Pulse-width modulation for switching mode power amplifiers

The application of highly efficient switching mode power amplifiers can be significantly broadened to amplify a wider class of time-varying envelope signals provided a suitable pulse train is synthesized for amplification. Pulse-Width Modulation (PWM) is one method for encoding the source signal into a binary level pulse train and can be implemented at baseband or directly at radio frequency (RF) as RFPWM. This paper evaluates the performance of baseband PWM and RFPWM for switching mode power amplifiers. It is shown that baseband PWM has significant advantage in terms of important pulse modulation parameters: coding efficiency, average transition frequency, and pulse width. Thus, switching-mode amplifiers with baseband PWM are expected to achieve relatively better performance. Verified at 800 MHz with a W-CDMA signal and a class-D amplifier constructed using GaN FET devices, 33.1 dBm output power and 59.2% drain efficiency is obtained for baseband PWM, comparing to 31.8 dBm output power and 35.3% drain efficiency obtained with RFPWM. On the other hand, baseband PWM has much higher demands on the bandpass filter (BPF) used at the output of the power amplifier for spur filtering and signal restoring. A class-F parallel amplification topology with three-level interleaving pulse modulation is proposed to suppress the odd-order output spurs and to ease the BPF design.

I. Introduction

Efficiency of RF power amplifiers (PAs) is of major concern when designing wireless transmitters, and the most efficient RF PAs operate in switching mode. However, when used for modulated signals with a time-varying envelope, switching mode PAs result in excessive spectral growth and loss of modulation accuracy. Thus, the desire to use highly efficient switching mode PAs yet achieving high linearity has been the subject of various research efforts [1]–[5].