A Zero Bias Pixel Sensor and its Zero-Bias Column Buffer-Direct-Injection Circuit
Two pixel sensors, namely active pixel sensor (APS) and pseudo-active pixel sensor (PAPS), are reviewed to show that APS suffers from dark current while PAPS suffers from leakage current. Then a new pixel sensor called zero bias pixel sensor (ZBPS) in which only two MOS switches in addition to the photodiode are used, one for connecting the pixel’s photodiode to a column bus and the other for bypassing it. A zero-bias column buffer-direct-injection (ZCBDI) circuit, which is similar to a regulated cascode amplifier, is used to control the voltage at column bus at zero. All ZBPS pixels are guaranteed to work at zero voltage at all times to eliminate the dark current as well as leakage current. A case of a 10 µm x 10 µm ZBPS pixel designed with standard 0.18 µm CMOS process is studied through simulation. This pixel generates a photocurrent within a range from 1 pA to 100 nA. To handle a large variation of photocurrent while maintaining zero column voltage, the ZCBDI is designed using differential cascode, common source, and buffer stages and then compensated for 50 degree phase margin. Transient simulation shows that the pixel steady state response time is around 1.406 ms, leading to at most 5.5 frames per second for an image of 128 x 128 ZBPS pixels. The fill factor of ZBPS for this case is around 59%.
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