Folded Cascode Instrumentation Amplifier for High Common-Mode Rejection
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Solution Overview
Problem
Prior instrumentation amplifiers suffer from large input capacitance due to the Miller effect, leading to distortion and instability, and lack effective common mode rejection when the load is not symmetric, resulting in unequal current supply and distortion.
Innovation Solution
The implementation of a differential folded cascode amplifier circuit with multiple feedback networks and a current buffer stage, including external gain setting circuitry and dependent current sources, to provide feedback and stabilize the amplifier, reducing input capacitance and enhancing common mode rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional instrumentation amplifier topology is used, then the circuit is simple to implement, but the input capacitance becomes large due to the Miller effect
Solution Approach 1:
The patent transforms the traditional voltage-amplification topology into a current-amplification topology. By changing the fundamental operating parameter from voltage to current, the Miller effect is eliminated because the high-impedance node is converted to a low-impedance current node, thereby reducing input capacitance while maintaining circuit simplicity
Solution Approach 2:
The patent substitutes the voltage-based amplification mechanism with a current-based amplification mechanism. The input differential stage directly outputs current signals that are amplified in the folded cascode stage, replacing the traditional voltage amplification path that caused large input capacitance
2Object-affected harmful factors
If folded cascode circuit is used to reduce input capacitance, then the gain increases and input capacitance decreases, but the circuit complexity increases
Solution Approach 1:
The patent merges the differential input stage with the folded cascode amplification stage into a unified current-mode architecture. The input differential transistors directly drive the folded cascode devices, eliminating intermediate voltage conversion stages and reducing overall circuit complexity while maintaining low input capacitance
Solution Approach 2:
The folded cascode stage serves multiple functions simultaneously: it provides current amplification, maintains low input capacitance, and enables easy stabilization. The same circuit structure achieves gain, impedance transformation, and capacitance reduction without requiring separate dedicated circuits for each function
3Adaptability or versatility
If feedback network without ground reference is used, then the circuit operates with floating load, but common mode signal cannot be sensed and common mode rejection is poor
Solution Approach 1:
The patent introduces a ground-referenced feedback network that acts as an intermediary between the floating differential output and the common mode sensing requirement. The feedback resistors connect to ground, enabling the circuit to sense common mode signals while still maintaining floating load operation capability
Solution Approach 2:
The patent adds the ground reference dimension to the feedback network while keeping the signal path differential. By separating the signal dimension (differential) from the reference dimension (ground), the circuit can simultaneously achieve floating load operation and common mode rejection through the ground-referenced feedback path
4Adaptability or versatility
If asymmetric load is connected to ground, then the output stage can drive ground-referenced loads, but unequal currents are supplied causing distortion and common mode component
Solution Approach 1:
The patent employs ground-referenced feedback networks that sense the actual output voltages and feed them back to the input stage. This feedback mechanism automatically compensates for asymmetric load conditions by adjusting the differential input signals to maintain equal current distribution, thereby eliminating distortion and common mode components while driving ground-referenced loads
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.