Historically, amplifier class designations seemed to be related to how amplifier devices were biased—that is, the percentage of the input-signal swing over which they conducted. That worked for classes A, B, AB, and C.
Today, class designators can only tell us how recently a new class was invented. Here is the current lineup of audio-amplifiers:
- Class A: single-ended; amplifier device is biased about the center of the input-signal swing.
- Class B: push-pull; each device conducts over half the input-signal swing.
- Class AB: push-pull; each device conducts over slightly more than half the input-signal swing to simplify crossover matching.
- Class D: was originally developed for motor-speed control. In audio, class D amps are generally push-pull; devices that are driven between cutoff and saturation as the input signal modulates pulse duty-cycle. Output is filtered-down to audio band.
- Classes G and H are like class AB amplifiers with multiple power rails. In class H, the input signal modulates the power-rail voltages.
- Classes C and E are radio-frequency power-amplifier configurations.