PURPOSE:To control the current of a light emitting element to be optimum by negative- feedbacking the current of the light outputted emitting element through rays of light outputted from the light emitting element so that the D.C. output voltage of photodetectihg element becomes a reference voltage. CONSTITUTION:The output from a phototransistor 2 is inputted to the negative phase input of an amplifier 3, and its in-phase input is determined by resitors 11 13, and the said output is held by a capacitor 6 as a reference voltage for a certain time constant. In the D.C. operation, the output from the amplifier 3 is negative-feedbacked to the light emitting diode through the base of a light-emitting-diode-driving transistor 7 to control the current of the diode so that the in-phase input to the amplifier 3 from the transistor 2 becomes the reference voltage. When the transmission between the light emitting diode 1 and the transistor 2 is poor, the reference voltage is lowered by the collector-voltage of the transistor 7 through the resistor 11 in order that the amplifier 3 is not cut off. But in the A.C. operation, the negative feedback is made less by a capacitor 8 and a resistor 9, and the share of variation occurring at the boundary between a magnetic tape and a reader tape is outputted to the next stage through a capacitor 10.
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