Patent ID: 7479999

Claim:
A photo-detecting device comprising: a photodetector for detecting incident light; an input J FET for reading out the sensing signal from the photodetector; an amplifier for amplifying the signal obtained by the input FET; a feed-back circuit for feeding the output of the amplifier back to the gate of the input FET through a feed-back capacitor; a reset circuit for resetting the feed-back capacitor; a reset MOS FET in the reset circuit for discharging the feed-back capacitor to reset the capacitor; a circuit of a switch and resistor; wherein the same voltage level as the gate voltage of the input FET element is applied to the source (or drain) of the reset MOS FET through a resistor, a circuit of switch S 2 and resistor is connected between the source (or drain) of the reset MOS FET and the feed-back capacitor, and the reset MOS FET and switch S 2 are controlled so that the reset MOS FET is turned “on” and “off” while the switch S 2 is “on”.