Class-F power amplifiers with maximally flat waveforms

TLDR

Class‑F power amplifiers shape drain waveforms with harmonic resonators to boost efficiency, but at VHF and higher frequencies parasitic elements make ideal tuning hard, so designers must control impedances at a limited set of harmonics. This paper derives the fundamental relationships between the Fourier coefficients of the waveforms and the amplifier’s performance. The authors calculate maximally flat Fourier coefficients up to the fifth harmonic and tabulate how including different harmonics in the voltage and current waveforms affects performance. Adding harmonics raises efficiency from about 50 % (class A) toward 100 % and increases power‑output capability by up to 27 %.

Abstract

Class-F power amplifiers (PA's) employ harmonic-frequency resonators to shape their drain or collector waveforms to improve efficiency. Generally, the output network must present the drain with either an open or short circuit at the harmonic frequencies. At VHF and higher frequencies, the drain capacitance, lead inductance, lead length, and dispersion make implementation of reasonably ideal tuned circuits difficult. However it is possible to control the impedances at a finite number of harmonics. This note first derives the basic relationships among the Fourier coefficients of the waveforms and the performance of the amplifier. Fourier coefficients for maximally flat waveforms are then derived for inclusion of up to the fifth harmonic. Amplifier performance is then tabulated as a function of which harmonics are included in the voltage and current waveforms. Efficiency increases from 50% of class A toward 100% as harmonics are added. Power-output capability increases by up to 27%.