Classifications of Power Amplifiers

A Class A Amplifier is¹ "an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows at all times."

A Class B Amplifier is¹ "an amplifier in which the grid bias is approximately equal to the cutoff value so that the plate current is approximately zero when no exciting grid voltage is applied, and so that plate current in a specific tube flows for approximately one half of each cycle when an alternating grid voltage is applied."

A Class C Amplifier is¹ "an amplifier in which the grid bias is appreciably greater than the cutoff value so that the plate current in each tube is zero when no alternating grid voltage is applied, and so that plate current flows in a specific tube for appreciably less than one-half of each cycle when an alternating grid voltage is applied."

When the suffix 1 is used it denotes that the grid is not driven positive and that grid current does not flow. The suffix 2 denotes the opposite.