The envelope-simulation method, developed earlier for large-signal simulation [time domain (TRAN)] is extended to include small-signal envelope simulation (ac) and dc sweep simulation (steady state for a range of carrier frequencies). The model is derived for amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM) modulation schemes and is demonstrated on a piezoelectric transformer circuit. The model is based on the equivalent circuit approach and can be run on any modern electronic circuit simulator. An excellent agreement was found between the simulation results according to the new unified envelope model, full simulation, and experimental results.