Class C Principle of Operation

This is an over simplified RF power stage. It lacks stability but it'd have to do for an example. Let's start with some AF topologies. Class A works by conducting the whole 360dgs wave cycle. Maximum efficiency is 50%. Class B works by conducting only half of the cycle (180dgs). Efficiency is better at 78.5%. Class AB takes the best of both and the transistor conducts a little over half the wave (181dgs-200dgs). Efficiency is still good at 70%. With class C amps the active device conducts for a very small fraction of the whole wave leading to very hard cut-off distortion. Class C amps are therefore not used as audio amplifiers. They're used as a radio frequency amplifier where an output filtering can restore the original waveform, no matter how distorted. As you see after 5 order filtering the original signal is completely restored and undistorted, without harmonics. If the output is not filtered your transmission will be heard on every even number of the fundamental. This means that if you're transmitting on 100Mhz, your station will be picked up on 200Mhz 300Mhz 400Mhz and so on. This is very dirty and contaminates the electromagnetic spectrum, which is against the law. The ITU-T states that every transmitter must have output filtering. Class C amps have a very high efficiency of 80%+ (more than class B and less than class D).

published 10 years ago