RF and Microwave Circuit Design

13Power Amplifiers and Distributed Amplifiers

13.1 Introduction

Power amplifiers are essential devices in any RF or microwave transmission system. Their primary function is to amplify the transmitted signals to a level that will overcome losses in the transmission channel. Since a power amplifier will normally be the last active device in the transmitter chain, any distortion produced within the amplifier will be transmitted to the channel. Distortion will arise due to non-linearities within the amplifier, and consequently issues relating to non-linearity will be the main focus of this chapter, rather than the circuit design of power amplifiers which is outside the scope of the present textbook.

The issue of non-linearity is particularly important for modern, complex digital modulation schemes, which result in high peak-to-average power ratios. These schemes require a high degree of linearity within the power amplifier if error rates are to be kept at an acceptably low level.

The chapter commences with a theoretical discussion of the unwanted frequency components generated by amplifiers with a non-linear power transfer response, and in particular on the influence of third-order distortion products. This is followed by a discussion of the principal methods used to linearize the performance of a power amplifier. Since it is often not possible to achieve the desired level of output power from a single, linearized power amplifier, it may be necessary to combine the outputs of ...