Active Pixel Sensor Circuit Simple version: 3 transistors (pixel reset, source follower, access), one photodiode Not shown: Row read circuitry, a timed buffer connected to v_out and activated a set time after pixel access

Active Pixel Sensor Circuit Operation: –Reset transistor gate pulsed, photodiode junction cap. charged up, source follower output follows –Photocurrent starts discharging, SF output follows –Access transistor gate pulsed, output cap.charged up –Bias current discharges output voltage linearly until the set read time for v_out

Active Pixel Sensor Circuit Can model photodiode with junction cap. and ideal current source in parallel

Active Pixel Sensor Circuit Design issues: –Transistor sizes should be as small as possible for the maximum photodiode/pixel ratio (“fill factor”) –Transistor sizes should be chosen carefully for enough current, SF gain, and isolation of SF output from the pixel output –Higher bias current: readout needs to be a smaller time after access is pulsed; timing gets tighter, reset period can be set smaller –Larger reset period: Better resolution, tighter readout timing required