Linearity and efficiency enhancement techniques in microwave transmitters

In microwave power amplifier (PA) design, high linearity is required in order to maintain the integrity of the amplified, modulated signals. High spectral efficiency in a communication signal can be achieved by the use of non-constant envelope modulations, which place particularly high demands on the PA linearity. High power efficiency is also increasingly important, given the drive to control CO emissions and reduce energy consumption. Simple measures to improve linearity, such as operating a Class A amplifier well backed off from saturation, result in very poor efficiency. The trade-off between linearity and efficiency can be improved by adopting a more complex amplifier topology or incorporating the amplifier into a more complex transmitter architecture. This paper presents a review of existing and recently developed techniques for the enhancement of PA linearity and efficiency. It explains the basic principles behind the techniques, provides comparisons of their relative advantages and disadvantages and discusses possible future directions.