Differential Amplifier Common-Mode Loop Stabilization With Buffered Injection
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Solution Overview
Problem
Fully differential amplifiers face common mode instability issues due to inadequate frequency compensation, which can lead to oscillations or voltage rise towards power rails, and existing solutions that stabilize the common mode loop often affect the differential loop's stability and increase implementation area.
Innovation Solution
A stabilization block measures the common mode component at the output of the input stage and injects a proportional current into each path to stabilize the common mode loop, using a buffer to isolate the differential loop and avoid distortion, thereby creating a dominant pole without affecting the differential loop's stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If common mode stabilization circuitry is added to fully differential amplifiers, then common mode stability is improved, but device complexity increases
Solution Approach 1:
The patent divides the common mode stabilization function into separate blocks for measuring common mode voltage and injecting stabilization current. This segmentation allows independent optimization of each function and reduces overall circuit complexity by modularizing the stabilization mechanism.
Solution Approach 2:
The patent introduces an intermediary stabilization block that measures common mode voltage and injects proportional current into the input stage paths. This intermediary mechanism stabilizes the common mode loop without directly interfering with the differential signal paths, thereby maintaining differential loop stability while reducing overall system complexity.
2Reliability
If current is injected into the output paths to stabilize the common mode loop, then common mode stability is improved, but the differential loop may be affected causing output signal distortion
Solution Approach 1:
The patent applies local quality by injecting stabilization current only into the input stage paths rather than the output paths. This localized approach stabilizes the common mode loop at the source without affecting the output signal quality, thereby preventing distortion while maintaining stability.
Solution Approach 2:
The stabilization block acts as an intermediary that measures common mode voltage and injects proportional current into the input stage paths. This intermediary mechanism stabilizes the common mode loop without directly interfering with the differential signal paths, thereby maintaining differential loop stability while reducing overall system complexity.
3Reliability
If existing common mode stabilization solutions are implemented, then common mode stability is improved, but implementation area increases
Solution Approach 1:
The patent merges the common mode measurement and stabilization functions into a single integrated block that operates within the existing amplifier structure. By combining these functions and utilizing the existing input stage paths, the implementation area is minimized while achieving effective common mode stabilization.
Solution Approach 2:
The stabilization block measures the common mode component internally and generates the stabilization current proportionally within the same block. This self-service approach eliminates the need for separate external stabilization circuits, thereby reducing the overall implementation area while maintaining stability.
Data Source
AI summary
A fully differential amplifier circuit provided according to an aspect of the present invention contains a stabilization block to measure the common mode component at the output of an input stage, and to inject a current proportionate to the common mode component into each of a pair of paths forming the output of the input stage to stabilize a feedback loop formed by the input stage, an output stage and a common mode feedback block. In an embodiment, the stabilization block contains a buffer to receive the measured common mode component and to provide a buffered output. The injected current is generated based on the buffered output. Due to the presence of the buffer, the differential loop may not be affected by injection of the additional current, thereby avoiding any distortions in the output signal.