Operating Mode Class A-B-C-D powe amplifier system panel

Class A Operating Mode

One device conducts for the whole of the

 a.c. cycle.

Conduction angle = 360 °.

The Class A stage must be biased at a

current greater than the amplitude of the

signal current.

Class B Operating Mode

Two devices, each conducting for half

 of the a.c. cycle.

Conduction angle = 180 °. 

Class AB Operating Mode

Two devices, each conducting for just over

 half of the a.c. cycle.

Conduction angle > 180 ° but << 360 ° .

Class C Operating Mode

One device conducts a small portion of 

the a.c. cycle.

Conduction angle << 180 °. 

Class D Operating Mode

Each output device always either fully on or off – theoretically zero power dissipation. 

Example: The built-in speaker in a PC is driven by a Class D type “on/off’ circuit.