Folded Cascode Common-Mode Feedback for Large Output Swing
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Solution Overview
Problem
Existing common mode feedback circuits for fully differential amplifiers fail to maintain a constant output common mode voltage and effectively control differential output voltage swings, leading to signal errors and linearity issues, especially during large output voltage swings and auto-zero phases.
Innovation Solution
A common mode feedback circuit with a tracking circuit coupled to both outputs of a folded cascode stage, using symmetric tracking capacitors and switches to generate and control common mode output signals, ensuring precise tracking and control of the common mode voltage without short-circuiting the amplifier outputs during auto-zeroing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a common mode feedback circuit is used to keep outputs within operational range, then output voltage control is improved, but output common mode voltage stability deteriorates
Solution Approach 1:
The feedback circuit is segmented into two independent paths: one path (first feedback circuit) controls the positive output voltage, and the other path (second feedback circuit) controls the negative output voltage. Each path has its own feedback transistor and control mechanism, allowing independent optimization of each output while maintaining overall common mode voltage stability.
2Stability of the object's composition
If common mode feedback control is applied, then output common mode voltage is maintained, but differential output voltage accuracy deteriorates due to introduced errors
Solution Approach 1:
The feedback circuit uses asymmetric transistor sizing where the width-to-length ratio of the first feedback transistor differs from that of the second feedback transistor. This asymmetric design compensates for inherent circuit imbalances and minimizes differential error introduction while maintaining common mode voltage control.
3Adaptability or versatility
If common mode feedback operates with large output voltage swings, then output swing range is improved, but common mode voltage tracking accuracy deteriorates
Solution Approach 1:
The feedback circuit dynamically adjusts its operation based on the output voltage swing conditions. The circuit transitions between different operating regions and uses adaptive biasing to maintain tracking accuracy across the full range of output voltage swings, from small signal to large signal conditions.
Data Source
AI summary
A differential amplifier includes a differential input pair (2A) coupled to a folded cascode stage (2B) and a common mode feedback circuit (34) including a tracking circuit (30A) coupled to first (Vout−) and second (Vout+) outputs of the folded cascode stage (2B). The first and second outputs are coupled to first terminals of first (31A) and second (31B) tracking capacitors which have second terminals on which a first common mode output signal (VCM1) is produced and also are coupled to first terminals of third (32A) and fourth (32B) tracking capacitors, respectively, which have second terminals on which a second common mode output signal (VCM2) is produced. The first and third tracking capacitors are discharged by first (27A) and second (27B) switches that directly couple the first and second outputs to first and second inputs of a common mode feedback amplifier (4). A desired common mode output voltage (VCM-IN) is applied to a third input of the common mode feedback amplifier. The switches are opened to cause the first and second common mode output voltages to be generated, causing a common mode feedback control signal (VCMFB) to be generated for biasing the folded cascode stage.