Bucket background subtraction circuit for charge-coupled devices

Abstract

There is disclosed a circuit for removing the DC or background representing charge component of a signal to be passed through a charge coupled device shift register without degrading the information carried by the AC component of the signal. Such a circuit may, for example, be used to remove the constant background illumination component of the input from a semiconductor imaging device to increase the contrast ratio of the image signal. The circuit comprises two electrode defined potential wells or buckets formed in a semiconductor substrate with a control gate between them and a transfer gate which controls the flow of the charges in the first bucket to a P-N junction. Like the well defining electrodes, the control gate and transfer gate are electrodes separated from the semiconductor substrate by an insulation layer. The transfer gate functions as a channel switch to control the flow of the background charges from the first bucket through the P-N junction to an external drain in discrete quantized increments under the control of an appropriate clock voltage on the transfer electrode. This subtraction of the DC component from the signal passing through the shift register results in a reduction in the size of the storage electrodes required to transmit a given quantity of signal charge through the shift register for fixed clock voltages and thereby makes more economical use of the chip area.