Folded Cascode Instrumentation Amplifier for Asymmetrical Loads
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Solution Overview
Problem
Prior instrumentation amplifiers suffer from large input capacitance due to the Miller effect, leading to increased distortion and common mode components in output signals, especially when the load connected to ground is not symmetrical, which affects the stability and precision of audio signal processing.
Innovation Solution
The proposed solution involves 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 compensate for asymmetry and stabilize the circuit.
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
Solution Approach 1:
The patent changes the circuit topology from traditional operational amplifier-based design to folded cascode architecture, fundamentally altering the electrical parameters including input capacitance characteristics. This parameter change eliminates the Miller effect multiplication while maintaining circuit functionality.
Solution Approach 2:
The patent replaces the traditional voltage-feedback mechanism with a current-mode folded cascode amplifying stage, substituting the operational mechanism to achieve lower input capacitance. The current-mode operation avoids the voltage amplification that causes Miller effect capacitance multiplication.
2Quantity of substance
If folded cascode circuit is used to reduce input capacitance, then the gain is higher and input capacitance is lower, but the common mode rejection is degraded when load is asymmetrical
Solution Approach 1:
The patent introduces multiple feedback paths including a first feedback network from the first output to the first input and a second feedback network from the second output to the second input. These feedback mechanisms actively compensate for common mode signals and maintain rejection performance even with asymmetrical loads connected to ground.
Solution Approach 2:
The patent explicitly addresses asymmetrical load conditions by providing separate feedback networks for each output channel, allowing independent compensation for each side. This asymmetric feedback configuration maintains common mode rejection despite the inherent asymmetry in the loading conditions.
3Reliability
If multiple feedback networks are added to improve common mode rejection, then the common mode rejection ratio is enhanced, but the device complexity increases
Solution Approach 1:
The patent divides the feedback function into separate networks: a first feedback network for the first output channel and a second feedback network for the second output channel. Each network is independently configured, allowing modular implementation and simplifying the design and analysis of each feedback path while achieving overall enhanced common mode rejection.
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.