Charge sensing circuit

Claim 1

In combination, a charge sensing device including a substrate of one conductivity type and a region of opposite conductivity type located therein forming a P-N junction, means for reversely biasing said P-N junction including a source of voltage and a first transistor including a control electrode and a pair of output electrodes, said P-N junction and the output electrodes of said first transistor connected in series circuit with said source of voltage, means for establishing and maintaining a substantially constant voltage with respect to said substrate on said region of opposite conductivity type and thereby producing a substantially zero rate of charge accumulation in said region of opposite conductivity type including a high gain differential amplifier having a non-inverting input terminal, an inverting input terminal, and an output terminal, the voltage at said output terminal being responsive to the difference in voltage at said input terminals, said non-inverting terminal being provided with a reference potential, said inverting terminal being connected to said region of opposite conductivity type, said output terminal being connected through a first feedback path to the control electrode of said first transistor, a capacitor connected between said control electrode and said substrate, means for sensing a change in the rate of charge flow to said region of opposite conductivity type corresponding to a signal applied to said charge sensing device which includes a second feedback path having an impedance connected between said output terminal and said region of opposite conductivity type, whereby the potential on said region of opposite conductivity type remains substantially constant and an output voltage which is proportional to said signal is produced at said output terminal.