pnp transistor 65 decreases therefore the current flowing into resistor 83 decreases so that the electric potential at the non inverting inputs of operational amplifiers 62 63 declines further the electric potential at the inverting inputs of operational amplifiers 62 63 declines by an imaginary short cut to become the same as the electric potential at the non inverting inputs therefore the electric potential at the connection between the emitter of npn transistor 68 and resistor 72 declines as a result the current flowing through resistor 72 which is the collector current ic3 of npn transistor 68 decreases so that the voltage divided voltage vx rises similarly the electric potential at the connection between the emitter of npn transistor 69 and resistor 73 declines by the decline of the electric potential at the inverting input of operational amplifier 63 as a result the current flowing through resistor 73 which is the collector current ic4 of npn transistor 69 decreases so that the reference voltage vr rises on the other hand the non inverting input of operational amplifier 81 is applied to the inverting input of operational amplifier 61 where the voltage of the inverting input of operational amplifier 61 is restricted at a predetermined voltage va by an imaginary short cut further the inverting input of operational amplifier.