AM/PM distortion in nonlinear circuits [power amplifier applications]

In a typical communication circuit, high power amplifiers are commonly used to increase the strength of a transmitted signal. Such amplifiers often operate near saturation and, hence, give rise to nonlinear effects that can degrade performance. The nonlinear effects of an amplifier at a specified frequency may be quantified by the AM/AM and AM/PM, the amplitude and phase modulations or distortions as functions of the amplitude of the input signal. Common design practices frequently attempt to ameliorate the effects of these distortions by providing compensation in preceding stages. Unfortunately, such practices often introduce as much distortion as they remove and unfortunately increase the complexity of the design. The following pages give a description of the design issues and results from a study that indicated that the introduction of nonlinear operation into stages preceding the amplifier had no benefit. Rather, the introduction of such nonlinearity gave rise to additional problems, and linear operation of these stages was preferable.