Single-Ended Folded Cascode Amplifier With Common-Mode Cancellation
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Solution Overview
Problem
Existing instrumentation amplifiers face challenges with high common mode rejection ratio degradation and elevated input capacitance, leading to signal distortion, particularly due to imbalances in collector currents and Miller effect.
Innovation Solution
A single-ended instrumentation amplifier with a folded cascode configuration, including an input differential stage, a current mirror, and an external gain setting stage with resistor feedback networks, which splits and sums output current mode signals to provide cancellation currents and voltages, thereby maintaining independent current flow and reducing common mode signal impact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a differential pair with feedback resistor is used to correct collector current imbalance, then common mode rejection is improved, but input capacitance increases due to Miller effect causing signal distortion
Solution Approach 1:
The amplifier is divided into separate functional stages: input differential stage, folded cascode amplifying stage, and output stage. This segmentation isolates the high-gain amplification function from the input stage, preventing Miller effect from affecting input capacitance while maintaining common mode rejection through the differential configuration.
Solution Approach 2:
The folded cascode stage acts as an intermediary between the input differential pair and the output stage. It provides current amplification and level shifting without requiring high voltage gain at the input stage, thereby eliminating the need for large feedback resistors that would increase input capacitance through Miller effect.
2Manufacturing precision
If three gain blocks are used in instrumentation amplifier configuration, then gain precision is improved, but device complexity increases
Solution Approach 1:
Multiple functions are merged into fewer stages. The folded cascode stage simultaneously provides current amplification, level shifting, and impedance transformation, eliminating the need for separate gain blocks while maintaining precise gain control through the external resistor network.
Solution Approach 2:
The folded cascode amplifying stage performs multiple functions: it amplifies the differential signal, shifts voltage levels, and provides current-to-voltage conversion. This multi-functionality reduces the overall number of stages needed while maintaining precision through the external gain-setting resistors.
3Measurement precision
If feedback resistor is used to sense and correct collector current imbalance, then common mode rejection is improved, but device complexity and resistor matching precision requirements increase
Solution Approach 1:
The folded cascode configuration inherently provides current balancing through its structure. The cascode transistors automatically equalize collector currents by providing high impedance paths, eliminating the need for external feedback resistors and complex correction circuitry while maintaining common mode rejection.
Data Source
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AI summary
An instrumentation amplifier (300) configured for providing high common mode rejection and low distortion includes an input differential pair (303, 304) configured to receive a differential input voltage and differential feedback voltage and a folded cascode amplifying stage (310, 311) configured to receive output current mode signals provided from the input differential pair (303, 304). A current mirror (312) is configured to mirror output current mode signals provided from said folded cascode amplifying stage (310, 311). An external gain setting configuration may include a resistor feedback network, which includes a first resistor (306) being connected between feedback inputs of said input differential pair (303, 304), a second resistor (307) between an output terminal of the current mirror (312) and a first feedback input of said input differential pair (303, 304), a third resistor (305) between a common terminal and a second feedback input of said input differential pair (303, 304).