There are four types of audio amplifiers - Class A, Class B, Class AB and Class D amplifiers:

Class A: in which the current in each active device supplying the load current is greater than zero throughout each cycle of the signal for all values of load current.

Class B: in which the current in each active device supplying the load current is equal to zero for exactly one-half of each cycle of load current.

Class AB: in which the current in at least one of the active devices supplying the load current is zero for some part of each cycle of load current for some range of values of load current not exceeding the value defined by the rated output power or voltage and the rated load impedance.

Class D: in which the current in each active device supplying the load is switched from zero to a maximum value by a carrier signal, modulation of which conveys the useful signal.