Stability analysis of power amplifiers under mismatching effects

A method is presented to predict the impact of these mismatches on the small/large-signal stability properties of power amplifiers. Using concepts from bifurcation theory, expressions are derived for the stability boundaries, which depend on the termination loads. The boundary calculation does not require any optimization of the circuit parameters. Instead, it is based on a small-signal analysis about either dc or the large-signal periodic solution (depending on the operation conditions) with a very low computational cost. In the large-signal stability analysis, the sidebands with an influence on the stability properties are identified, these depending on the particular amplifier topology and its filtering effects. Next, the stability boundaries are calculated with the conversion-matrix approach. The low computational cost enables a fast tracing of instability regions on the Smith chart and analysis of the evolution of these regions versus the input-drive power. The method has been validated with pole-zero identification and with measurements.