Class C Amplifier

The active element (transistor) of a class C power amplifier conducts for less than one half cycle of the input signal. The conduction angle has a usual range of 80° to 120° and is less than one-half cycle, which indicates that it is less than 180°. Although the efficiency is greatly enhanced, the smaller conduction angle results in significant distortion. A Class C amplifier's theoretical maximum efficiency is roughly 90%. The Class C designs are not employed in audio applications because of the significant distortion. The Class C amplifier is most frequently used in RF (radio frequency) circuits like RF oscillators, RF amplifiers, etc., where additional tuned circuits are used to recover the original input signal from the Class C amplifier's pulsed output, minimizing the impact of the amplifier's distortion on the output. A tank circuit, consisting of inductor L1 and capacitor C1, helps to extract the necessary signal from the transistor's pulsed output. The active element (transistor)'s function in this system is to generate a sequence of current pulses in response to the input and cause them to flow through the resonant circuit. L1 and C1 values are chosen in such a way that the resonant circuit oscillates at the input signal's frequency. All other frequencies are muted since the resonant circuit only oscillates at one frequency (often the carrier frequency), allowing the appropriate frequency to be extracted using a properly tuned load. Using additional filters, harmonics or noise in the output signal can be removed.