Class C amplifiers conduct for less than 50% of the input signal. The active device in case of class C amplifiers is so biased that it conducts for less than one half of the input cycle, that is, it operates for less than 180o of the input cycle (refer to figure below).
Input and output waveforms of class C amplifiers
Class C amplifiers have high efficiency upto 90%. However, they are associated with a very high level of distortion at the output.
Class C amplifiers operate in the following two modes.
• Untuned mode class C amplifier
• Tuned mode class C amplifier
Class C amplifiers are mostly they are operated in the tuned mode.
Figure below shows a tuned class C amplifier configuration. The tuned circuit provides a full cycle of the output signal at the fundamental or the resonant frequency of the tuned circuit. Hence, the unwanted frequencies are drastically suppressed. Other residual harmonics can be removed by using a filter.
Tuned class C amplifier
In class C amplifiers only a small portion of the input cycle is passed through the amplifier. This distorts the input signal and hence class C amplifiers are not used for audio applications.
Class C amplifiers are generally used as RF and IR amplifiers in RF transmitters and are primarily not intended for use as large signal or power amplifiers.
In applications where linearity is not the main issue and efficiency is of primary concern, class C, class D, class E and class F amplifiers are used.