Hewlett-packard Company

Joey Doernberg

Travis N Blalock

Thomas Hornak

Richard A Baumgartner

Circuitry and circuitry layout are provided to achieve a high percentage of photoreceiver area to total area and to stabilize the voltage at the base node of a phototransistor. Voltage stabilization is achieved by a servo circuit in which a negative feedback loop from the base node to an emitter node maintains a bias point, so that photocurrent is efficiently delivered to charge transfer circuitry. In the preferred embodiment, the base node is connected to a gate of a first transistor having a drain that is connected to a source of constant current and to a gate of a second transistor that functions as a source follower. The source of the second transistor is connected to the emitter node of a phototransistor. As photocurrent is generated by the reception of light, an integration capacitor is charged. The relatively high percentage of photoreceiver area to total area is achieved by contributions from a number of factors, including the design of the servo circuit, use of a single transfer amplifier to service an entire row of phototransistors, and pairing the phototransistor circuits in order to permit efficient sharing of common circuit paths and circuit components.

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San Carlos, CA, United States of America

Joey Doernberg