Abstract: A circuit is disclosed for obtaining a quantity of electrical charge carriers equal to and representative of the algebraic difference between the values of two original quantities of electrical charge carriers. The circuit includes two charge coupled device input shift registers. Each of the two shift registers incorporates at least three potential well electrodes and interposed transfer electrodes such that charge packets can be shifted from well to well. One potential well in each of the two shift registers is operated in a floating gate electrode mode, and the two potential wells are connected at a common node which services as the input to a third or output charge coupled device shift register.