The class-C amplifier

A bipolar transistor or FET can be biased past cutoff or pinchoff, and it will still work as a power amplifier (PA), provided that the drive is sufficient to overcome the bias during part of the cycle. You might think, at first, that this bias scheme couldn’t possibly result in amplification. Intuitively, it seems as if this could produce a marginal signal loss, at best. But in fact, if there is significant driving power, class-C operation can work very well. And, it is more efficient than any of the aforementioned methods. 
Class-C PAs are never linear, even for amplitude modulation on a signal. Because of this, a class-C circuit is useful only for signals that are either full-on or full-off. Continuous- wave (CW), also known as Morse code, and radioteletype (RYTY) are examples of such signals. Class-C PAs also work well with frequency modulation (FM) because the amplitude never changes.
A class-C PA needs a lot of drive. The gain is fairly low. It might take 300 W of radio-frequency (RF) drive to get 1 kW of RF power output. That’s a gain of only a little over 5 dB. Nonetheless, the efficiency is excellent, and class-C operation is common in CW, RTTY, or FM radio transmitters.