Dual-Loop CMFB Circuit for Fast Turn-On 2-Stage Op-Amps
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Solution Overview
Problem
Common-mode feedback (CMFB) stability is challenging in operational amplifiers due to differing unity gain bandwidths and phase margin requirements, particularly in 2-stage miller compensated op-amps used in pipelined-SAR ADCs, necessitating fast turn-on during amplification phases.
Innovation Solution
A feedforward-based common-mode feedback scheme with two parallel switched-capacitor CMFB loops: a larger loop for both stages and a smaller loop for the first stage, providing higher gain and slower operation, and a faster loop with lower gain, respectively, to enhance stability and phase margin.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a single CMFB loop is used in 2-stage miller compensated op-amps, then the circuit complexity is low, but the common-mode unity gain bandwidth is insufficient and phase margin deteriorates
Solution Approach 1:
The patent divides the single CMFB loop into two separate loops: a first CMFB loop that provides high gain and a second CMFB loop that provides fast response. This segmentation allows each loop to be optimized for different functions, resolving the contradiction between bandwidth speed and circuit complexity by distributing the CMFB functionality across multiple specialized loops rather than attempting to achieve all functions in a single loop.
Solution Approach 2:
The patent implements dynamic switching between two CMFB loops based on operational requirements. The first loop is engaged when high gain is needed, while the second loop is engaged when fast response is required. This dynamic approach allows the system to adapt its CMFB characteristics in real-time, achieving both high bandwidth and acceptable complexity through conditional activation of different loop configurations.
2Use of energy by moving object
If amplifier stages are turned off during sampling phase to conserve power, then power consumption is reduced, but the op-amp requires fast turn-on capability which conflicts with CMFB stability
Solution Approach 1:
The patent prepares the CMFB loops in advance during the sampling phase when the amplifier stages are off. The second CMFB loop is pre-configured and ready to quickly activate when the amplification phase begins. This preliminary preparation ensures that when power is conserved during sampling, the system can still achieve fast turn-on during amplification without compromising CMFB stability, as the feedback mechanism is already in place and ready to operate.
3Device complexity
If common-mode and differential signal paths share the same poles and zeroes, then the circuit structure is simplified, but CMFB stability becomes difficult to achieve due to different unity gain bandwidths
Solution Approach 1:
The patent segments the CMFB functionality into two distinct loops with different characteristics. The first loop is designed with parameters optimized for stability and gain, while the second loop is designed for speed and bandwidth. By separating these functions into distinct loops rather than using a single shared path, the patent resolves the contradiction between structural simplicity and stability, allowing each loop to independently optimize its poles and zeroes for its specific function.
Data Source
AI summary
In accordance with various embodiments of the present disclosure, an amplifier is provided. In some embodiments, the amplifier comprises a first amplifier stage, a second amplifier stage, a common mode sense amplifier stage, a first common mode feedback (CMFB) loop involving the first amplifier stage, the second amplifier stage, and the common mode sense amplifier stage, and a second CMFB loop involving only the first amplifier stage.