Chapter 3 - Class-F Power Amplifiers

Abstract

Chapter 3 describes the different Class-F techniques using lumped and transmission-line elements including a quarterwave transmission line. Highly efficient Class-F operation of the power amplifier can be obtained by applying biharmonic or polyharmonic modes when an additional single-resonant or multi-resonant circuit tuned to the odd harmonics of the fundamental frequency is added to the load network. An infinite number of odd-harmonic resonators results in an idealized Class-F mode with a square voltage waveform and a half-sinusoidal current waveform at the device output terminal providing ideally 100% collector (or drain) efficiency. The Class-F mode with a quarterwave transmission line resulting in ideal voltage and current waveforms is analyzed for shunt and series connections of the quarterwave line. In Class-F power amplifiers analyzed in the frequency domain, the fundamental and harmonic load impedances are optimized by short-circuit termination and open-circuit peaking to control the voltage and current waveforms at the device output to obtain maximum efficiency. The effects of the transistor saturation resistance and parasitic shunt capacitance are described. Design examples and practical RF and microwave Class-F power amplifiers are given and discussed.