Fully Differential Amplifier Input VICM Feedback for Common-Mode Noise
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Solution Overview
Problem
Fully-differential amplifiers (FDAs) face limitations in independently controlling input-side operating voltage levels, leading to potential common-mode voltage or current issues that can exceed allowable ranges, compromising differential-mode gain and signal-to-noise ratio, especially in applications like differential transimpedance amplifiers with high common-mode noise.
Innovation Solution
The implementation of a fully-differential amplifier with both input and output common-mode voltage control circuits, using error amplifiers and averaging networks to adjust common-mode voltages based on target voltages, ensuring the common-mode signals remain within acceptable limits and optimizing the differential-mode gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If fully-differential amplifier is used to provide differential output, then common-mode signal suppression is improved, but input common-mode voltage control capability deteriorates
Solution Approach 1:
The amplifier is segmented into independent common-mode control circuits for both input and output sides. The input common-mode control circuit includes an error amplifier that receives the target input common-mode voltage and the actual input common-mode voltage, generating an error signal that controls the input common-mode voltage independently. Similarly, the output common-mode control circuit operates independently to control output common-mode voltage, allowing each side to be optimized separately while maintaining overall differential performance.
Solution Approach 2:
Error amplifiers are introduced as intermediary components between the target common-mode voltage inputs and the actual common-mode voltage nodes. These error amplifiers act as mediators that compare the desired target voltage with the actual voltage and generate correction signals through feedback networks, enabling precise control of common-mode voltages without interfering with the differential signal path.
2Adaptability or versatility
If common-mode voltage control is added to FDA, then common-mode voltage control capability is improved, but device complexity increases
Solution Approach 1:
The common-mode control circuits are merged with the existing differential amplifier structure by integrating error amplifiers and feedback networks into the overall circuit architecture. The input common-mode control circuit is combined with the differential input stage, and the output common-mode control circuit is combined with the differential output stage, allowing common-mode control functionality to be added without requiring completely separate independent circuits.
Solution Approach 2:
The error amplifiers serve multiple functions: they control the common-mode voltage levels while also providing feedback for differential signal amplification. The feedback networks are designed to simultaneously manage both common-mode and differential-mode signals, allowing the same circuit components to perform multiple functions and reducing the need for additional dedicated elements.
Data Source
AI summary
A fully-differential amplifier (FDA) includes a core differential amplifier and a common-mode input voltage control circuit. The core differential amplifier includes differential inputs. The common-mode input voltage control circuit is coupled to the differential inputs. The common-mode input voltage control circuit is configured to generate an error signal as a difference of an input common mode voltage at the differential inputs and a target common mode input voltage (VICM); and to adjust the input common mode voltage to the VICM based on the error signal.