the flip flop vr1 voltage drop between the b e path of the transistor 216 resistance of the resistor 212 12 as seen from the above equation the magnitude of the discharge current i2 depends on the output voltage 250 of the f v circuit 25 if the resistances of the resistors 211 and 212 upon commencing the discharge of the capacitor 217 the output voltage d from the opamp 218 descends as shown in fig1 to reach the bias voltage vr1 at this time a positive pulse voltage appears at the output of the comparator 219 and serves as a reset input signal when receiving at the input terminal r the flip flop circuit 23 is reset so that the output voltage e becomes a low level the output voltage e of high level thus obtained corresponds to the operation result of the operation circuit 21 when the output voltage from the flip flop circuit 23 becomes high level base current is fed to the transistor 261 of the negative going detecting circuit 26 the output voltage of the f v circuit 25 has a value larger than vro and the voltage 250 exceeds the conduction voltage of the transistor 261 the output voltage e of the operation circuit therefore a signal caused by the output voltage e of the operational amplifier 21 is applied through the resistor 263 to the base of the transistor 261 to thereby turn on the transistor 261 with this charge stored in the capacitor 264 with the polarity.