Patent ID: 8692919

Claim:
A photo-sensor comprising: a phototransistor for receiving light to generate a photo-current and to output a current generated by amplifying the photo-current; a first MOSFET for detecting the current from the phototransistor; and a second MOSFET having a one non-controlling terminal connected to a base of the phototransistor, wherein the second MOSFET is turned OFF when a light quantity incident in the phototransistor is smaller than a predetermined value, and wherein the first MOSFET operates in a saturation region under a condition that the incident light quantity of the phototransistor is larger than a predetermined value, and wherein a gate potential of the second MOSFET is fixed so that the second MOSFET is turned ON when the light quantity incident in the phototransistor changes so as to be larger than the predetermined value during a sensor operation period for outputting the photo-current from the phototransistor.