CMOS input buffer with high speed and low power

• US 5,304,867 A
• Filed: 12/12/1991
• Issued: 04/19/1994
• Est. Priority Date: 12/12/1991
• Status: Expired due to Term

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Original Assignee: AT&T, Inc.
• Inventors: Morris, Bernard L.
• Primary Examiner(s): Westin, Edward P.
• Assistant Examiner(s): Sanders, Andrew
• Application Number:
  US07/806,890
• Time in Patent Office: 859 Days
• Field of Search: 307/448, 307/443, 307/451, 307/548, 307/550
• US Class Current: 326/71
• CPC Class Codes: H03K 19/0013 in field effect transistor circuits
• CPC Class Codes: H03K 19/018521 of complementary type, e.g. CMOS

Abstract

Prior-art high speed TTL-to-CMOS input buffers consume a large amount of power supply current through the input transistors when the input voltage is held at a mid-range level between V_DD and V_SS (e.g., 2.0 volts). The inventive input buffer includes a resistance in series with the p-channel pull-up transistor on the input inverter, in order to limit this current. In addition, to retain high operating speed, a p-channel shunt transistor is placed in parallel with the resistance, and controlled by the buffer output signal. This shunt transistor effectively bypasses the resistance from the circuit when the buffer output goes low, thereby providing high operating speed.

Claims

1. An integrated circuit having an input buffer comprising an input inverter having a p-channel field effect transistor serially connected with an n-channel field effect transistor that are coupled to receive an input signal, and an output inverter coupled to a buffer output node,

Characterized in that said buffer further comprises a current-limiting transistor biased to conductor and directly connected in series between a controlled electrode of a first one of the complementary transistors of said input inverter and a power supply conductor;

and still further comprises a shunt transistor having controlled electrodes connected in parallel with said current-limiting transistor and having a control electrode coupled to said buffer output node.

2. The integrated circuit of claim 1 wherein said current-limiting transistor is a field effect transistor having its gate connected to a power supply voltage conductor.

3. The integrated circuit of claim 2 wherein said field effect transistor is a p-channel transistor.

4. The integrated circuit of claim 3 wherein said p-channel transistor is an enhancement mode device, and has its gate connected to a negative power supply voltage conductor.

5. The integrated circuit of claim 4 wherein said output inverter comprises a p-channel field effect transistor serially connected with an n-channel field effect transistor.

6. The integrated circuit of claim 2 wherein said field effect transistor is an n-channel transistor.

7. The integrated circuit of claim 6 wherein said n-channel transistor is an enhancement mode device, and has its gate connected to a positive power supply voltage conductor.

8. The integrated circuit of claim 7 wherein said output inverter comprises a p-channel field effect transistor serially connected with an n-channel field effect transistor.

10. The integrated circuit of claim 2 wherein said current-limiting field effect transistor has a width-to-length ratio (W/L) in the range of from 3/200 to 3/20.

9. An integrated circuit having an input buffer comprising:

a first inverter comprising an n-channel field effect transistor having its source connected to a negative power supply conductor, its gate connected to an input node, and its drain connected to a first inverter output node; and

a p-channel field effect transistor having its gate connected to said input node, its drain connected to said first inverter output node, and its source connected to a given node;

a second inverter comprising an n-channel and a p-channel field effect transistor having their gates connected to said first inverter output node, their sources connected to negative and positive power supply voltage conductors, respectively, and their drains connected to a buffer output node;

Characterized in that said buffer further comprises a resistance p-channel transistor having its drain connected to said given node, its source connected to a positive power supply voltage conductor, and its gate connected to a negative power supply voltage conductor;

and still further comprises a shunt p-channel transistor having its drain connected to said given node, its source connected to said positive power supply voltage conductor, and its gate connected to said buffer output node.

11. The integrated circuit of claim 9 wherein said resistance p-channel field effect transistor has a width-to-length ratio (W/L) in the range of from 3/200 to 3/20.