Common-Mode Feedback Capacitors for Stable Differential Amplifiers
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Solution Overview
Problem
The stability of common mode feedback loops in differential amplifiers is often poor, leading to instability and performance degradation due to inadequate control of the common mode component of the output signal.
Innovation Solution
The proposed amplifier circuit includes a telescopic amplifier with pole-splitting and feedforward capacitors, a common mode feedback amplifier, and a differential source follower, which improves the phase margin and stability of the common mode feedback loop by connecting capacitors between the common mode feedback input and outputs of the telescopic amplifier, and between the common mode feedback amplifier and the amplifier circuit outputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a common mode feedback loop is used to control the common mode component of the output, then the control of common mode signal is improved, but the stability of the feedback loop deteriorates
Solution Approach 1:
The patent introduces pole-splitting capacitors that change the frequency response parameters of the feedback loop. By adding capacitive elements between specific nodes (common mode feedback input and differential outputs), the loop gain and phase characteristics are modified, creating pole-splitting effect that improves stability margin while maintaining common mode control functionality
Solution Approach 2:
The pole-splitting capacitors act as intermediary elements that mediate between the common mode feedback path and the differential output signals. These capacitors introduce controlled coupling that modifies the feedback loop characteristics without directly altering the common mode control mechanism, thereby improving stability while preserving control effectiveness
Data Source
AI summary
An amplifier circuit. In some embodiments, the amplifier circuit includes: a telescopic amplifier, and a common mode feedback amplifier. The telescopic amplifier has a first signal input, a second signal input, a first output, a second output, a common mode feedback input, a first pole-splitting capacitor, and a second pole-splitting capacitor. The common mode feedback amplifier has an output connected to the common mode feedback input of the telescopic amplifier. The first pole-splitting capacitor is connected between the common mode feedback input of the telescopic amplifier and the first output of the telescopic amplifier, and the second pole-splitting capacitor is connected between the common mode feedback input of the telescopic amplifier and the second output of the telescopic amplifier.