High signal to noise ratio optical image reader

An optical image reader for reading a document, comprising:

• an array of photosensing elements comprising a plurality of photosensing elements;
• a signal common line;
• a dummy common line;
• a plurality of signal switches each corresponding to each photosensing element, each switch being composed of a complementary MOS switch comprising a P channel MOS switch element and an N channel MOS switch element connected in parallel with each other, each switch having a first switch terminal, a second switch terminal, a first control terminal and a second control terminal, the first switch terminal of each signal switch being connected to the corresponding photosensing element and the second switch terminal thereof being commonly connected to the signal common line;
• a plurality of dummy switches each combining with the corresponding signal switch to constitute a pair of switches, each dummy switch being composed of a complementary MOS switch comprising a P channel MOS switch element and an N channel MOS switch element connected in parallel thereto, each dummy switch also having a first switch terminal, a second switch terminal, a first control terminal and a second control terminal, the first switch terminal of each dummy switch being commonly connected to the dummy common line, the second switch terminal thereof keeping an open state, the first control terminal thereof being connected to the first control terminal of the corresponding signal switch, and the second control terminal thereof being connected to the second control terminal of the corresponding signal switch;
• a plurality of inverters each corresponding to each pair of switches, each inverter having an input terminal and an output terminal, the input terminal of each inverter being connected to the first control terminals of the corresponding signal switch and dummy switch, and the output terminal thereof being connected to the second control terminals of the corresponding signal switch and dummy switch;
• a scanning pulse generating means for providing scanning pulses to each pair of switches comprising the signal switch and dummy switch, having plural output terminals each being connected to the input terminal of the corresponding inverter and the first switch terminal of the corresponding signal switch and dummy switch;
• and
• a differential amplifier having two input terminals being connected to the signal common line and the dummy common line respectively for detecting the difference between an output delivered from the signal common line and an output delivered from the dummy common line.