AMPLIFIERS and POWER AMPLIFIERS

What is an Amplifier?
A device which is used to increase the power of a signal by use of an external energy source is a Amplifier.
Almost every electronic device contains at least one stage of amplification, so you will be seeing amplifiers in many devices that you work on.
CLASSIFICATION OF AMPLIFIERS
Most amplifiers can be classified in two ways:
1)Classification by their function
In this they are basically voltage amplifiers or power amplifiers.
2)Classification is by their frequency response.

POWER AMPLIFIERS and TYPES
Class A— Linear
Class B— AB Linear* (Complementary)
Class C— Nonlinear (RF, Tuned)
Class D— Switching (Linear Audio)

CLASS A Amplifier
Output device(s) conduct through 360 degrees of input cycle (never switch off) – A single output device is possible. The device conducts for the entire waveform. Class A amplifiers have the general property that the output device(s) always carry a significant current level, and hence have a large Quiescent Current.
The Quiescent Current is defined as the current level in the amplifier when it is producing an output of zero.

General Characterstics:
1)Class A amplifiers vary the large Quiescent Current in order to generate a varying current in the load, hence they are always inefficient in power terms.
2)These amps run hot, as the transistors in the power amp are on and running at full power all the time.
3)There is no condition where the transistor(s) is/are turned off. That doesn’t mean that the amplifier is never or can never be turned off; it means the transistors doing the work inside the amplifier have a constant flow of electricity through them. This constant signal is called “bias”.
Disadvantage: The Class-A amplifier consumes a high and constant DC power.

CLASS B Amplifier
Output devices conduct for 180 degrees (1/2 of input cycle) – for audio, two output devices in “push-pull” must be used.
In this amp, the positive and negative halves of the signal are dealt with by different parts of the circuit. The output devices continually switch on and off.


General Characteristics:
The input signal has to be a lot larger in order to drive the transistor appropriately.
This is almost the opposite of Class A operation
There have to be at least two output devices with this type of amp. This output stage employs two output devices so that each side amplifies each half of the waveform.
Each output device is on for exactly one half of a complete signal cycle.
Advantage
Tthe dissipate lesser power than the Class A amplifiers.
Disadvantage:
The sound quality is not as good, as there is a lot of “crossover” distortion, as one output device turns off and the other turns on over each signal cycle.

CLASS AB Amplifier
Halfway (or partway) between the above two examples (181 to 200 degrees typical) – also requires push-pull operation for audio.
Class AB operation has some of the best advantages of both Class A and Class B built-in. Its main benefits are sound quality comparable to that of Class A and efficiency similar to that of Class B.


General Characteristics:
1)Class AB amps operate in Class A at lower output levels.
2)The output bias is set so that current flows in a specific output device for more than a half the signal cycle but less than the entire cycle.
CLASS C Amplifier
In Class-C conduction is for less than 180 degrees (100 to 150 degrees typical) – Radio Frequencies only – cannot be used for audio.

Class D
These amplifiers are also called “digital” amplifiers.
They are better termed “switching” amplifiers, because here the output devices are rapidly switched on and off at least twice for each cycle.
Depending on their switching frequency, they may be “switched on” or “off” millions of times a second.Class D audio uses a fixed, high frequency signal having pulses that vary in width based on input signal amplitude
In practice, they have 80-90% efficiency.This efficiency gain is at the cost of high-fidelity.
Class D amplifiers are generally used for non-high-fidelity, or subwoofer applications.
Few more types such as CLASS G and CLASS H amplifiers are also available.