Stability analysis of nonlinear circuits driven with modulated signals

Nonlinear microwave circuits are usually simulated with the harmonic-balance method, which is insensitive to the stability properties of the solution obtained. Therefore, this method must be complemented with stability-analysis techniques, in order to check the physical existence of the solution. Existing techniques assume a periodic excitation, usually consisting of a single tone. However, practical circuits such as power amplifiers are generally be driven by modulated signals or multiple input tones. This paper presents a methodology for the stability analysis of nonlinear circuits in the presence of modulation signals. This methodology is based on the determination of the time-varying poles associated to an input-output representation of the system. In the presence of modulated signals, this representation is obtained by linearizing the envelope-transient system that rules the circuit behavior about the modulated steady-state solution. The formulation and methodology have been applied to a practical power-combined amplifier at 3 GHz, with very good results in the comparison with independent simulations and experimental results.