The circuit is biased such that M1 turns on if the peak value of Vin raises Vx above Vth. Figure shows that Vx exceeds Vth for only a fraction of the period, as if M1 were stimulated by a narrow pulse. As a result, the transistor delivers a narrow pulse of current to the output every cycle. In order to avoid large harmonic levels at the antenna, the machine network must provide some filtering. In fact, the input impedance of the matching network is also designed to resonate at the frequency of interest, thereby making the drain voltage sinusoid.

The distinction between class C and class B power amplifier is in the conduction angle. As the angle decreases, the transistor is on for a smaller fraction of the period, thus dissipating less power. For the same reason, the transistor delivers less power to the load.

If the drain current of M1 is assumed to be the peak section of a sinusoid and the drain voltage is a sinusoid having a peak amplitude of Vdd, then the efficiency can be calculated as -

This relation suggests an efficiency of 100% as conduction angle approaches to zero.