ÷ V_C

Common Emitter - I_C ÷ V_C

Transfer Characteristics:- Common Base - I_E ÷ I_C

Common Emitter - I_B ÷ I_C

with the characteristics of the different transistor configurations given in the following table:

Characteristic
Common
Base Common
Emitter Common
Collector

Input impedance
Low Medium High

Output impedance
Very High High Low

Phase Angle
0^o 180^o 0^o

Voltage Gain
High Medium Low

Current Gain
Low Medium High

Power Gain
Low Very High Medium

Unijunction Transistor

noreply@blogger.com (Sabarish) — Sat, 19 Sep 2009 18:05:00 +0000
INTRODUCTION:-
The basic structure of a unijunction transistor (UJT) is shown in Fig.1. It is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. Contacts are then made to the device as shown; these are referred to as the emitter, base 1 and base 2 respectively. Fig.2 shows the schematic symbol used to denote a UJT in circuit diagrams. For ease of manufacture alternative methods of making contact with the bar have been developed, giving rise to the two types of structure - bar and cube.

DIAGRAMS..

The equivalent circuit shown in Fig.4 has been developed to explain how the device works, and it is necessary to define the terms used in this explanation.
R_BB is known as the interbase resistance, and is the sum of R_B1 and R_B2:
R_BB = R_B1 + R_B2
N.B. This is only true when the emitter is open circuit.
V_RB1 is the voltage developed across R_B1; this is given by the voltage divider rule:
R_B1
V_RB1 =
R_B1 + R_B2
Since the denominator of equation 2 is equal to equation 1, the former can be rewritten as:
R_B1
VRB1 = x V_BB
R_BB
The ratio R_B1 / R_BB is referred to as the intrinsic standoff ratio and is denoted by (the Greek letter eta).
If an external voltage V_e is connected to the emitter, the equivalent circuit can be redrawn as shown in Fig..
If Ve is less than V_RB1, the diode is reverse biased and the circuit behaves as though the emitter was open circuit. If however V_e is increased so that it exceeds V_RB1 by at least 0.7V, the diode becomes forward biased and emitter current Ie flows into the base 1 region. Because of this, the value of R_B1 decreases. It has been suggested that this is due to the presence of additional charge carriers (holes) in the bar. Further increase in V_e causes the emitter current to increase which in turn reduces R_B1 and this causes a further increase in current. This runaway effect is termed regeneration. The value of emitter voltage at which this occurs is known as the peak voltage V_P and is given by: V_P = _AVV_BB + V_D
The characteristics of the UJT are illustrated by the graph of emitter voltage against emitter current.

As the emitter voltage is increased, the current is very small - just a few microamps. When the peak point is reached, the current rises rapidly, until at the valley point the device runs into saturation. At this point R_B1 is at its lowest value, which is known as the saturation resistance.
The simplest application of a UJT is as a relaxation oscillator, which is defined as one in which a capacitor is charged gradually and then discharged rapidly. The basic circuit is shown in Fig.7; in the practical circuit of Fig.8 R3 limits the emitter current and provides a voltage pulse, while R2 provides a measure of temperature compensation. Fig. 9 shows the waveforms occurring at the emitter and base 1; the first is an approximation to a sawtooth and the second is a pulse of short duration.

The operation of the circuit is as follows: C1 charges through R1 until the voltage across it reaches the peak point. The emitter current then rises rapidly, discharging C1 through the base 1 region and R3. The sudden rise of current through R3 produces the voltage pulse. When the current falls to I_V the UJT switches off and the cycle is repeated.
It can be shown that the time t between successive pulses is given by:
V_BB - V_V
t + R1C ln secs (5) Megaohms. C in µF.
V_BB - V_P

The oscillator uses a 2N2646 UJT, which is the most readily available device, and is to operate from a 10V D.C. power supply.
From the relevant data sheet the specifications for the 2N2646 are:
V_EB2O I_E(peak) P_TOT(max) I_P(max) I_V(max) Case style TO18
30V 2A 300mw 5µA 4ma 0.56 - 0.75
It is important that the value of R1 is small enough to allow the emitter current to reach I_P when the capacitor voltage reaches V_P and large enough so that the emitter current is less than I_V when the capacitor discharges to V_V. The limiting values for R1 are given by:
V_BB - V_P V_BB - V_V
R1(max) = and R2(min) =
I_P I_V
From the specifications for the 2N2646 the average value of is 0.56 + 0.75/2 = 0.655. Substituting this value in equation (4) and assuming V_D = 0/7V: V_P = 0.655 x 10 + 0.7 = 7.25V.
So R1(max) = 10 - 7.25/5µA = 550K, and if V_V = approx VBB/10,
R1(min) = 10 - 1/4mA = 2.25K.
If we choose a value for R1 somewhere between these limits, e.g. lOK, the value of C can be calculated from equation.
If f = 1MHz, t = 1/f = 1msec. V_BB - V_P = 10 - 7.25 = 2.75 and V_BB - V_V = 10 - 1 = 9
t

Rearranging equation(5) to make C the subject: C = V_BB - V_V
R1 ln
V_BB - V_P

0.001
so C = = approx 84nF.
10⁴ ln (9/2.75)
Because of component and UJT tolerances it is sufficient in most circumstances to use an approximate formula: f = 1/CR, which assumes that is 0.63 - well within 5% of the average value for the 2N2646. In practice one would use a variable resistance (or a variable resistance in series with a fixed resistance) for R1 so that the frequency of oscillation could be adjusted to give the required value.
R2 is not essential; if it is included, a value of 470 ohms is appropriate for the 2N2646. The value of R3 should be small in comparison with R_BB, with which it is in series, so as to prevent it from affecting the value of the peak voltage. A value of 47 ohms or thereabouts is satisfactory.

COMMON EMITTER CONFIUGRATION

noreply@blogger.com (Sabarish) — Sat, 19 Sep 2009 16:53:00 +0000
INTRODUCTION..

The COMMON-EMITTER CONFIGURATION (CE) is the most frequently used configuration in practical amplifier circuits, since it provides good voltage, current, and power gain. The input to the CE is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, making the emitter the element "common" to both input and output. The CE is set apart from the other configurations,
because it is the only configuration that provides a phase reversal between input and output signals.

CIRCUIT DIAGRAM..

PURPOSE....

The COMMON-BASE CONFIGURATION (CB) is mainly used for impedance matching, since it has a low input resistance and a high output resistance. It also has a current gain of less than 1. In the CB, the input is applied to the emitter, the output is taken from the collector, and the base is the element common to both input and output

Norton's Theorem

noreply@blogger.com (Sabarish) — Thu, 03 Sep 2009 16:13:00 +0000

Norton's Theorem

In some ways Norton's Theorem can be thought of as the opposite to "Thevenins Theorem", in that Thevenin reduces his circuit down to a single resistance in series with a single voltage. Norton on the other hand reduces his circuit down to a single resistance in parallel with a constant current source. Norton's Theorem states that "Any linear circuit containing several energy sources and resistances can be replaced by a single Constant Current generator in parallel with a Single Resistor". As far as the load resistance, R_L is concerned this single resistance, R_S is the value of the resistance looking back into the network with all the current sources open circuited and I_S is the short circuit current at the output terminals as shown below.

Norton's equivalent circuit.

The value of this "Constant Current" is one which would flow if the two output terminals where shorted together while the source resistance would be measured looking back into the terminals, (the same as Thevenin).

For example, consider our now familiar circuit from the previous section.

To find the Nortons equivalent of the above circuit we firstly have to remove the centre 40Ω load resistor and short out the terminals A and B to give us the following circuit.

When the terminals A and B are shorted together the two resistors are connected in parallel across their two respective voltage sources and the currents flowing through each resistor as well as the total short circuit current can now be calculated as:

with A-B Shorted Out

If we short-out the two voltage sources and open circuit terminals A and B, the two resistors are now effectively connected together in parallel. The value of the internal resistor Rs is found by calculating the total resistance at the terminals A and B giving us the following circuit.

Find the Equivalent Resistance (Rs)

Having found both the short circuit current, Is and equivalent internal resistance, Rs this then gives us the following Nortons equivalent circuit.

Nortons equivalent circuit.

Ok, so far so good, but we now have to solve with the original 40Ω load resistor connected across terminals A and B as shown below.

Again, the two resistors are connected in parallel across the terminals A and B which gives us a total resistance of:

The voltage across the terminals A and B with the load resistor connected is given as:

Then the current flowing in the 40Ω load resistor can be found as:

which again, is the same value of 0.286 amps, we found using Kirchoff's circuit law in the previous tutorials.

Nortons Analysis Summary.

The basic procedure for solving Nortons Analysis equations is as follows:

1. Remove the load resistor R_L or component concerned.
2. Find R_S by shorting all voltage sources or by open circuiting all the current sources.
3. Find I_S by placing a shorting link on the output terminals A and B.
4. Find the current flowing through the load resistor R_L.

Thevenins Theorem

noreply@blogger.com (Sabarish) — Thu, 03 Sep 2009 15:29:00 +0000
Thevenins Theorem states that "Any linear circuit containing several voltages and resistances can be replaced by just a Single Voltage in series with a Single Resistor". In other words, it is possible to simplify any "Linear" circuit, no matter how complex, to an equivalent circuit with just a single voltage source in series with a resistance connected to a load as shown below. Thevenins Theorem is especially useful in analyzing power or battery systems and other interconnected circuits where it will have an effect on the adjoining part of the circuit.
Thevenins equivalent circuit.

As far as the load resistor R_L is concerned, any "One-port" network consisting of resistive circuit elements and energy sources can be replaced by one single equivalent resistance Rs and voltage Vs, where Rs is the source resistance value looking back into the circuit and Vs the open circuit voltage at the terminals.

For example, consider the circuit from the previous section.

Firstly, we have to remove the centre 40Ω resistor and short out (not physically as this would be dangerous) all the emf´s connected to the circuit, or open circuit any current sources. The value of resistor Rs is found by calculating the total resistance at the terminals A and B with all the emf´s removed, and the value of the voltage required Vs is the total voltage across terminals A and B with an open circuit and no load resistor Rs connected. Then, we get the following circuit.

Find the Equivalent Resistance (Rs)

Find the Equivalent Voltage (Vs)

We now need to reconnect the two voltages back into the circuit, and as V_S = V_AB the current flowing around the loop is calculated as:

so the voltage drop across the 20Ω resistor can be calculated as:

V_AB = 20 - (20Ω x 0.33amps) = 13.33 volts.

Then the Thevenins Equivalent circuit is shown below with the 40Ω resistor connected.

and from this the current flowing in the circuit is given as:

which again, is the same value of 0.286 amps, we found using Kirchoff´s circuit law in the previous tutorial.

Thevenins theorem can be used as a circuit analysis method and is particularly useful if the load is to take a series of different values. It is not as powerful as Mesh or Nodal analysis in larger networks because the use of Mesh or Nodal analysis is usually necessary in any Thevenin exercise, so it might as well be used from the start. However, Thevenins equivalent circuits of Transistors, Voltage Sources such as batteries etc, are very useful in circuit design.

Summary.

The basic procedure for solving Thevenins Analysis equations is as follows:

1. Remove the load resistor R_L or component concerned.
2. Find R_S by shorting all voltage sources or by open circuiting all the current sources.
3. Find V_S by the usual circuit analysis methods.
4. Find the current flowing through the load resistor R_L.

SR Flip Flop

noreply@blogger.com (Sabarish) — Wed, 02 Sep 2009 15:12:00 +0000

An SR Flip-Flop can be considered as a basic one-bit memory device that has two inputs, one which will "SET" the device and another which will "RESET" the device back to its original state and an output Q that will be either at a logic level "1" or logic "0" depending upon this Set/Reset condition. A basic NAND Gate SR flip flop circuit provides feedback from its outputs to its inputs and is commonly used in memory circuits to store data bits. The term "Flip-flop" relates to the actual operation of the device, as it can be "Flipped" into one logic state or "Flopped" back into another.

The simplest way to make any basic one-bit Set/Reset SR flip-flop is to connect together a pair of cross-coupled 2-input NAND Gates to form a Set-Reset Bistable or a SR NAND Gate Latch, so that there is feedback from each output to one of the other NAND Gate inputs. This device consists of two inputs, one called the Reset, R and the other called the Set, S with two corresponding outputs Q and its inverse or complement Q.
SR Flip Flop Component Outline
SR Flip Flop Circuit Diagram
"Set" State

Consider the circuit shown above. If the input R is at logic level "0" (R = 0) and input S is at logic level "1" (S = 1), the NAND Gate Y has at least one of its inputs at logic "0" therefore, its output Q must be at a logic level "1" (NAND Gate principles). Output Q is also fed back to input A and so both inputs to the NAND Gate X are at logic level "1", and therefore its output Q must be at logic level "0". Again NAND gate principals. If the Reset input R changes state, and now becomes logic "1" with S remaining HIGH at logic level "1", NAND Gate Y inputs are now R = "1" and B = "0" and since one of its inputs is still at logic level "0" the output at Q remains at logic level "1" and the circuit is said to be "Latched" or "Set" with Q = "1" and Q = "0".
"Reset" State

In this second stable state, Q is at logic level "0", Q = "0" its inverse output Q is at logic level "1", not Q = "1", and is given by R = "1" and S = "0". As gate X has one of its inputs at logic "0" its output Q must equal logic level "1" (again NAND gate principles). Output Q is fed back to input B, so both inputs to NAND gate Y are at logic "1", therefore, Q = "0". If the set input, S now changes state to logic "1" with R remaining at logic "1", output Q still remains LOW at logic level "0" and the circuit's "Reset" state has been latched.
Truth Table for this Set-Reset Function

It can be seen that when both inputs S = "1" and R = "1" the outputs Q and Q can be at either logic level "1" or "0", depending upon the state of inputs S or R BEFORE this input condition existed. However, input state R = "0" and S = "0" is an undesirable or invalid condition and must be avoided because this will give both outputs Q and Q to be at logic level "1" at the same time and we would normally want Q to be the inverse of Q. However, if the two inputs are now switched HIGH again after this condition to logic "1", both the outputs will go LOW resulting in the flip-flop becoming unstable and switch to an unknown data state based upon the unbalance. This unbalance can cause one of the outputs to switch faster than the other resulting in the flip-flop switching to one state or the other which may not be the required state and data corruption will exist. This unstable condition is known as its Meta-stable state.
Then, a bistable latch is activated or Set by a logic "1" applied to its S input and deactivated or Reset by a logic "1" applied to its R. The SR Latch is said to be in an "invalid" condition (Meta-stable) if both the Set and Reset inputs are activated simultaneously.

As well as using NAND Gates, it is also possible to construct simple 1-bit SR Flip-flops using two NOR Gates connected the same configuration. The circuit will work in a similar way to the NAND gate circuit above, except that the invalid condition exists when both its inputs are at logic level "1" and this is shown below.
NOR Gate SR Flip-flop

Superposition Theorem

noreply@blogger.com (Sabarish) — Sun, 30 Aug 2009 13:37:00 +0000
Superposition Theorem
Superposition theorem is one of those strokes of genius that takes a complex subject and simplifies it in a way that makes perfect sense.

The strategy used in the Superposition Theorem is to eliminate all but one source of power within a network at a time, using series/parallel analysis to determine voltage drops (and/or currents) within the modified network for each power source separately. Then, once voltage drops and/or currents have been determined for each power source working separately, the values are all “superimposed” on top of each other (added algebraically) to find the actual voltage drops/currents with all sources active.
Let’s look at our example circuit and apply Superposition Theorem to it:

Since we have two sources of power in this circuit, we will have to calculate two sets of values for voltage drops and/or currents, one for the circuit with only the 28 volt battery in effect. . .

. . . and one for the circuit with only the 7 volt battery in effect

When re-drawing the circuit for series/parallel analysis with one source, all other voltage sources are replaced by wires (shorts), and all current sources with open circuits (breaks).

Analyzing the circuit with only the 28 volt battery

Analyzing the circuit with only the 7 volt battery

Super Imposing Voltage

Super Imposing current

Final Circuit

REVIEW
The Superposition Theorem states that a circuit can be analyzed with only one source of power at a time, the corresponding component voltages and currents algebraically added to find out what they’ll do with all power sources in effect.

To negate all but one power source for analysis, replace any source of voltage (batteries) with a wire; replace any current source with an open (break).

Multiplexer

noreply@blogger.com (Sabarish) — Sat, 29 Aug 2009 16:22:00 +0000

Introduction
Multiplexers are used in building digital semiconductor such as CPUs and graphics controllers. In these applications, the number of inputs is generally a multiple of 2 (2, 4, 8, 16, etc.), the number of outputs is either 1 or relatively small multiple of 2, and the number of control signals is related to the combined number of inputs and outputs. For example, a 2-input, 1-output mux requires only 1 control signal to select the input, while a 16-input, 4-output mux requires 4 control signals to select the input and 2 to select the output.
Multiplexers are also used in communications; the telephone network is an example of a very large virtual mux built from many smaller discrete ones. Instead of having a direct connection from every telephone to every telephone - which would be physically impossible - the network "muxes" individual telephones onto one of a small number of wires as calls are placed. At the receiving end, a demultiplexer, or "demux", chooses the correct destination from the many possible destinations by applying the same principle in reverse.
There are more complex forms of multiplexers. Time-division multiplexers, for example, have the same input/output characteristics as described above, but instead of having a control signal, they alternate between all possible inputs at precise time intervals. By taking turns in this manner, many inputs can share one output. This technique is commonly used on long distance phone lines, allowing many individual phone calls to be spliced together without affecting the speed or quality of any individual call. Time-division multiplexers are generally built as semiconductor devices, or chips, but can also be built as optical devices for fiber optics applications.
Even more complex are code-division multiplexers. Using mathematical techniques developed during World War II for cryptographic purposes, they have since found application in modern cellular networks. Generally referred to by the acronim "CDMA" - Code Division Multiple Access - these semiconductor devices work by assigning each input a unique complex mathematical code. Each input applies its code to the signal it receives, and all signals are simultaneously sent to the output. At the receiving end, a demux performs the inverse mathematical operation to extract the original signals.
Example
The circuit symbol for the above multiplexer is:
Two input multiplexer

One circuit I've received a number of requests for is the multiplexer circuit. This is a digital circuit with multiple signal inputs, one of which is selected by separate address inputs to be sent to the single output. It's not easy to describe without the logic diagram, but is easy to understand when the diagram is available.
The multiplexer circuit is typically used to combine two or more digital signals onto a single line, by placing them there at different times. Technically, this is known as time-division multiplexing.
Input A is the addressing input, which controls which of the two data inputs, X0 or X1, will be transmitted to the output. If the A input switches back and forth at a frequency more than double the frequency of either digital signal, both signals will be accurately reproduced, and can be separated again by a demultiplexer circuit synchronized to the multiplexer.
This is not as difficult as it may seem at first glance; the telephone network combines multiple audio signals onto a single pair of wires using exactly this technique, and is readily able to separate many telephone conversations so that everyone's voice goes only to the intended recipient. With the growth of the Internet and the World Wide Web, most people have heard about T1 telephone lines. A T1 line can transmit up to 24 individual telephone conversations by multiplexing them in this manner.
A very common application for this type of circuit is found in computers, where dynamic memory uses the same address lines for both row and column addressing. A set of multiplexers is used to first select the row address to the memory, then switch to the column address. This scheme allows large amounts of memory to be incorporated into the computer while limiting the number of copper traces required to connect that memory to the rest of the computer circuitry. In such an application, this circuit is commonly called a data selector.
Multiplexers are not limited to two data inputs. If we use two addressing inputs, we can multiplex up to four data signals. With three addressing inputs, we can multiplex eight signals.

NPN transistors
Code	Structure	Case style	I_C max.	V_CE max.	h_FE min.	P_tot max.	Category (typical use)	Possible substitutes
BC107	NPN	TO18	100mA	45V	110	300mW	Audio, low power	BC182 BC547
BC108	NPN	TO18	100mA	20V	110	300mW	General purpose, low power	BC108C BC183 BC548
BC108C	NPN	TO18	100mA	20V	420	600mW	General purpose, low power
BC109	NPN	TO18	200mA	20V	200	300mW	Audio (low noise), low power	BC184 BC549
BC182	NPN	TO92C	100mA	50V	100	350mW	General purpose, low power	BC107 BC182L
BC182L	NPN	TO92A	100mA	50V	100	350mW	General purpose, low power	BC107 BC182
BC547B	NPN	TO92C	100mA	45V	200	500mW	Audio, low power	BC107B
BC548B	NPN	TO92C	100mA	30V	220	500mW	General purpose, low power	BC108B
BC549B	NPN	TO92C	100mA	30V	240	625mW	Audio (low noise), low power	BC109
2N3053	NPN	TO39	700mA	40V	50	500mW	General purpose, low power	BFY51
BFY51	NPN	TO39	1A	30V	40	800mW	General purpose, medium power	BC639
BC639	NPN	TO92A	1A	80V	40	800mW	General purpose, medium power	BFY51
TIP29A	NPN	TO220	1A	60V	40	30W	General purpose, high power
TIP31A	NPN	TO220	3A	60V	10	40W	General purpose, high power	TIP31C TIP41A
TIP31C	NPN	TO220	3A	100V	10	40W	General purpose, high power	TIP31A TIP41A
TIP41A	NPN	TO220	6A	60V	15	65W	General purpose, high power
2N3055	NPN	TO3	15A	60V	20	117W	General purpose, high power
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors
Code	Structure	Case style	I_C max.	V_CE max.	h_FE min.	P_tot max.	Category (typical use)	Possible substitutes
BC177	PNP	TO18	100mA	45V	125	300mW	Audio, low power	BC477
BC178	PNP	TO18	200mA	25V	120	600mW	General purpose, low power	BC478
BC179	PNP	TO18	200mA	20V	180	600mW	Audio (low noise), low power
BC477	PNP	TO18	150mA	80V	125	360mW	Audio, low power	BC177
BC478	PNP	TO18	150mA	40V	125	360mW	General purpose, low power	BC178
TIP32A	PNP	TO220	3A	60V	25	40W	General purpose, high power	TIP32C
TIP32C	PNP	TO220	3A	100V	10	40W	General purpose, high power	TIP32A
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.

Input Characteristics:-	Common Base -	I_E ÷ V_EB
	Common Emitter -	I_B ÷ V_BE

Output Characteristics:-	Common Base -	I_C ÷ V_C
	Common Emitter -	I_C ÷ V_C

Transfer Characteristics:-	Common Base -	I_E ÷ I_C
	Common Emitter -	I_B ÷ I_C

Characteristic	Common Base	Common Emitter	Common Collector
Input impedance	Low	Medium	High
Output impedance	Very High	High	Low
Phase Angle	0^o	180^o	0^o
Voltage Gain	High	Medium	Low
Current Gain	Low	Medium	High
Power Gain	Low	Very High	Medium