TABLE OF CONTENTS In the power amplifier modes so far considered, the RF current and voltage swings have been carefully constrained to keep inside the strongly nonlinear boundaries; the device currents have not been allowed to exceed their saturation points, and the voltages have not been allowed to clip or distort in any way. This has been quite deliberate, and design rules have been based upon the presumption that either form of limiting action would be undesirable. This is a reasonable assumption when the application in question is a modern wireless communications system; linearity is usually a critical specification which has to be met, regardless of the possibilities for higher power and efficiency which may incur more strongly nonlinear behavior. Depending on the modulation system, and the specific application for the PA in the system, there are actually many situations in which linearity, especially amplitude linearity, can be traded for efficiency and RF power output. These applications range from single channel, constant envelope systems such as those using FSK and GMSK which can tolerate high levels of amplitude distortion, through to intermediate cases such as QPSK and DQPSK systems which can tolerate significant amounts of amplitude distortion, within the constraints of spectral mask regulations.
This chapter considers the behavior of conventional Class A, AB, and B modes in overdrive condition. It will be seen that two distinct effects of overdrive require separate consideration; the saturation of device current when heavily driven at the input can result in higher RF power but...
