Multi-Path Common Mode Feedback for High Speed Multi-Stage Amplifiers

Abstract

The invention includes methods and systems for providing a multi-path common mode feedback loop in an amplifier system. Embodiments include techniques for dividing a common mode feedback current path to provide a slow common mode feedback current path and a fast common mode feedback current path. The slow and fast paths are configured for controlling common mode feedback current within a small bandwidth.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from consideration of the following detailed description and drawings in which:

FIG. 1 (prior art) is a schematic diagram representative of a circuit known in the arts for providing common mode feedback in a two stage amplifier system;

FIG. 2 is a simplified schematic diagram illustrating examples of methods and systems having a multi-path common mode feedback loop according to preferred embodiments of the invention; and

FIG. 3 is a schematic diagram of an example of a system embodying a preferred method of implementing a multi-path common mode feedback loop in an amplifier system according to the principles of invention.

Claims

1. A system for common mode feedback in a fully differentiated multi-stage amplifier system, comprising: a multi-path common mode feedback loop configured for narrow bandwidth and large bias current.

2. A system according to claim 1 wherein the multi-path common mode feedback loop comprises a slow path.

3. A system according to claim 1 wherein the multi-path common mode feedback loop comprises a slow path having a low pass filter configured to ensure that the slow path is inactive at approximately the unity gain bandwidth of the system.

4. A system according to claim 1 wherein the multi-path common mode feedback loop comprises a slow path having a low pass filter.

5. A system according to claim 1 wherein the multi-path common mode feedback loop comprises a fast path.

6. A system according to claim 1 wherein the bandwidth of the multi-path common mode feedback loop is described by the ratio of the transconductance gain of a fast path and the compensating capacitance of the fully differentiated multi-stage amplifier system.

7. A multi-path common mode feedback system for a high speed multi-stage amplifier, comprising: a slow path further comprising a series RC low pass filter operably coupled to a slow path transistor; anda fast path further comprising a fast path transistor;wherein the slow path and fast path are configured for controlling a common mode feedback current within a small bandwidth.

8. A system according to claim 7 wherein the fast path is configured for controlling common mode loop bandwidth.

9. A system according to claim 7 wherein the slow path and fast path are configured in combination for controlling DC performance.

10. A system according to claim 7 wherein the bandwidth of the multi-path common mode feedback loop is described by the ratio of the transconductance gain of the fast path and the compensating capacitance of the high speed multi-stage amplifier.

11. A method for controlling a common mode feedback loop for a multi-stage amplifier system, comprising the steps of: dividing the common mode feedback current path; therebyproviding a slow common mode feedback current; andproviding a fast common mode feedback current path;wherein the slow path and fast path are configured for controlling a common mode feedback current within a narrow bandwidth.

12. A method according to claim 11 wherein the bandwidth of the multi-path common mode feedback loop is described by the ratio of the transconductance gain of the fast path and the compensating capacitance of the multi-stage amplifier system.

13. A method according to claim 11 further comprising the step of configuring the fast path for controlling system bandwidth.

14. A method according to claim 11 further comprising the step of configuring the slow path and fast path in a combination for controlling DC performance of the system.

15. A method according to claim 11 further comprising the step of configuring the slow path for inactivation in response to the condition of approaching unity gain bandwidth.

16. A multi-path common mode feedback system comprising: a fast path having a fast path transistor, the fast path configured for determining the bandwidth of the common mode feedback loop; anda slow path having a slow path transistor operably coupled to a low pass filter, the low pass filter having a 3 dB bandwidth of about one-tenth of the fast path bandwidth;wherein the slow path and fast path are configured to operate in combination to determine the direct current carrying capacity of the common mode loop, and wherein the bandwidth of the loop is determined by how the current is divided between the fast path and the slow path.