Common-Mode Feedback Amplifier With Reduced Loop Gain Stability
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Solution Overview
Problem
Conventional differential amplifiers face instability issues due to high open-loop gain and require significant power consumption, especially in common-mode feedback circuits, which complicates the design and increases costs due to large geometric area requirements for capacitors.
Innovation Solution
A differential amplifier design with a reduced loop gain in the common-mode feedback circuit, achieved by introducing a feedback factor in the feedback path, maintains unity closed-loop gain while improving phase margin and stability, and reduces power consumption by increasing resistance in the feedback impedance portion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high open-loop gain is used in the common-mode feedback circuit, then the stability is improved, but the power consumption increases significantly
Solution Approach 1:
The patent changes the gain parameter of the common-mode feedback amplifier by introducing a feedback factor (beta) less than one. This is achieved by using a voltage divider network (resistors R1 and R2) to attenuate the feedback signal, thereby reducing the loop gain from A106 to A106×beta. This parameter change allows the circuit to achieve adequate stability with lower power consumption by operating the amplifier in a linear region without requiring excessive gain.
Solution Approach 2:
The patent introduces negative feedback through the voltage divider network connected to the inverting input of the differential amplifier. The feedback factor beta = R2/(R1+R2) determines the amount of feedback, creating a closed-loop system that stabilizes the common-mode voltage while controlling the loop gain. This feedback mechanism allows the system to maintain stability with reduced gain requirement.
2Reliability
If large compensation capacitor is used to improve stability, then the phase margin is improved, but the geometric area and device complexity increase
Solution Approach 1:
The patent changes the loop gain parameter to be less than one, which fundamentally alters the stability characteristics of the system. By reducing the loop gain through the feedback factor beta, the system achieves adequate phase margin with a smaller compensation capacitor. The reduced gain lowers the bandwidth requirement, allowing the use of smaller capacitive values while maintaining stability.
3Reliability
If conventional common-mode feedback circuit is used, then the common-mode stability is achieved, but the device complexity and manufacturing cost increase
Solution Approach 1:
The patent makes the differential amplifier serve multiple functions: it provides both the main signal amplification and the common-mode feedback control. By using the same differential amplifier for both purposes and adding a simple voltage divider network, the circuit achieves common-mode stability without requiring a separate dedicated common-mode feedback amplifier, thereby reducing overall device complexity.
Solution Approach 2:
The patent introduces a voltage divider network (resistors R1 and R2) as an intermediary element that simplifies the feedback path. This intermediate network attenuates the feedback signal to achieve the desired loop gain without requiring complex control circuits, making the overall design simpler and more manufacturable.
Data Source
AI summary
A circuit is provided for use with a reference voltage. The circuit includes a voltage source, a common-mode feedback amplifier and a feedback impedance portion. The common-mode feedback amplifier may be connected to the voltage source and may be arranged to receive the reference voltage. The common-mode feedback amplifier may include an input stage, an output stage, a positive input, a negative input and an output. The output may be connected to the feedback impedance portion. The feedback impedance portion may additionally be connected to one of the positive input and the negative input. A feedback factor, based on the feedback impedance portion, is less than one.