Output universality in maximum efficiency linear power amplifiers

A general linear passive termination is considered for a typical field effect power transistor such that the first m odd harmonics, excluding the fundamental frequency, are open circuited with the remaining harmonics short-circuited. Under this termination, with the appropriate resistive termination at the fundamental frequency, it is shown that an optimum maximum efficiency of is universally achieved independent of non-linearities in the transistor. Furthermore, where higher ordered harmonics are terminated in finite reactive impedances, which is the case with any realizable network, it is shown that the same maximum efficiency is obtained with the correct complex termination at the fundamental frequency. A prototype network is then defined including the output capacitance of the transistor and synthesized in a lowpass form which, when terminated in a shunt resonant circuit and load resistor, will provide the correct impedances at the fundamental and all of the harmonics. Remarkably, this optimum network has a simple formula for the element values in the general (2m+1) th degree network and a rigorous proof is presented in the appendix. Copyright © 2003 John Wiley & Sons, Ltd.