Differential amplifier having an improved slew rate

Abstract

A differential amplifier receives a differential input signal and generates an output signal at an output node. An auxiliary circuit coupled to the differential amplifier operates to improve slew rate response. In quiescent and small signal situations with respect to the differential input signal, the auxiliary circuit does not alter or change operation of the differential amplifier. However, in situations where a large signal change is experienced with respect to the differential input signal, the auxiliary circuit functions to speed up the sourcing and sinking current to/from the output node. A stability compensation capacitor coupled to the output node is accordingly more quickly charged or discharged and an improvement in slew rate performance of the differential amplifier is experienced.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained by reference to the accompanying drawings wherein:

FIG. 1 is a circuit diagram of a conventional and traditional, prior art, differential amplifier;

FIG. 2 is a graph of voltage versus time with respect to the output voltage of the differential amplifier of FIG. 1;

FIG. 3 a graph of voltage versus time with respect to an input voltage square wave;

FIG. 4 is a circuit diagram of a differential amplifier in accordance with an embodiment of the present invention; and

FIG. 5 is a graph of voltage versus time with respect to the output voltage of the differential amplifier of FIG. 4.

Claims

1. A circuit for connection to a compensation capacitor associated with a differential amplifier, comprising: a current source circuit coupled to sense differential amplifier operation and source current to the compensation capacitor in response to detecting that the differential amplifier is operating responsive to a first high input voltage differential; anda current sink circuit coupled to sense differential amplifier operation and sink current from the compensation capacitor in response to detecting that the differential amplifier is operating responsive to a second high input voltage differential.

2. The circuit of claim 1 wherein the current source circuit comprises: a current source transistor having a conduction path coupled to the compensation capacitor and a control terminal; anda sensing circuit having an input measuring a current sourced by the differential amplifier to an output node and an output coupled to the control terminal of the current source transistor.

3. The circuit of claim 2 wherein the output of the sensing circuit causes the current source transistor to turn on in response to the sensing of current being sourced by the differential amplifier.

4. The circuit of claim 1 wherein the current sink circuit comprises: a current sink transistor having a conduction path coupled to the compensation capacitor and a control terminal; anda sensing circuit having an input measuring a current sunk by the differential amplifier from an output node and an output coupled to the control terminal of the current sink transistor.

5. The circuit of claim 4 wherein the output of the sensing circuit causes the current sink transistor to turn on in response to the sensing of current being sunk by the differential amplifier.

6. The circuit of claim 1 wherein the circuit is fabricated as an integrated circuit.

7. A differential amplifier, comprising: a differential amplifier stage having a differential voltage input and a current output node;a compensation capacitor coupled to the current output node; anda slew rate enhancement stage coupled to the differential amplifier stage and the compensation capacitor, the slew rate enhancement stage operable to selectively source/sink current to/from the current output node in addition to any current sourced/sunk to/from the current output node by the differential amplifier stage itself.

8. The differential amplifier of claim 7 wherein the slew rate enhancement stage comprises: a charge source circuit coupled to sense differential amplifier stage operation and source the additional current to the compensation capacitor in response to detecting that the differential amplifier stage is operating responsive to a first high differential voltage input; anda charge sink circuit coupled to sense differential amplifier stage operation and sink the additional current from the compensation capacitor in response to detecting that the differential amplifier stage is operating responsive to a second high differential voltage input.

9. The differential amplifier of claim 7 wherein the slew rate enhancement stage comprises: a controllable current circuit operable to selectively source/sink current to/from the compensation capacitor; anda circuit for sensing whether current is being sourced/sunk to/from the current output node by the differential amplifier stage itself and in response thereto control the operation of the controllable current source to assist in sourcing/sinking current so as to improve slew rate during swings in differential voltage input.

10. The differential amplifier of claim 7 wherein the differential amplifier is fabricated as an integrated circuit.

11. A differential amplifier, comprising: a differential amplifier stage having differential voltage inputs, a current source transistor coupled to a current output node and a current sink transistor coupled to the current output node;a compensation capacitor coupled to the current output node;a current source sensor circuit coupled to the current source transistor and operable to detect when current is being sourced to the current output node by the differential amplifier stage itself;a current sink sensor circuit coupled to the current sink transistor and operable to detect when current is being sunk from the current output node by the differential amplifier stage itself;an additional current source transistor also coupled to the current output node and operable responsive the current source sensor circuit to source additional current to the current output node; andan additional current sink transistor also coupled to the current output node and operable responsive the current sink sensor circuit to sink additional current to the current output node.

12. The differential amplifier of claim 11 wherein the current source sensor circuit comprises: a first copy transistor with a common control terminal connection to the current source transistor of the differential amplifier stage; anda first current supply coupled at a first node in series with the first copy transistor, the first node coupled to control the additional current source transistor.

13. The differential amplifier of claim 12 wherein the current sink sensor circuit comprises: a second copy transistor with a common control terminal connection to the current sink transistor of the differential amplifier stage; anda second current supply coupled at a second node in series with the second copy transistor, the second node coupled to control the additional current sink transistor.

14. The differential amplifier of claim 11 wherein the differential amplifier is fabricated as an integrated circuit device.

15. A differential amplifier, comprising: a differential amplifier stage having a differential voltage input and a current output node;a compensation capacitor coupled to the current output node; anda capacitor charge/discharge stage coupled to the differential amplifier stage and the compensation capacitor, the capacitor charge/discharge stage operable to detect when the differential amplifier stage is operating responsive to a high differential voltage input and respond thereto by assisting the differential amplifier stage in more quickly charging/discharge the compensation capacitor than the differential amplifier stage is capable of doing by itself.

16. The differential amplifier of claim 15 wherein the capacitor charge/discharge stage comprises: a charging circuit operable response to a first high differential voltage input swing to source a first current in addition to a differential amplifier stage supplied second current to the current output node; anda discharging circuit operable response to a second, opposite, high differential voltage input swing to sink a third current in addition to a differential amplifier stage sunk fourth current to the current output node.

17. The differential amplifier of claim 16 wherein: the charging circuit includes a first current sensor operable to detect differential amplifier stage supplied second current and source a first current in response thereto; andthe discharging circuit includes a second current sensor operable to detect differential amplifier stage sunk fourth current and sink the third current in response thereto.

18. The differential amplifier of claim 17 wherein: the first current sensor functions to compare the second current to a first reference current and generate a first control signal in response to the comparison; andthe second current sensor functions to compare the fourth current to a second reference current and generate a second control signal in response to the comparison.

19. The differential amplifier of claim 18 wherein: the charging circuit further includes an auxiliary current source having a control terminal coupled to receive the first control signal and generate the first current in response thereto; andthe discharging circuit further includes an auxiliary current sink having a control terminal coupled to receive the second control signal and generate the second current in response thereto.

20. The differential amplifier of claim 19 wherein the auxiliary current source and auxiliary current sink each comprise a transistor.

21. The differential amplifier of claim 15 wherein the differential amplifier is fabricated as an integrated circuit device.