Continuous-Time Common-Mode Feedback for Wide Swing Linearity
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Solution Overview
Problem
Conventional common-mode feedback circuits face limitations in providing wide swing, good linearity, wide bandwidth, and low systematic offsets, with switched capacitor circuits suffering from reduced bandwidth due to parasitic sensitivity, resistor averaged circuits compromising output swing and phase margin, and differential difference amplifier circuits having poor linearity.
Innovation Solution
A continuous time common-mode feedback module that includes a common-mode resolver and a control voltage generating module, utilizing transistors and resistors to generate a common-mode current and feedback voltage, with a method involving differential amplifier stages to provide a wide swing and good linearity, low input capacitance, and high output impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If switched capacitor CMFB circuit is used, then common mode control is achieved, but bandwidth is reduced due to parasitic sensitivity and capacitor effects
Solution Approach 1:
The patent replaces the switched capacitor mechanism with a continuous-time resistive network. Instead of using switches and capacitors that introduce parasitic effects and bandwidth limitations, the invention uses a resistive averaging network that operates continuously without switching, thereby eliminating the parasitic sensitivity and capacitor-induced bandwidth reduction while maintaining common mode control functionality.
Solution Approach 2:
The patent eliminates the periodic switching action inherent in switched capacitor circuits. By transitioning to a continuous-time operation mode where the resistive network continuously averages the differential outputs, the system removes the periodic sampling effect that limits bandwidth, achieving both common mode control and wide bandwidth simultaneously.
2Reliability
If resistor averaged CMFB circuit is used, then instantaneous control and minimum common mode offset are achieved, but output swing is reduced due to buffering requirements
Solution Approach 1:
The patent extracts and removes the buffer stage from the conventional resistor averaged CMFB circuit. By taking out the buffering component that was introduced to compensate for output impedance reduction, the invention directly achieves the desired common mode control with minimum offset while preserving the full output swing capability, as the buffer is the element that limited the swing.
Solution Approach 2:
The patent changes the operating parameters of the resistive network to optimize the trade-off between output impedance and output swing. By carefully selecting resistor values and configuring the resistive averaging network, the system achieves sufficient output impedance for stable common mode control while maintaining large output swing, eliminating the need for buffering that would otherwise reduce the swing.
3Reliability
If differential difference amplifier CMFB circuit is used, then output buffering is provided, but linearity deteriorates
Solution Approach 1:
The patent replaces the differential difference amplifier architecture with a simpler resistive averaging network. This substitution eliminates the complex transistor-based differential amplification that introduces non-linearities, while the resistive network provides inherent linearity in the common mode averaging process. The output buffering function is maintained through the existing circuit topology without requiring the non-linear differential amplifier stage.
4Manufacturing precision
If channel lengths of input transistors are increased to improve linearity, then linearity improves, but swing is reduced and compensation is required for stability
Solution Approach 1:
The patent substitutes the transistor-based differential amplifier with a resistive averaging network. This replacement fundamentally changes the mechanism for achieving linearity - instead of relying on long-channel transistors to reduce non-linear effects, the resistive network inherently provides linear operation. This substitution simultaneously preserves output swing and eliminates the need for stability compensation that would be required with long-channel transistor designs.
Data Source
AI summary
A continuous time common mode feedback module is capable of operating in a wide range of input voltages. The common mode feedback module includes a common mode detector and an amplifier for computing and amplifying the difference of a reference voltage and a common mode voltage of a first input signal and a second input signal. The common-mode feedback module includes a common mode resolver and a control voltage generating module coupled to each other to provide a common mode feedback voltage. The common mode feedback module provides a good linearity and a wide bandwidth, without compensation requirements. The common mode feedback module also provides small process corner dependence of bias current and a common mode offset.