Patent ID: 6909462

Claim:
A photosensor circuit comprising: light-electricity converting means which converts a light signal into electricity, an n-channel MOS transistor which has a logarithmic characteristic in its weakly reversed condition and converts a sensor current generated by said light-electricity converting means into a detection voltage which has a logarithmic characteristic, a capacitor which in connection to a detection terminal of said n-channel MOS transistor, and initializing means which applied to reset voltage to a gate of said n-channel MOS transistor, thereby lowering an independence between a drain and a source to control charging and discharging of said capacitor; wherein an n-channel MOS transistor is a depletion type n-channel MOS transistor (QD 1 ), which has a logarithmic characteristic in its weakly reversed condition and stays in a high impedance condition without a gate voltage being applied; and a low impedance condition is created between the drain and the source of said depletion type channel MOS transistor (QD 1 ) when a drain voltage (VD) or a predetermined voltage (VS) lower than the drain voltage (VD) is applied as the gate voltage (VG) for said depletion type n-channel MOS transistor (QD 1 ).