Wideband Active Load Modulation in RF Power Amplifiers

Abstract

For almost a century, active load modulation techniques have been used to increase the backoff efficiency of high frequency power amplifiers. The Doherty architecture, which is based on active load modulation, has become the standard power amplifier architecture in cellular base station transmitters and makes a significant contribution to the energy-efficient operation of cellular mobile radio networks today. In recent times, the increasing availability of ever more powerful digital signal processing also led to a paradigm shift in the field of transmitter systems. The application of complex digital predistortion algorithms opens up new ways to overcome some of the bandwidth limitations that are inherent to classic power amplifier architectures based on active load modulation and thus allows to exploit the efficiency advantages of active load modulation also for the increasingly important wideband operation. In this work, a universal and illustrative method of analysis of multi-harmonic load modulation spaces is developed and the possibilities and characteristics of hybrid modes within the ‘Doherty-Outphasing continuum’ with regard to wideband operation are investigated. In the last part of the work, the theoretical findings are verified using a wideband Doherty PA demonstrator with a peak output power of 300 W in the frequency range of 680 to 1020 MHz.