PURPOSE: To generate an output voltage with constant amplitude by providing the feedback winding of a feedback oscillating circuit with the voltage dividing circuit composed of a diode, an accumulator, a resistance, and a feedback control element, and controlling the extent of positive feedback according to the voltage division ratio to the feedback resistance.

CONSTITUTION: The feedback winding of the feedback oscillating ciruit composed of a transistor TR3 and a transformer 2 is provided with the voltage dividing circuit composed of a diode D8, an accumulator 9, a variable resistance 11, a constant voltage source diode D10, a resistance 12, and TR7. The voltage of the winding 2C develops a DC voltage, which corresponds to an oscillation output, across the accumulator 9 through the D8. This voltage is applied to the positive feedback extent control TR7 through the resistance 11 and D10. The positive feedback current of a feedback extent adjusting resistance 6 is shunt to the TR7 according to equivalent resistance to regulate the extent of positive feedback to the base of the TR3 as a driving element. Therefore, the voltage developed across the winding 2c is held constant to obtain the constant amplitude of the output voltage of the oscillating circuit.