Optimized delta sigma modulation for Class-S power amplifiers based on GaN switching transistors

A novel modulation concept based on Delta-Sigma modulation dedicated for a Class-S Amplifier based on GaN technology is presented. This paper includes system considerations of efficient modulation schemes for GaN switching transistors, which suffer from many limits. The analyses are focused on evaluating the impact of modulation scheme on the switching conditions of the transistors. Moreover practical figures of merits are introduced for defining optimum modulation strategy. Simulations are performed for a carrier frequency of 890.88 MHz for UMTS 5 MHz bandwidth signal. Results describe the main differences and limits of modulation schemes dedicated for switching mode power amplifiers.

I. Introduction

Main research efforts push towards the realization of efficient Switch Mode Power Amplifiers (SMPA), which can offer very high output power, despite the linear classes of amplifiers which offer high output power and low distortions. The general Class-S PA shown in fig. 1 indicates main parts of circuit, where the input single bit signal converter is the key point of the whole amplifier. SMPAs topology requires two state driving signals, because transistors are engaged as switches and operate in ON and OFF states. There are very few techniques of modulation signals for SMPA's, but they are based on different pulse length modulation. The Modulation (DSM) is based on quantised pulse lengths, and the shortest pulse widths are connected with the sampling frequency which determines maximum expected bit rate for the whole amplifier. Transistor maximum switching frequency is still the main limit of the Class-S PA architecture and determines maximum bit rate of the input signal. The proper modulation strategy optimized for used transistor technology is the key point of Class-S amplifier research.