The class-C  amplifier

A bipolar transistor or FET is biased past cutoff or pinchoff, and it will still work as a power amplifier (PA), given that the drive is sufficient to overcome bias during part of the cycle. You might think, at 1st, that this bias scheme could not possibly result in amplification. Intuitively, it seems as if this could generate a marginal signal loss, at its best. But actually, if there is significant driving power, class-C operation can work well. And, it is more efficient than the aforementioned methods. The operating points for class C are labeled in the figures given below.

Class-C PAs are not linear, even for amplitude modulation on the signal. Due to this, a class-C circuit is useful only for signals which are either full-on or full-off. Continuous-wave (CW), also called as Morse code, and radioteletype (RYTY) are examples of such type of signals. Class-C PAs work well with the frequency modulation (FM) as the amplitude never changes.A class-C PA requires a lot of drive. The gain is quite low. It may take 300 W of radio-frequency (RF) drive to get 1 kW of RF power output. That is a gain of only a little over 5 dB. Nonetheless, efficiency is excellent, and class-C operation is common in FM radio transmitters.