What circuit is required at the output of an RF switching amplifier?

RF switching amplifiers, such as Class D, E, or F, operate by toggling the output devices rapidly between fully on and fully off. This hard switching action generates a waveform that behaves much like a square wave, which is inherently rich in harmonic energy. Because transmitting these heavy harmonics would cause severe out-of-band interference, a robust output filter is strictly required. A low-pass network smooths the switching waveform back into a clean sine wave before it reaches the antenna.