A highly linear CMOS APS circuit for column-parallel readout with improved gain and process tolerance

This paper presents a CMOS active pixel sensor circuit that ensures high linearity in a voltage-mode readout while exhibiting improved small-signal gain and process tolerance. The proposed circuit results in a maximum nonlinearity of 0.00013% in a 180 nm CMOS process while operating in TT corner, which is four and three orders smaller than an NMOS-input source follower and a 5-T OTA buffer, respectively. The small-signal voltage gain of our circuit is almost unity and around 25% higher than the NMOS-input source follower. The proposed circuit exhibits the least variation in its small-signal gain and linearity across corners compared to the source follower and the 5-T OTA buffer. The improvement is mainly due to the negative feedback employed in the amplifier used in the proposed circuit. The paper also presents a transistor-level placement scheme that suits the column-parallel readout architecture.