If in the generic switching stage of class E stage, the load network provides a high termination impedance at the second or third harmonics, the voltage waveform across the switch exhibits sharper edges than a sinusoidal wave, thereby reducing the power loss in the transistor. Such a circuit is called a class F stage.

The tank consisting of $L_1$ and $C_1$ resonates at twice or three times the input frequency, approximating an open circuit. As shown, $V_x$ approaches a rectangular waveform with the addition of third harmonic.

We tune impedance to odd harmonics to be an open circuit to dissipate no harmonic power but to allow odd harmonics in voltage waveform.

Also, we tune even harmonics to short circuit to avoid dissipating power.

An ideal class F power amplifier has a Square Voltage Waveform and 100% efficiency.

Disadvantages are - output capacitance of device not naturally absorbed into the network, it needs inductor to tune it. Also, difficult to control more than 5th harmonic as resonators are lossy, diminishing the overall efficiency.