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Existing methods for the large-signal stability analysis of microwave circuits assume a periodic excitation, usually consisting of a single tone. However, practical circuits such as power amplifiers will generally be driven by modulated signals or multiple input tones. This paper presents a procedure for the stability analysis of linear time-varying systems, applicable to circuits excited with nonperiodic input signals. The procedure is based on the determination of the time-varying poles associated to an input-output representation of the system. Under modulated signals, this representation is obtained by linearizing the envelope-transient system that rules the circuit behavior. The formulation and methodology have been applied to a simple nonlinear circuit with arbitrary excitation and a practical power-combined amplifier at 3 GHz with very good agreement in comparison with independent simulations and experimental results.