A low-voltage PWM CMOS imager with small pixel size using an in-pixel gate-common comparator

Abstract

We propose a new pixel structure of pulse-width-modulation (PWM) CMOS active pixel sensor with a small pixel size performing a pixel-level single-slope A/D conversion to realize a low and single power supply operation. The pixel is composed of only three transistors without pixel-circuit sharing, and has a gate-common amplifier that compares a photodiode voltage on the gate node with a ramp signal on the source for A/D conversion. We have fabricated a 128×96-pixel prototype imager with an on-chip ramp generator and bootstrap circuits in a 0.35-µm CMOS technology, and successfully demonstrated image acquisition with a 1.4-1.8-V single power supply.