A high-sensitivity CCD image sensor using source follower circuit with actively controlled gain characteristics

Abstract

A high-sensitivity charge coupled device (CCD) image sensor that utilizes a new output structure has been developed. The structure utilizes an enhancement inverter, a coupling capacitor, and resistors in addition to the conventional source follower. The output of the source follower is inverted and ac component of that signal is fed back to the gate of the load transistor of source follower to modulate the impedance. The overall gain improvement can be estimated from simple small-signal analysis and the results match well to the circuit simulation. A 1/4 in. 250,000 pixels color CCD image sensor was fabricated employing the new output circuit. The fabrication process is same to that for conventional CCD image sensors since the inverter in this circuit consists of enhancement mode transistors. It is observed that 14% of sensitivity improvement in the CCD image sensor is achieved by the new structure and the linearity shows no distortion compared to conventional image sensors in the full range of imaging illumination.