CMOS switch with linearized gate capacitance

• US 6,522,187 B1
• Filed: 03/12/2001
• Issued: 02/18/2003
• Est. Priority Date: 03/12/2001
• Status: Expired due to Term

• Current Assignee: Analog Devices International Unlimited Company (Analog Devices, Inc.)
• Original Assignee: Linear Technology Corporation (Analog Devices, Inc.)
• Inventors: Sousa, Joseph L.
• Primary Examiner(s): Callahan, Timothy P.
• Assistant Examiner(s): ENGLUND, TERRY LEE
• Application Number:
  US09/804,348
• Time in Patent Office: 708 Days
• Field of Search: 327/378, 327/382, 327/381, 327/437, 327/408, 327/391
• US Class Current: 327/391
• CPC Class Codes: G11C 27/02 Sample-and-hold arrangements G11C27/04 takes precedence
• CPC Class Codes: H03K 17/162 without feedback from the output circuit to the control circuit

Abstract

A CMOS switch with compensation circuitry that maintains linearized gate capacitance, said switch capable of selectively processing a signal independent of changes to gate capacitance current. The switch passes signals which are substantially insensitive to changes in source impedance. Thus, the switch processes an analog signal with a minimum of distortion as a result of gate capacitance currents.

Claims

1. A compensation circuit for a CMOS switch, the compensation circuit comprising:

a nonlinear gate capacitance compensation circuit coupled to the CMOS switch; and

biasing circuitry for providing a substantially fixed bias voltage that is coupled to the nonlinear gate capacitance compensation circuit, the biasing circuitry that provides the substantially fixed bias voltage to the nonlinear gate capacitance compensation circuit when the CMOS switch is in an ON state.

2. The compensation circuit of claim 1 further comprising an input voltage (Vin) and a voltage output (Vout), wherein the nonlinear gate capacitance compensation circuit comprises a first drain-source connected switch coupled between Vin and the CMOS switch and a second drain-source connected switch coupled between Vout and the CMOS switch on either side of the CMOS switch.

3. The compensation circuit of claim 2 wherein the first drain-source connected switch comprises a channel width that is about half the size of the channel width of the CMOS switch.

4. The compensation circuit of claim 2 wherein the first drain-source connected switch is a drain-source connected CMOS switch configured at a reverse polarity to the CMOS switch.

5. The compensation circuit of claim 2 wherein the second drain-source connected switch comprises a channel width that is about half the size of the channel width of the CMOS switch.

6. The compensation circuit of claim 2 wherein the second drain-source connected switch is a drain-source connected CMOS switch configured at a reverse polarity to the CMOS switch.

7. The compensation circuit of claim 1 further comprising an input voltage (Vin) and a voltage output (Vout), wherein the nonlinear gate capacitance compensation circuit comprises a drain-source connected switch coupled between Vin and the CMOS switch on a first side of the CMOS switch.

8. The compensation circuit of claim 2 wherein an N-channel portion of the first drain-source connected switch and an N-channel portion of the second drain-source connected switch are configured to turn ON substantially simultaneously to a turn-OFF of a P-channel portion of the CMOS switch.

9. The compensation circuit of claim 2 wherein a P-channel portion of the first drain-source connected switch and a P-channel portion of the second drain-source connected switch are configured to turn ON substantially simultaneously to a turn-OFF of an N-channel portion of the CMOS switch.

10. The compensation circuit of claim 1 further comprising an input voltage (Vin) and a voltage output (Vout), wherein the nonlinear gate capacitance compensation circuit comprises a drain-source connected switch coupled between Vout and the CMOS switch on one side of the CMOS switch.

11. The compensation circuit of claim 2 wherein the biasing circuitry provides the bias voltage to the first drain-source connected switch and the second drain-source connected switch to substantially cancel variations in a process of forming the CMOS switch and a temperature of operation of the CMOS switch.

12. A method of using a gate capacitance compensation circuit for maintaining a substantially constant gate capacitance of a CMOS switch, the gate capacitance compensation circuit being biased with a fixed bias voltage, the CMOS switch having an N-channel device and a P-channel device, and the CMOS switch receiving an input voltage, the method comprising:

compensating a non-linear gate capacitance current in the N-channel device at a first threshold level of the input voltage; and

compensating a non-linear gate capacitance current in the P-channel device at a second threshold level of the input voltage.

13. The method of claim 12, the compensating in the N-channel device comprising providing two opposite channel polarity P-channel capacitance currents.

14. The method of claim 13, wherein the providing two opposite channel polarity P-channel capacitance currents further comprises providing each of the opposite channel polarity P-channel capacitance currents having half the value of the non-linear gate capacitance current in the N-channel device at the first threshold.

15. The method of claim 13, wherein the two opposite channel polarity P-channel capacitance currents are substantially independent of process variations and operating temperature.

16. The method of claim 12, the compensating in the P-channel device comprising providing two opposite channel polarity N-channel capacitance currents.

17. The method of claim 16, wherein the providing two opposite channel polarity N-channel capacitance currents further comprises providing each of the opposite channel polarity N-channel capacitance currents having half the value of the non-linear gate capacitance current in the P-channel device at the second threshold.

18. The method of claim 16, wherein the two opposite channel polarity N-channel capacitance currents are substantially independent of process variations and operating temperature.

19. A method of using a gate capacitance compensation circuit for maintaining a gate capacitance of a CMOS switch substantially constant, the gate capacitance compensation circuit being biased with a fixed bias voltage, a first drain-source connected switch being coupled between an input voltage and the CMOS switch and a second drain-source connected switch being coupled between the CMOS switch and a voltage output, the method comprising:

turning on an N-channel portion of the first drain-source connected switch and an N-channel portion of the second drain-source connected switch substantially simultaneously to turning OFF a P-channel portion of the CMOS switch; and

turning on a P-channel portion of the first drain-source connected switch and a P-channel portion of the second drain-source connected switch substantially simultaneously to turning OFF an N-channel portion of the CMOS switch.

20. A method of using a gate capacitance compensation circuit for maintaining a gate capacitance of a CMOS switch substantially constant, the gate capacitance compensation circuit being biased with a fixed bias voltage, a first drain-source connected switch being coupled between an input voltage and the CMOS switch and a second drain-source connected switch being coupled between the CMOS switch and a voltage output, the method comprising:

turning ON an N-channel portion of the first drain-source connected switch and an N-channel portion of the second drain-source connected switch, and turning OFF a P-channel portion of the CMOS switch at a first level of the input voltage to the CMOS switch; and

turning ON a P-channel portion of the first drain-source connected switch and a P-channel portion of the second drain-source connected switch, and turning OFF an N-channel portion of the CMOS switch at a second level of the input voltage.

21. A multiplexer circuit comprising:

a plurality of CMOS switches;

a nonlinear gate capacitance compensation circuit for compensating non-linear gate capacitance current in at least one of the plurality of CMOS switches; and

wherein the gate capacitance compensation circuit is biased with a fixed bias voltage.