Differential Amplifier Feed-Forward Control for Common Mode Rejection
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Solution Overview
Problem
Differential amplifiers face challenges in maintaining ideal common mode rejection performance due to component mismatch and thermal variations, leading to significant reduction in common mode rejection ratio.
Innovation Solution
A differential amplifier with a feed-forward path that adjusts the target common mode output voltage as a function of the common mode input signal, using a resistor-based feedback network to control the common mode output voltage and improve linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If component mismatch and thermal variations are present in the differential amplifier, then the amplifier can operate with standard components, but the common mode rejection ratio is significantly reduced
Solution Approach 1:
The patent applies preliminary action by measuring the common mode input voltage before amplification and using this measurement to pre-adjust the output common mode voltage. The feed-forward path calculates the required output common mode voltage based on the measured input common mode voltage and the known differential gain, allowing the system to compensate for component mismatch before it affects the output signal. This proactive compensation improves common mode rejection ratio without requiring higher precision components.
2Reliability
If component mismatch occurs in the feedback network, then the amplifier can be manufactured with standard components, but the common mode rejection ratio deteriorates
Solution Approach 1:
The patent implements feedback by measuring the actual output common mode voltage and comparing it with the target common mode voltage calculated from the feed-forward path. The error signal is used to adjust the output, creating a closed-loop system that compensates for resistor mismatch in the feedback network. This feedback mechanism ensures that even with standard precision resistors, the amplifier maintains high common mode rejection ratio by continuously correcting for manufacturing variations.
Data Source
AI summary
An amplifier circuit having improved common mode rejection is provided. This can be achieved by estimating the common mode value of an input signal and using this to adjust a target common mode voltage at the output of the amplifier. This can help avoid the differential gain becoming modified by the common mode voltage.