Patent ID: 7486322

Claim:
A light sensor circuit working by producing in a photoelectric converting element a photo current proportional to a quantity of light falling thereon and converting the photo current into a voltage signal by using a sub-threshold region characteristic of a transistor having a logarithmic output characteristic in a weak inverse state and outputting a sensor signal corresponding to the converted voltage signal, wherein said light sensor circuit is provided with a switching transistor for charging/discharging a parasite capacitor of the photoelectric converting element and an electric charge remaining in the parasite capacitor is removed by turning on the switching transistor before detecting a light signal by the photoelectric converting element, and wherein a turning-off voltage Vsw(off) of the switching transistor is expressed as Vsw(off)≦VG(Q 1 )−Vth(Q 1 )−Vth(Q 4 ) wherein VG(Q 1 ) is a gate voltage of the transistor for logarithmic characteristic conversion, Vth(Q 1 ) is a threshold voltage of the transistor for logarithmic characteristic conversion and Vth(Q 4 ) is a threshold voltage of the switching transistor.