Folded Cascode Instrumentation Amplifier With Multi-Loop CMRR Control
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Solution Overview
Problem
Instrumentation amplifiers face issues with large input capacitance due to the Miller effect, leading to distortion and common mode signal generation, especially when the load connected to ground is not symmetrical, which affects the accuracy and stability of the output signal.
Innovation Solution
The implementation of a balanced instrumentation amplifier with a differential folded cascode amplifying stage and multiple feedback networks, including resistor feedback loops and current buffers, to provide high common mode rejection and low distortion, utilizing a dependent current source and current followers to enhance signal precision and stability.
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 becomes large due to the Miller effect
Solution Approach 1:
The amplifier is divided into multiple independent stages: differential input stage, intermediate amplification stage, and output stage. Each stage is optimized separately to control input capacitance while maintaining overall simplicity. The differential input stage uses a specific topology that isolates the high-capacitance elements from the input terminals.
Solution Approach 2:
An intermediate amplification stage is introduced between the input and output stages. This intermediary stage buffers the input from the output, preventing the Miller effect from propagating back to the input terminals while still providing the necessary gain.
2Measurement precision
If folded cascode circuit is used to reduce input capacitance, then the gain and stability improve, but the device complexity increases
Solution Approach 1:
The folded cascode topology is applied selectively to specific stages where it provides the most benefit (intermediate and output stages), rather than the entire amplifier. This segmented application reduces input capacitance while minimizing the increase in circuit complexity.
Solution Approach 2:
Different circuit topologies are applied to different stages based on their specific requirements. The differential input stage uses a simpler topology optimized for low input capacitance, while the intermediate and output stages use folded cascode for high gain and stability.
3Adaptability or versatility
If asymmetrical load connected to ground is used, then the output stage can drive different load conditions, but common mode signal is generated and distortion increases
Solution Approach 1:
Multiple feedback paths are implemented that sense both differential and common mode signals. The feedback networks are designed to provide equal feedback for both polarities, ensuring that asymmetrical loads do not generate common mode signals at the output while maintaining the ability to drive different load conditions.
Solution Approach 2:
The circuit is designed with intentional asymmetry in the feedback network configuration to compensate for asymmetrical loads. The feedback paths are adjusted to balance the common mode rejection despite the load asymmetry, allowing versatile load driving without generating common mode signals.
Data Source
AI summary
An instrumentation amplifier configured for providing high common mode rejection is described and includes an input differential stage configured to receive a differential input voltage and a folded cascode amplifying stage configured to receive output current mode signals provided from the input differential pair. A plurality of feedback networks is provided to improve the input stage. The amplifier may operate to provide an enhanced common mode rejection ratio of a single gain block in the instrumentation amplifier. 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.