PURPOSE: To enable connection with a piezoelectric transformer without requiring the adjustment of a high-frequency circuit even when the resonance frequency of the piezoelectric transformer is changed by mounting the digital type self-excited type high-frequency circuit consisting of an amplitude detecting section, an active filter section, a phase meter section and an output section.

CONSTITUTION: An amplitude detecting section 6 installed on the grounding side of a piezoelectric transformer 1 detects the peak value of currents flowing into the piezoelectric transformer 1 and the phase relationship of voltage and currents, and detects the electric output of the piezoelectric transformer 1 and the magnitude of the amplitude of mechanical vibrations. An active filter section 7 functions as a resonance type band-pass filter by a plurality of amplifiers and an inductive element and a capacitive element inserted between the amplifiers. Since output impedance is determined by the internal impedance of a switching element as a switching circuit and an output section 9 has low impedance, input impedance is matched with the input impedance of the piezoelectric transformer 1 having input impedance of several ohms, thus allowing a drive by large power.