Abstract: An improved oscillatory circuit in which the output of a radio frequency transformer is coupled from a half bridge squarewave amplifier to piezoelectric ultrasonic transducers through a serially connected feedback network. The feedback network employs a resistance connected in parallel with the serially connected combination of a capacitance, a feedback inductance and a feedback transformer. The improved feedback network provides a variable phase shift in feedback current to the half bridge squarewave amplifier tending to hold the frequency generated therein at a target frequency above anti-resonance.