Light intensity deterioration detecting circuit with compensation for resistance errors

摘要

A light intensity deterioration detecting circuit which includes a photodiode which receives light from a laser diode and having a cathode connected to a power supply and an anode connected to a first end of a resistor. A reference current source is connected between the power supply and a first input of a comparator which has the first input connected to a first end of a resistor and a second input connected to the first end of the resistor, and which compares a comparison voltage provided at the first end of the resistor and a comparison voltage provided at the second end of the resistor to output an output voltage on the basis of the comparison result. Errors of a resistance ratio are reduced to a negligible level if relatively large errors are produced in individual resistances, thus correctly detecting deterioration of a current to light output characteristic of the laser diode without an offset control.