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Audio Amplifier Classes

Class A
The transistor amp conducts for the entire cycle of input signal. The conduction angle is 360 deg. These amps run hot, as the transistors in the power amp are on all the time, but the upside is high sound quality.

Class B
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. These amps run cooler, but the sound quality is not as pure.

Class AB
These amps work by biasing the transistor amp at a non-zero DC current much smaller than the peek current of the signal source. The second transistor conducts during negative half cycle of waveform and the currents from the 2 transistors are combined at the load. A compromise between sound quality of Class A and efficiency of Class B. Most modern amp designs employ this method.

Class D

There are really two types of Class D amplifiers.
Analog-Controlled Class D: Switching amplifiers with an analog input signal and an analog control system. Normally some degree of feedback error correction is presentDigitally-Controlled Class D: Amplifiers with a digitally generated control that switches a power stage. No error control is present. Those that do have an error control can be shown to be topologically equivalent to an analog-controlled class D with a DAC in front. Class G
The power supply voltage in thee amps changes from a lower level to a higher level when larger output swings are required. This is accomplished by utilizing a single class AB output stage connected to two power supply rails by a diode, or a transistor switch. For the majority of program material, the output stage is connected to the lower supply voltage, but it automatically switches to the higher rails for large signal peaks. One other approach uses 2 class AB output stages, each one connected to a different power supply voltage. The strength of the input signal determining the corresponding signal path.