 The most efficient valve-based RF amplifiers operate Class C. If used with no tuned circuit in the output, this would distort the input signal, producing harmonics.However, class C amplifiers normally use a high Q output network which removes the harmonics, leaving an undistorted sine wave identical to the input waveform.Class C is suitable only for amplifying signals with a constant amplitude, such as FM, FSK, and some CW (Morse code) signals.