OFFSET CURRENT CANCELING CIRCUIT

PURPOSE:To eliminate an offset share in a current mode by providing a transistor controlled to always cause a current at a peak value to flow and subtracting the current of a phototansistor from the current at the peak value. CONSTITUTION:A current mirror is composed of transistors Q1, Q2 and Q3. A connection is executed so that the current can flow sufficiently to the base of the Q1 in order that the current. Which flows in the series circuit of a phototransistor Q0 and the Q1, is exclusively controlled by the Q0 and that the current can flow sufficiently to the base of a Q4 is order that the current, which flows in the series circuit of the transistors Q2 and Q4, is exclusively controlled by the Q2. A voltage, which enlarges sufficiently a constant at the time of a CR and corresponds to the peak value of the current of the Q2, is held in a capacitor C. Thus, the current to flow to a Q5 is maintained at the peak value of the current to flow to the Q0. The current to flow to a Q7 is the remainder of the current to flow to the Q5 subtracted from the current to flow to the Q3, and here, the current is obtained in which an offset value is canceled.