New insight into subharmonic oscillation mode of GaAs power amplifiers under severe output mismatch condition

Abstract

This paper provides new insight into the cause of subharmonic generation in GaAs MESFET power amplifiers under severe output mismatch conditions. A nonlinear modeling methodology is developed to identify the root cause of subharmonic oscillations and provides a means to implement circuit techniques to suppress this mode of oscillation without degrading the desired power amplifier performance. The primary mechanism for low-order subharmonics generated as a result of large signal drive and severe output mismatch is shown to be parametric. This modeling-based technique is enabled through the use of a time domain MESFET model which achieves excellent convergence by avoiding the discontinuity in high-order derivatives that is typical of implementations that use conditional functions. This technique provides the design engineer for the first time the ability to determine the large signal parametric stability of a power amplifier operating into a severe mismatch and implement circuit solutions prior to MMIC fabrication.