Protection circuit

Claim 2

In combination: at least one enhancement-type metal oxide semiconductor transistor of one conductivity type which is connectable to a source of bias voltage and which is operative only when said bias voltage is applied thereto; and at least one depletion-type metal oxide semiconductor transistor of opposite conductivity type which is connectable to a source of bias voltage, which is conductive only when said bias voltage is not applied thereto, and which is nonconductive only when said bias voltage is applied thereto; said depletion-type transistor being coupled to said enhancement-type transistor so that said depletion-type transistor shunts any static voltage applied to said enhancement-type transistor prior to the application of said bias voltage to said enhancement-type and depletion-type transistors.

Claim 3

The invention according to claim 1 wherein said transistors each have a drain electrode, a gate electrode, and a source electrode; wherein the source and drain electrodes of said depletion-type transistor are connected between the gate electrode and one of the source and drain electrodes of said enhancement-type transistor; and wherein said gate electrode of said depletion-type transistor and the other one of said source and drain electrodes of said enhancement-type transistor are connected to a terminal which is connectable to said source of bias voltage.

Claim 4

The invention according to claim 2 wherein said one of said source and drain electrodes of said enhancement-type transistor are connected to a second terminal which is connectable to a point of reference potential; wherein said protection circuit further includes a third terminal connectable to an input signal, and means, including a resistance, for coupling said third terminal to the junction of said depletion-type transistor and the gate electrode of said enhancement-type transistor; and wherein said static voltage is applied to said enhancement-type transistor, in the absence of said depletion-type transistor, by being built up between said second and third terminals.

Claim 5

On an integrated circuit chip which includes first, second, and third terminals respectively connectable to a source of bias voltage, a source of data voltage, and a point of reference voltage, and which further includes at least one enhancement-type field effect transistor of one conductivity type having a source electrode, a drain electrode, and a gate electrode, there being means for coupling said source and drain electrodes between said first and third terminals, said enhancement-type transistor being operable only when said source of bias voltage and point of reference voltage are connected to said first and third terminals respectively, said enhancement-type transistor being susceptible of being rendered permanently inoperative due to static voltage which can build up between said second and third terminals, a static voltage protection circuit for said enhancement-type transistor comprising: at least one depletion-type field effect transistor of opposite conductivity type having a source electrode, a drain electrode, and a gate electrode; means for coupling said source and drain electrodes of said depletion-type transistor between said gate electrode of said enhancement-type transistor and the one of said source and drain electrodes of said enhancement-type transistor which is coupled to said third terminal; means including a resistor for coupling said second terminal to the junction of said depletion-type transistor and the gate of said enhancement-type transistor; and means for coupling the gate electrode of said depletion-type transistor to said first terminal.

Claim 6

On an integrated circuit chip which includes first, second, and third terminals respectively connectable to a source of bias voltage, a source of data voltage, and a point of reference voltage, and which further includes at least one enhancement-type field effect transistor of one conductivity type having a source electrode, a drain electrode, and a gate electrode, there being means for coupling said source and drain electrodes between said first and third terminals, said enhancement-type transistor being operable only when said source of bias voltage and point of reference voltage are connected to said first and third terminals respectively, said enhancement-type transistor being susceptible of being rendered permanently inoperative due to static voltage which can build up between said second and third terminals, a static voltage protection circuit for said enhancement-type transistor comprising: at least one depletion-type field effect transistor of opposite conductivity type having a source electrode, a drain electrode, and a gate electrode; means for coupling said source and drain electrodes of said depletion-type transistor between said gate electrode of said enhancement-type transistor and the one of said source and drain electrodes of said enhancement-type transistor which is coupled to said third terminal; means including a resistor for coupling said second terminal to the junction of said depletion-type transistor and the gate of said enhancement-type transistor; and means for coupling the gate electrode of said depletion-type transistor to said first terminal.

Claim 7

The invention according to claim 4 wherein said transistors are metal oxide semiconductor transistors and wherein said one type conductivity is P-channel and said opposite type conductivity is N-channel.

Claim 8

The invention according to claim 4 wherein said protection circuit further includes: a second depletion-type field effect transistor of said opposite conductivity having a source, a drain, and a gate electrode, means for coupling said source and drain electrodes of said second depletion-type transistor between the end of said resistor remote from the gate electrode of said enhancement-type transistor and said one of said source and drain electrodes of said enhancement-type transistor, and means for coupling said gate electrode of said second depletion-type transistor to said first terminal; and wherein said means for coupling said second terminal to said junction includes a second resistor coupled between said remote end of said first resistor and said second terminal.

Claim 9

The invention according to claim 6 wherein said protection circuit further includes a diode coupled between said second and third terminals, said diode being poled to be reverse-biased by the application of said source of data voltage to said second terminal.