Common-Mode Feedback Amplifier Compensation for Stable Phase Margin
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Solution Overview
Problem
Fully differential operational amplifiers with common-mode feedback circuits face instability issues due to noise injection, leading to ringing or oscillation in common-mode output voltages, which existing compensation methods address by sacrificing gain and bandwidth for stability.
Innovation Solution
The introduction of an additional pole and zero in the frequency response of the common-mode feedback circuit using a combination of capacitors and resistors, specifically through a compensating capacitor and resistor configurations, enhances stability while maintaining amplifier gain and improving phase margins.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If source degeneration compensation is applied by providing a resistor coupled between two transistors at two input terminals of the CMFB amplifier, then circuit stability is enhanced, but amplifier gain is reduced
Solution Approach 1:
The patent introduces a compensating capacitor as an intermediary element coupled between the common-mode detector and the CMFB amplifier input terminals. This capacitor acts as a mediator that provides frequency-dependent compensation, adding a zero to the transfer function that improves stability without the gain penalty associated with resistive source degeneration.
2Stability of the object's composition
If current reduction compensation is applied by reducing the control current of the FD Op-Amp to 1/N, then circuit stability is enhanced, but amplifier bandwidth is reduced
Solution Approach 1:
The patent changes the compensation parameter from current reduction to capacitive compensation. By introducing a compensating capacitor with specific capacitance value, the system achieves stability through parameter optimization rather than current reduction, thereby maintaining the full bandwidth capability of the operational amplifier.
3Stability of the object's composition
If existing compensation methods are applied to the CMFB circuit, then circuit stability is improved, but phase margins are reduced
Solution Approach 1:
The patent implements feedback compensation by coupling the compensating capacitor between the common-mode detector output and the CMFB amplifier input. This creates a feedback path that introduces a zero in the transfer function, increasing phase margins and improving stability without the trade-offs of conventional compensation methods.
Data Source
AI summary
A common-mode feedback circuit is provided. An amplifier with a common-mode feedback circuit is compensated by adding a compensating unit so that the amplifier totally has two poles and one zero in its frequency response. Accordingly, the gain of the amplifier is not sacrificed, and both the stability and the phase margin of the circuit are improved.