Folded Cascode Instrumentation Amplifier for High CMRR and Low Distortion
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Solution Overview
Problem
Existing instrumentation amplifiers face challenges with high common mode rejection ratio and distortion due to input capacitance variations and imbalances in collector currents, which affect signal quality.
Innovation Solution
A single-ended instrumentation folded cascode amplifier design incorporating an input differential stage, a folded cascode amplifying stage, a current mirror, and an external gain setting stage with resistor feedback networks to provide high common mode rejection and low distortion, utilizing current-controlled current sources and transistors as current dividers to manage feedback and cancellation currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a differential pair with emitter resistors is used to provide feedback, then distortion is reduced, but common mode rejection ratio deteriorates due to current imbalances
Solution Approach 1:
The patent introduces an intermediary current mirror circuit that senses the collector currents of the differential pair and provides compensating feedback signals. This mediator enables independent control of collector currents without directly affecting the emitter feedback path, thus maintaining both low distortion and high common mode rejection ratio simultaneously
2Stability of the object's composition
If Miller effect compensation is applied to reduce input capacitance, then stability is improved, but bandwidth is reduced
Solution Approach 1:
The patent extracts the Miller effect compensation function from the main signal path by implementing it in the feedback network rather than directly at the input. This separation allows the compensation to stabilize the amplifier without significantly impacting the input bandwidth, as the compensation acts on the feedback signal rather than the input signal directly
3Manufacturing precision
If three gain blocks are used in the instrumentation amplifier, then gain precision is improved, but device complexity increases
Solution Approach 1:
The patent merges the functions of multiple gain blocks into a single operational amplifier by implementing precise gain control through the feedback network. The feedback resistors and the differential pair configuration work together to provide the necessary gain precision that would otherwise require multiple separate gain stages, thus reducing overall device complexity
Data Source
AI summary
An instrumentation amplifier configured for providing high common mode rejection and low distortion is described and includes an input differential pair configured to receive a differential input voltage and differential feedback voltage and a folded cascode amplifying stage configured to receive output current mode signals provided from the input differential pair. A current mirror is configured to mirror output current mode signals provided from said folded cascode amplifying stage. An external gain setting configuration may include a resistor feedback network, which includes a first resistor being connected between feedback inputs of said input differential pair, a second resistor between an output terminal of the current mirror and a first feedback input of said input differential pair, a third resistor between a common terminal and a second feedback input of said input differential pair.