Effect of second and third harmonic input impedances in a class-F amplifier

Abstract

In this paper, the design of a class-F radio frequency power amplifier with a multiharmonic input transmission line network is presented. Harmonic signal components at the gate come from several sources including nonlinear device capacitances and imperfect output harmonic terminations that create harmonic components that are fed back to the gate through the gate-drain capacitance. The effect of these harmonic generation mechanisms and the potential to shape the gate waveform to improve power efficiency are investigated. The study shows that a second harmonic short is most beneficial and the effect of a third harmonic termination is less significant. The concepts are applied to the design of a 10W GaN class-F amplifier and the design is supported by theoretical, simulation and experimental results. The fabricated design has a measured drain efficiency of 78.8% at an output power of 40.5dBm for a frequency of 990 MHz. The amplifier was also tested with a 8.8 dB peak-to-average power ratio 5MHz WCDMA signal. With the modulated signal, the adjacent channel power ratio was ¡33:1 dBc at a drain e±ciency of 46.1% without predistortion correction.