Differential Amplifier Common-Mode Chopping for Higher PSRR
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Solution Overview
Problem
Fully differential amplifiers face performance degradation due to resistor mismatches in feedback paths, which adversely affect their power supply rejection ratio (PSRR) and increase sensitivity to power supply variations.
Innovation Solution
A fully differential amplifier circuit with a common mode loop circuit and chopper switches that provide a local feedback loop to drive input resistors to a fixed common mode voltage, independent of the power supply, and utilize a chopper clock signal with a variable duty cycle to equalize current flow between positive and negative inputs, reducing PSRR dependence on resistor mismatches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If resistor mismatches occur in feedback paths, then the amplifier gain is maintained, but the power supply rejection ratio (PSRR) deteriorates and sensitivity to power supply variations increases
Solution Approach 1:
The patent introduces a common mode feedback loop that senses the common mode voltage at the amplifier inputs and actively regulates it to a fixed reference voltage. This feedback mechanism compensates for resistor mismatches in the differential feedback paths, maintaining high PSRR despite component variations. The common mode feedback works alongside the differential feedback to independently control the common mode operating point.
Solution Approach 2:
The patent employs chopper switches as intermediary elements that periodically sample and transfer voltages between different circuit nodes. These chopper switches, controlled by a variable duty cycle clock signal, enable the common mode feedback loop to regulate the common mode voltage without directly interfering with the differential signal path, thus maintaining both high gain and high PSRR.
2Reliability
If a common mode loop circuit with chopper switches is added, then the power supply rejection ratio is enhanced, but the device complexity increases
Solution Approach 1:
The patent merges the common mode feedback functionality with the existing differential amplifier structure by sharing operational amplifiers and feedback paths. The common mode loop utilizes the same power supply and grounding infrastructure, reducing the need for completely separate circuitry. This integration approach enhances PSRR while minimizing the increase in overall device complexity.
Solution Approach 2:
The patent uses periodic chopper switching at a variable duty cycle to implement the common mode regulation. This periodic action allows the complex feedback function to be achieved through simple, repetitive switching operations rather than continuous complex analog circuitry. The chopper switches periodically connect and disconnect circuit elements to enforce the common mode voltage constraint.
Data Source
AI summary
In a general aspect, a system can include a fully differential amplifier circuit that includes a first amplifier, and first and second feedback paths. The first feedback path can provide a feedback path from a positive output of the first amplifier to a negative input of the first amplifier. The second feedback path can provide a feedback path from a negative output of the first amplifier to a positive input of the first amplifier. The system can include a chopper clock circuit configured to output a variable duty cycle chopper clock signal. The system can include a common mode loop circuit including a second amplifier and chopper switches. The common mode loop circuit can be configured as a local feedback loop for the first amplifier. The chopper switches can be configured to receive the chopper clock signal and control current flow into the positive and negative inputs.