PURPOSE: To generate a single phase AC call signal output by interrupting a DC power supply inputted to a transformer in response to a change in the rectangular wave output of an oscillator having the basic frequency of a call signal output.

CONSTITUTION: A call signal output 10 is full-wave-rectified by a full wave rectifier circuit 11 and divided by a voltage dividing circuit 12. Error amplifiers(EA) 14, 15 amplify a difference from a DC reference voltage 13 and its output is inputted to PWM circuits 16, 17 a high frequency oscillator 18 is connected to one terminal, outputs of the error amplifier(EA) 14, 15 and a triangle carrier are compared to implement pulse width modulation control. A call signal fundamental frequency is set by a rectangular wave low frequency oscillator 1 and the operation of DRV circuits 3, 4 and conversion sections 7, 8 provided respectively on the positive and the negative sides is switched for the positive and the negative polarity and the output waveform of the call signal device is generated. The outputs of the PWM circuits 16, 17 are sent to the DRV circuit 3 or 4 as a driving pulse in response to the positive or negative time of the oscillation period of a switching device 2.