Switched-Capacitor Amplifier Reset Feedback for Stable Bandwidth
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Solution Overview
Problem
Conventional switched-capacitor gain amplifiers face stability issues due to varying feedback factors during reset/sampling and amplification phases, leading to design challenges and increased costs for compensation techniques like costly reference circuitry and auto-zero methods.
Innovation Solution
A two-stage switched-capacitor amplifier design with control logic generating phase-specific control signals to manage switches and feedback paths, creating a shorter feedback path during the reset phase, which stabilizes the circuit and reduces capacitive load, thereby enhancing operational bandwidth and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional feedback paths are used during reset phase, then the amplifier can operate, but stability deteriorates due to high feedback factor (beta=1) causing excessive open loop bandwidth (900 MHz) and phase margin issues
Solution Approach 1:
The feedback path is segmented into two distinct configurations: during reset phase, a first feedback path connects the output to the inverting input, while during amplification phase, a second feedback path is established. This segmentation allows independent optimization of feedback factor for each phase, resolving the stability-bandwidth contradiction.
Solution Approach 2:
The feedback factor is made dynamic by switching between different feedback paths based on operational phase. Control logic generates phase-specific control signals that dynamically reconfigure the feedback network, enabling beta=1 during reset for stability and beta=Cf/(Cf+Cs+Cp) during amplification for appropriate bandwidth, eliminating the need to compromise either parameter.
2Speed
If feedback factor is reduced during amplification phase, then bandwidth is improved, but stability deteriorates during reset phase where beta must be 1
Solution Approach 1:
The feedback network is segmented into phase-specific paths: the first feedback path provides beta=1 during reset phase for stability, while the second feedback path provides reduced beta during amplification phase for extended bandwidth. This segmentation resolves the contradiction by allowing each phase to have optimized feedback characteristics.
Solution Approach 2:
The feedback configuration undergoes periodic switching between two states synchronized with the operational phases. During reset phase, full feedback is applied; during amplification phase, reduced feedback is applied. This periodic reconfiguration allows the system to achieve both stability and bandwidth performance at different times in the operational cycle.
3Reliability
If costly reference circuitry is used to compensate stability issues, then stability is improved, but device complexity and cost increase
Solution Approach 1:
The amplifier uses its own output signal to provide feedback during reset phase, eliminating the need for external reference circuitry. The control logic automatically generates phase-specific control signals that enable the feedback path, allowing the circuit to self-regulate its stability without additional costly components.
Solution Approach 2:
The feedback network serves multiple functions: during reset phase, it provides stability through beta=1 feedback; during amplification phase, it enables bandwidth extension through reduced feedback. This multi-functionality eliminates the need for separate reference circuits or auto-zero techniques, reducing overall device complexity.
4Measurement precision
If auto-zero techniques are employed to cancel offset, then offset cancellation is achieved, but device complexity and cost increase due to additional reference circuits
Solution Approach 1:
The feedback mechanism uses the amplifier's own output to automatically compensate for offset and low-frequency noise during reset phase, eliminating the need for separate auto-zero circuits or reference voltage generators. This self-service approach achieves offset cancellation while maintaining circuit simplicity.
Solution Approach 2:
The offset cancellation function is merged with the feedback mechanism. The same feedback path that provides stability during reset phase also performs offset cancellation by feeding back the output signal, combining two functions into one circuit element and reducing overall complexity.
Data Source
AI summary
A switch-capacitor (“SC”) amplifier includes a two-stage operational amplifier (“OP-AMP”), an input SC network, and a feedback SC network. The two-stage OP-AMP includes a first OP-AMP stage having an output coupled to an input of a second OP-AMP stage. The input SC network is coupled to an input of the first OP-AMP stage. The feedback SC network is configured to selectively couple the output of the first OP-AMP stage to the input of the first OP-AMP stage during a first phase of operation of the SC amplifier and to couple an output of the second OP-AMP stage to the input of the first OP-AMP stage during a second phase of operation of the SC amplifier.