| 摘要 |
PURPOSE:To prevent the variation in the temperature of the detecting characteristics of the photodetecting circuit having a light emitting element and a photodetector by forming a photodetector for receiving the light from the light emitting element in series with the light emitting element and controlling the output of the photodetector so as to be always constant. CONSTITUTION:The light emitting element 1 and the photodetector 2 form a photodetector PS1 for generating a control signal, and the light emitting element 3 and the photodetector 4 form a photodetector PS2 for generating a detection signal. The voltage produced at a resistor R2 due to the output current of the photodetector PS1 is applied through a resistor R3 to a differential amplifier 7, and compared with a voltage divided via resistors R4, R5 from the reference voltage Vs of a reference power source 6. The output of the amplifier 7 is applied to the base of a transistor 5. Since the transistor 5 is so controlled that the voltage across the resistor R2 becomes constant, even if the characteristics of the light emitting element and the photodetector are altered, the variation in the detecting characteristics can be prevented. |
