Overview

The understanding of RF power amplifiers is greatly helped by a background knowledge of the most important theoretical features of small-signal high-frequency amplifiers. A summary of the subject is well beyond the scope of this book; instead, a short overview is offered. For a full discussion of small-signal amplifiers, please refer to references [1 5].

Small signal implies that the signal amplitude is small enough such that a linear equivalent circuit (such as a hybrid-pi circuit or any linear two-port circuit with constant coefficients) can model the amplifier. RF power amplifiers function very differently from small-signal amplifiers. Power amplifiers operate with large signals, and the active devices display strong nonlinear behavior. The amplifier output may be modeled as an infinite power series consisting of nonlinear terms added to a linear term and a dc offset. The power series coefficients depend on the transistor operating point (dc bias point, or the average operating point) and are considered constant to changes in the input and output RF signal. A more realistic model could use the Volterra series, which allows for the inclusion of phase effects. However, all these models have a serious limitation in that they can only accurately model weak nonlinear circuits for which the power series coefficients are almost constant (a narrow operation zone around the dc-bias point). In a large-signal power amplifier, nonlinear effects are very strong because transistor parameters depend on many factors, including the input and output matching network configuration and the input and output signal amplitudes...