The invention comprises apparatus for comparing the filtered power transformer output waveform of a pulse-width-modulated inverter circuit with an AC reference voltage waveform and generating output voltage signals to control the inverter drive circuit to the extent necessary to produce an inverter output waveform substantially duplicating the reference voltage waveform and substantially void of low order harmonics. An anticipatory signal developed by a circuit which monitors the change in current with respect to time in a capacitor included in a filter circuit connected to the output of the power transformers provides optimum response of the apparatus to changes in the output waveform beyond prescribed units relative to the reference voltage waveform.