Stability criteria for power amplifiers under mismatch effects

Potential instability of power amplifiers (PAs) under mismatch effects is analyzed, with emphasis on the impact of the PA termination at the fundamental frequency. A large-signal version of the μ factor is derived, considering the lower and upper sideband of the fundamental frequency as virtual observation ports. This requires the application of the conversion matrix approach to each steady-state solution obtained with harmonic balance when varying the fundamental termination. To evaluate the impact of the fundamental output load, a contour technique is used to assemble loads giving the same μ value. A stabilization procedure is provided, based on the mapping of the mismatched load onto the stable region of the fundamental termination Smith chart. The analysis is extended to consider the baseband frequency, using three virtual observation ports.