drives each switching element power mos fet of the four quadrant chopper 12 in accordance with the outputs from the comparison circuits 24 and 28 and their inverted signals delayed by the dead time circuit 34 the comparison circuit 24 and inverter 30 corresponds to switching elements q a and u2212 q a of the four quadrant chopper 12 the comparison circuit 28 and inverter 32 corresponds to switching elements q b and u2212 q b of the four quadrant chopper 12 as shown in fig2 the pi control circuit 20 includes an operational amplifier 38 two resistors r 1 a variable resistor r 2 and a capacitor c 4 the output from the pi control circuit 20 is represented by expression 1 below a gain k 1 and integral time constant t 1 are respectively represented by expressions 2 and 3 below accordingly it is possible to adjust the gain k 1 by the variable resistor r 2 and adjust the integral time constant t 1 by the capacitor c 4 as shown in fig3 the carrier generating circuit 22 includes operational amplifiers 40 and 42 a variable resistor r 3 a resistor r 4 a variable resistor r 5 a resistor r 6 a variable resistor r 7 and a capacitor c 5 also the inverting amplification circuit 26 includes an operational amplifier 44 and resistors r 8 and r 9 the carrier generating circuit 22 generates a triangular carrier wave having a frequency as indicated by the carrier generating circuit 22 includes a hysteresis comparator using an operational amplifier 40 and an integrating circuit using an operational amplifier 42 the variable resistors r 3 r 5 and r 7.