Prediction of odd-mode instabilities under output mismatch effects

Abstract

A methodology is presented to predict odd-mode instability in power amplifiers under output mismatch effects, as in the case of amplifiers connected to an antenna. This kind of instability is often observed in power-combining configurations, due to their symmetry properties. Unlike the single-ended situation, there is a cancellation of odd multiples of the oscillation frequency at the circuit output, so there is no impact of the load impedance values at the sideband frequencies. The odd-mode instability depends on the terminations at the fundamental frequency and its harmonic terms, and can only be detected within the circuit unstable loop, instead of the output plane. Here a methodology for the prediction and suppression of odd-mode instabilities is presented. Low-pass filtering effects and the use of a shorted stub allow the stability analysis to be limited to the fundamental-frequency termination. Then, the stability boundaries are efficiently determined through bifurcation detection inside the unstable loop, using the magnitude and phase of the reflection coefficient as the analysis parameters. Results have been validated through pole-zero identification and experimental measurements.