Then the gain of an amplifier can be said to be the relationship that exists between the signal measured at the output with the signal measured at the input.
Class A - has low efficiency of less than 40% but good signal reproduction and linearity.
Class B - is twice as efficient as class A amplifiers with a maximum theoretical efficiency of about 70% because the amplifying device only conducts (and uses power) for half of the input signal.
Class AB - has an efficiency rating between that of Class A and Class B but poorer signal reproduction than class A amplifiers.
Class C - is the most efficient amplifier class as only a very small portion of the input signal is amplified therefore the output signal bears very little resemblance to the input signal. Class C amplifiers have the worst signal reproduction.