Adaptive Local Feedback in Differential Amplifiers for Low Noise
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Solution Overview
Problem
Differential amplifiers face performance degradation due to resistor mismatches and increased noise floor from local feedback loops, which affect power supply rejection ratio (PSRR) and introduce harmonic distortion, especially during large-signal processing.
Innovation Solution
An adaptive control circuit enables and disables a local feedback loop based on the magnitude of the input signal, using a control circuit to manage current flow and reduce dependency on resistor mismatches, and incorporates chopper switches to equalize current into the amplifier inputs, thereby reducing noise and harmonic distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a local feedback loop is implemented in a differential amplifier, then power supply rejection ratio (PSRR) is improved, but noise floor increases and harmonic distortion is introduced
Solution Approach 1:
The local feedback loop is made dynamically controllable through an enable signal that allows it to be activated or deactivated based on operating conditions. This dynamic control enables the system to optimize between PSRR improvement and noise/distortion reduction by selectively engaging the feedback loop when beneficial.
Solution Approach 2:
The system changes the operational state of the local feedback loop by modifying the enable signal parameter. This parameter change allows transition between different operating modes (feedback enabled/disabled) to optimize performance characteristics based on signal conditions and operating requirements.
2Reliability
If resistor values are increased to reduce mismatch effects, then PSRR improves, but noise floor increases
Solution Approach 1:
Different resistor pairs serve different functional purposes: feedback resistors (R1, R2) are optimized for PSRR with higher values, while input resistors (R3, R4) are optimized for noise performance with lower values. This local differentiation of resistor qualities allows simultaneous optimization of both PSRR and noise floor.
Solution Approach 2:
The local feedback loop provides dynamic compensation that reduces dependency on precise resistor matching. By actively correcting imbalances through the feedback mechanism, the system achieves good PSRR without requiring high-value resistors that would increase noise.
3Reliability
If a local feedback loop is always enabled, then PSRR is continuously improved, but noise floor increases during small-signal processing
Solution Approach 1:
The local feedback loop is dynamically enabled or disabled based on operating conditions through control of the enable signal. This dynamic operation allows the system to engage the feedback loop when PSRR improvement is needed while disengaging it during small-signal processing to minimize noise floor.
Solution Approach 2:
The feedback loop operates in periodic cycles of being enabled and disabled based on signal conditions. This periodic action allows the system to optimize performance for different operating modes, engaging the loop when beneficial and disengaging when it would degrade performance.
Data Source
AI summary
In a general aspect, a circuit can include an input circuit configured to receive an input signal, and an amplifier circuit coupled with the input circuit. The amplifier circuit can include an amplifier, and first and second feedback paths. The first feedback path can be from a positive output to a negative input of the amplifier, and the second feedback path can be from a negative output to a positive input of the first amplifier. The circuit can also include a loop circuit configured to provide a local feedback loop for the first amplifier and configured to control current flow into the positive input of the first amplifier and current flow into the negative input of the first amplifier. The circuit can also include a control circuit that is configured to enable the loop circuit in response to a magnitude of the input signal exceeding a threshold.