Folded Cascode Instrumentation Amplifier With Multi-Loop CMRR Feedback
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Solution Overview
Problem
Existing instrumentation amplifiers suffer from large input capacitance due to the Miller effect, leading to distortion and common mode component issues, particularly when the load connected to ground is not symmetrical, resulting in unequal current supply and distortion.
Innovation Solution
The proposed instrumentation amplifier employs a differential folded cascode amplifying stage with multiple feedback networks and a current buffer stage, utilizing resistor networks to provide feedback and cancellation currents, ensuring high common mode rejection and low distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional instrumentation amplifier topology is used, then the circuit structure is simple, but the input capacitance is large due to the Miller effect causing distortion
Solution Approach 1:
The patent changes the fundamental circuit topology from traditional operational amplifier-based design to folded cascode architecture, fundamentally altering the electrical parameters including input capacitance, gain characteristics, and frequency response. This parameter transformation resolves the Miller effect issue while maintaining circuit functionality.
Solution Approach 2:
The patent replaces the traditional two-stage operational amplifier mechanism with a folded cascode single-stage architecture, substituting the conventional signal processing mechanism with an alternative that achieves the same function without the harmful Miller effect.
2Adaptability or versatility
If the load connected to ground is not symmetrical, then the circuit can handle practical loading conditions, but the output stages supply unequal currents generating distortion and common mode components
Solution Approach 1:
The patent intentionally introduces asymmetry through the third feedback loop that references ground potential, creating an imbalance in the feedback networks that specifically counteracts the effects of asymmetrical loading. This controlled asymmetry compensates for external loading asymmetries, maintaining equal current supply to output stages despite unequal load conditions.
Solution Approach 2:
The patent implements a third feedback loop that senses the common mode voltage at the output and feeds it back to the input stage through a third resistor, creating a feedback mechanism that actively compensates for asymmetrical loading effects and maintains common mode rejection.
3Device complexity
If feedback networks have no reference to ground, then the circuit maintains simple feedback structure, but the networks do not sense common mode signal reducing common mode rejection
Solution Approach 1:
The patent introduces a third feedback loop that acts as an intermediary between the output stage and input stage, specifically designed to sense and feedback common mode signals. This intermediary feedback path enables common mode rejection without significantly complicating the overall feedback structure.
Solution Approach 2:
The patent segments the feedback function into three separate loops: two differential feedback loops for differential signal handling and one common mode feedback loop for common mode signal sensing. This segmentation allows each feedback network to be optimized for its specific function while maintaining overall system simplicity.
Data Source
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AI summary
An instrumentation amplifier (500) configured for providing high common mode rejection is described and includes an input differential stage (505, 506) configured to receive a differential input voltage and a folded cascode amplifying stage (514, 515) configured to receive output current mode signals provided from the input differential stage (505, 506). A plurality of feedback networks is provided to improve the input differential stage (505, 506). The amplifier may operate to provide an enhanced common mode rejection ratio of a single gain block in the instrumentation amplifier (500). In some examples, the circuitry may have a differential folded cascode amplifying stage which permits high precision and low distortion of amplified signals without degrading the common mode rejection ratio.