A class-C amplifier is one in which the operating point is selected so that the output current or voltage is zero for more than one half of an input sinusoidal signal cycle.This amplifier is the most efficient power amplifier.It can produce more load power than that of either class-A or class-B amplifier.A class-C amplifier has to be tuned to the sinusoidal frequency to amplify a sinusoidal signal.Due to this fact,the class-C amplifier is known as a tuned amplifier or a narrow band circuit. It mean that it can amplify only the resonant frequency and those frequencies which are closer to it.The class-C amplifiers are used to amplify the signals at radio frequencies to avoid the need for large inductors and capacitors in the resonant circuit.Hence,class-C amplifier is basically a radio-frequency(RF) power amplifier and not an audio power amplifier like class-A and class-B amplifier.
A class-C amplifier is biased to operate for less than 180 degree of the input signal cycle.However,the tuned circuit in the output will provide a full cycle of the output signal for the fundamental or resonant frequency of the tuned circuit of the output.Therefore,the use of such amplifiers is limited for a fixed frequency as take place in communication circuits.Operation of a class-C circuit is not intended primarily for a large signal or power amplifiers.
CHARACTERISTICS OF CLASS-C AMPLIFIER-Main characteristic of class-C amplifier are as follows-
(i) The class-C amplifier is the most efficient power amplifier and its overall efficiency,under certain conditions may approach even 100%.
(ii) Th output current flows only during a part of the positive (or negative) half-cycle of the input signal.This condition is obtained by biasing the transistor below cut-off.
(iii) The output signal does not resemble the input signal because it consist of narrow pulses.