Patent ID: 6025935
Filing Date: 2000-02-15
Classification: H04N

Abstract:
A contact image sensor circuit comprising:a plurality of chips butted end-to-end, each of said chips comprisinga plurality of sensing elements arranged in an array, each of said sensing elements comprises a phototransistor with a base storage capacitor formed by a base-to-collector capacitance of said phototransistor, a readout-multiplexing switch, a pre-charge switch, an isolation switch, and a reset switch,control and driving clocks to control a timing and a scan advance of said sensor circuit,a digital scanning shift register, andat least one buffer amplifier; whereinduring an integration phase of a scan cycle of the sensor circuit, said pre-charge switch, said readout-multiplexing switch, and said reset switch are open so that said base storage capacitor is isolated, causing a current proportional to an incident light detected to flow from a collector of each sensing element in an activated array to a base of each said sensing element, andduring a sampling phase of the scan cycle, said readout multiplexing switch is activated and closed in sequence by an output pulse of said digital scanning shift register, causing a current proportional to said detected incident light to flow into an output capacitor of an output video circuit, thereby enabling said output video circuit to read an output voltage of said phototransistor activated by said readout multiplexing switch, andduring a pre-charge phase of the scan cycle, said pre-charge switch of said phototransistor is closed and a background charge is injected into said base storage capacitor, said isolation switch of said phototransistor being open during said pre-charge phase so that said background charge does not effect said output voltage, andduring said pre-charge phase, said output capacitor is also reset, thereby readying said output video circuit to sample a next phototransistor in sequence, andduring a sampling phase of said next phototransistor in sequence, said activated phototransistor is reset; such thatsaid background charge applied during said pre-charge phase is reset through a base-to-emitter junction of said activated phototransistor, thereby eliminating any image-lag problem and replenishing any charge loss caused by any sub-threshold leakage current of said base-to-emitter junction, thereby also eliminating any low-light-level thresholding and improving a photo-response linearity of each of said phototransistors.