A high efficiency linear power amplifier for portable communications applications

The class AB power amplifier has been used widely in conjunction with non-constant envelope modulation systems such as QPSK, QAM and CDMA. Unfortunately, the "peak envelope power to average power" ratio (PEP/Pavg) of such systems combined with the need to maintain PEP below saturation generally results in poor DC to RF conversion efficiency. Techniques such as envelope tracking, envelope elimination and restoration (EER), or linear amplification using nonlinear components (LINC) are available to improve efficiency. However, these methods involve complex architectures, representing revolutionary as opposed to evolutionary efficiency solutions. Among manufacturers, the competition to extend the talk-time of portable communication products involves a careful weighing of cost, size and reliability trade-offs. As a result, the high efficiency power amplifier method of W. H. Doherty (1936) has experienced renewed interest, more than 60 years after its introduction. This paper describes a practical Doherty power amplifier using GaAs HBT devices. An overview of the deficiencies of common Class AB designs and a brief tutorial of the Doherty concept follows.