Generally, an amplifier or simply amp, is a device for increasing the power of a signal.

An appliance or circuit that increases the strength of a weak electrical signal without changing the other characteristics of the signal.

amplifier types

1. Electronic amplifiers

2. Magnetic amplifier

3. Rotating electrical machinery amplifier

4. Johnsen-Rahbek effect amplifier

5. Mechanical amplifiers

6. Optical amplifiers

Class A amps sound the best, cost the most, and are the least practical. They waste power and return very clean signals.

Class AB amps dominate the market and rival the best Class A amps in sound quality. They use less power than Class A, and can be cheaper, smaller, cooler, and lighter

Class D amps are even smaller than Class AB amps and more efficient, because they use high-speed switching rather than linear control.

Class A refers to an output stage with bias current greater than the maximum output current, so that all output transistors are always conducting current. The biggest advantage of Class A is that it is most linear, ie: has the lowest distortion.

The biggest disadvantage of Class A is that it is inefficient, ie: it takes a very large Class A amplifier to deliver 50 watts, and that amplifier uses lots of electricity and gets very hot.

Class B amps have output stages which have zero idle bias current. Typically, a Class B audio amplifier has zero bias current in a very small part of the power cycle, to avoid nonlinearities. Class B amplifiers have a significant advantage over Class A in efficiency because they use almost no electricity with small signals.

Class B amplifiers have a major disadvantage: very audible distortion with small signals. This distortion can be so bad that it is objectionable even with large signals. This distortion is called crossover distortion, because it occurs at the point when the output stage crosses between sourcing and sinking current. There are almost no Class B amplifiers on the market today.

Class C amplifiers are similar to Class B in that the output stage has zero idle bias current. However, Class C amplifiers have a region of zero idle current which is more than 50% of the total supply voltage.

The disadvantages of Class B amplifiers are even more evident in Class C amplifiers, so Class C is likewise not practical for audio amps.

Class AB amplifiers are almost the same as Class B amplifiers in that they have two driven transistors. However, Class AB amplifiers differ from Class B amplifiers in that they have a small idle current flowing from positive supply to negative supply even when there is no input signal. This idle current slightly increases power consumption, but does not increase it anywhere near as much as Class A. This idle current also corrects almost all of the nonlinearity associated with crossover distortion. These amplifiers are called Class AB rather than Class A because with large signals, they behave like Class B amplifiers, but with small signals, they behave like Class A amplifiers. Most amplifiers on the market are Class AB.