Power amplifier classes

In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's efficiency, linearity and other characteristics.

Broadly, as you go up the alphabet, the amplifiers become more efficient but less linear, and the reduced linearity is dealt with through other means.

The first classes, A, AB, B, and C, are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input.^[1] This metric is known as conduction angle (${\displaystyle \theta }$). A class-A amplifier is conducting through the entire period of the signal (${\displaystyle \theta =360}$°); class-B only for one-half the input period (${\displaystyle \theta =180}$°), class-C for much less than half the input period (${\displaystyle \theta <180}$°).

Class-D and E amplifiers operate their output device in a switching manner; the fraction of the time that the device is conducting may be adjusted so a pulse-width modulation output (or other frequency based modulation) can be obtained from the stage.

Additional letter classes are defined for special-purpose amplifiers, with additional active elements, power supply improvements, or output tuning; sometimes a new letter symbol is also used by a manufacturer to promote its proprietary design.

By December 2010, classes AB and D dominated nearly all of the audio amplifier market with the former being favored in portable music players, home audio and cell phone owing to lower cost of class-AB chips.^[2]

In the illustrations below, a bipolar junction transistor is shown as the amplifying device. However, the same attributes are found with MOSFETs or vacuum tubes.