Digital Communication Systems

APPENDIX

H  Nonlinear Solid-State Power Amplifiers

One of the most critical constraints imposed on the design of hand-held devices (terminals) in mobile radio communications is that of limited battery power. These devices are designed for the purpose of a certain battery life or time taken for recharging the battery; the corresponding electronic circuitry must therefore respect the underlying power budget. Moreover, a significant consumer of power in mobile radio is the transmit power amplifier. Attention must therefore be paid to solid-state power amplifiers in mobile radio, hence this appendix.

Another point to keep in mind is that power amplifiers are inherently nonlinear, regardless of where they are used in the design of communication systems. In this context we may classify nonlinearities into one of two types:

• low-pass or band-pass;
• memoryless or with memory.¹

In this appendix, we focus attention on band-pass nonlinearities.

H.1   Power Amplifier Nonlinearities

There are many amplifier designs, and they have been traditionally categorized in the electronics literature as Class A, Class B, Class AB, Class C, Class D, and so on, typically increasingly nonlinear. Although Class A is considered to be a linear amplifier, no amplifier is truly linear; what linearity means in this context is that the operating point is chosen such that the amplifier behaves linearly over the signal range. The drawback of the Class A amplifier is that it is power inefficient. Typically, 25% or ...