three classes of transistor as an amplifier....?

Question

please dont refer wikipedia.....and explain in easy language....taking me as a beginner

Answer 1

CLASS ‘A’ AMPLIFIER.

Fig. 16.4 This amplifier is one in which the bias voltage and the signal amplitude are such that the collector current flows continuously over the complete cycle (360 degrees). It also flows when no signal is present. In other words the transistor never goes into saturation and never cuts off.

Disadvantages:— Poor efficiency, 50% maximum. Has a tendency to overheat which may cause thermal runaway.

Advantages:— Cheap, only one transistor is required. Can be coupled to the next stage via a simple capacitor-resistor coupling or, it can be direct coupled to the next stage.

CLASS ‘B’ AMPLIFIER.

Fig.16.5. This is one in which the bias is set at the cut-off point. Collector current flows for one half (180 degrees) of the signal cycle.

Disadvantages:— More costly, two transistors are required for linear operation.

Possibility of cross-over distortion.

Requires a means of splitting the phase so that each transistor uses one half of each cycle.

Requires a transformer to reform the signal wave shape at the output.

Advantages:— Better efficiency, 65%. More power out.

The even harmonics are cancelled at the output.

Because of the extra components and cost a class B amplifier is mainly use for the power amp. (PA) stage.

This amplifier can be used at radio frequencies using only one transistor.

CLASS "C" AMPLIFIER.

This amplifier is strictly a radio frequency amplifier, it is only mentioned here for comparison.

It is biased below the cut-off point and collector current flows for less than 180 degrees.

It is a high efficiency amplifier, 75% or more and it generates harmonics. In other words it is not linear.

Disadvantages:— Cannot be used where the signal contains audio frequencies.

Advantages:— High efficiency, 65 to 85%. Can be used to multiply the frequency of the signal, this is accomplished by ‘picking out’ a harmonic with a suitable tuned circuit.

Answer 2

class A means that the signal is amplified on the linear part of the transistors operation curve..



this means that the input signal is reproduced with almost 100 percent purity



this type of amplifier is used for low power applications such as pre amps where less amplification is needed but the signal has to be reproduced with as few defects as possible..



generally the load line is used for this If memory serves..

............c...........o

..........c

........c

......c...........o.........o,,,,,,,,,,,o

....c

..c...the o's represent output signal

c....................................o



........i

............i

........i......i's represent input signal

....i

........i

............i





this diagram is only crude to show what I mean by whole signal is amplfied



here is a more specific article on class A amps

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part2/page1.html



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Class B are more efficient power wise amps but they use the 0 voltage point as the Q point and have 2 transistors .. one transistor amplifies one half of the signal.... one transistor amplifies the other half..



the problem occurs when its switching polarity it doesnt always create a true reproduction of the signal and thus create DISTORTION..



http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part2/page2.html





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Class C is power amplifier operation..



basically it is biased so that only a portion of the curve is accurately reproduced.. and it drives the transistor far into cutoff for the other half of the signal.



with a Class C only half of the signal is amplified and thus more power can be used in amplification getting higher amplification



then at the end of the amplification circuit the other half of the signal is reproduced with the use of a TUNED circuit ..



this means that Class C amplifiers have a more limited band width than Class A because they require a Tuned circuit to reproduce the second half of the signal



this also means that the signal is NOT A TRUE REPRODUCTION OF THE INPUT signal



THIS MAY sound like a Class B but that is not true ... because Class B uses 2 transistors one to amplify the signal where Class A only amplifies less than ONE HALF of the signal..



Hope that helps you a bit ... email me if you need some type of clarification

Answer 3

In class A, the entire signal is amplified as an exact copy but a standing bias current flows at all times.



In class B, only half of the signal (e.g. the positive half cycles) is used to control one half of the output stage while the other half of the signal controls the other half of the output. This is often known as Push-pull and is common in audio amplifiers. As one half of the circuit is turned off at any time, the standing current is lower than class A.



In class C, only a small part of the signal is used to switch on a transistor (for example). Using a tuned circuit, the entire waveform is regenerated in amplified form. This is most commonly used in RF circuits and other where distortion is unimportant. Current only flows when the signal is active.

Answer 4

Transistors are certainly a pair of diodes. Diodes habit modern in one course and block it interior the different course. between the two diodes is the backside. it is likewise prevalent as a gate. A small voltage utilized to the backside facilitates modern to flow from one diode to the different. in terms of certainly transistors, the midsection leg is many times the backside. modern will flow from one outer leg to the different if the voltage of the backside rises to around 0.6V. The transistor will then carry as lots modern because it may cope with. yet otherwise of finding at a transistor is it being a sort of lever. A small volume of rigidity at one end will become a huge volume of rigidity on the different. The transistor acts as an amplifier because of the fact a small volume of modern on the backside may be used to regulate an great volume of modern flowing by the transistor. power transistors can carry sufficient modern that they want a warmth sink utilized. Transistors replaced radio tubes, that have been additionally amplifiers. Early radio tubes have been gentle bulbs wherein the filament became cut back. a third cord became placed between the 1st pair. A small volume of voltage utilized to the midsection cord brought about modern to leap between the severed filament leads. the backside became appearing like a lightening rod, attracting modern in direction of it and then passing it directly to the 2d lead. The voltage utilized to the backside got here from the radio antenna. vulnerable radio alerts brought about sufficient modern to flow by the radio tube to function the electromagnets in telegraphs (the 1st radio tubes basically carried Morse code). the belief became the quantity of power interior the radio sign became a strategies too small to function an electromagnet. Radio tubes or transistors are mandatory to advance the sign because of the fact their gates can administration particularly a lot of modern.