Which class of amplifier operates over less than 180 degrees of the cycle?

A Class C amplifier is biased well beyond its cutoff point, meaning that output current only flows during the peak of the input signal. This period of conduction corresponds to significantly less than 180 degrees of the 360-degree input cycle. Because they remain "off" for the majority of the cycle, Class C amplifiers are highly efficient, often exceeding 80% efficiency. However, they produce severe distortion and are non-linear, making them unsuitable for amplifying complex waveforms like SSB or audio. They are primarily used in constant-envelope RF transmission, such as CW or FM, where tuned tank circuits can filter out the resulting harmonics. Other classes operate longer: Class A conducts for a full 360 degrees, Class B for exactly 180 degrees, and Class AB for between 180 and 360 degrees.