Introduction

Most of this book has been concerned with the design of a single power amplifier gain stage, using a single transistor. Any practical power amplifier will be a subassembly consisting of several driver and gain stages, and the final power stage itself will probably use some form of power-combining network. This chapter addresses several of the specific topics which arise in designing complete amplifier assemblies. These topics include balanced and push-pull configurations, multistage design, power combining, biasing and stability considerations.

The previous focus on single transistor PA stages is easily justified. A PA stage with 10 dB of gain is most likely to consume around 80% of the DC power of the transmitter RF chain and dissipate about the same high percentage of heat. The transistor itself will be by far the most costly individual item, and may be over 50% of the total material cost of the final assembly. Due to its thermal requirements, the final stage will demand the heaviest design and manufacturing overheads, with physical size, mechanical integrity, and heat management requiring attention from a wide range of engineering and manufacturing services.

Indeed, with all these justifications in mind, the design of even the PA driver stage seems a relatively noncritical task. The contribution of the driver stages to the overall efficiency of the assembly is reduced by the factor of the final PA stage gain, and it is quite justifiable to design all the driver stages as simple Class A, or light AB, amplifiers. This...