Fully Differential Amplifier with Common-Mode Injection Rejection
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Solution Overview
Problem
Conventional differential amplifiers fail to effectively reject common mode signals, leading to unwanted signal components in output voltages, and often require additional components that increase cost and complexity.
Innovation Solution
A fully differential amplifier design incorporating a common gain-setting network with resistors to split and inject signals into the amplifying circuits, preventing common mode components from appearing in output signals, and utilizing a compensation circuit to amplify and suppress common mode inputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional differential amplifiers are used, then the circuit structure is simple, but common mode signals cannot be effectively rejected leading to unwanted signal components in output
Solution Approach 1:
The amplifier is divided into two separate single-ended amplifying circuits (first and second amplifying circuits) that process differential signals independently, rather than using a single differential amplifier. This segmentation allows each circuit to be optimized for common mode rejection while maintaining overall differential operation
Solution Approach 2:
A summing circuit is introduced as an intermediary component that combines the outputs of the two amplifying circuits and generates a common mode signal, which is then fed back through a compensation circuit to both amplifying circuits. This intermediary summing circuit enables active common mode rejection by providing feedback control
2Object-affected harmful factors
If additional components are added to improve common mode rejection, then common mode signal rejection improves, but cost and complexity increase
Solution Approach 1:
The compensation circuit serves multiple functions simultaneously: it generates common mode feedback signals, adjusts common mode gain, and stabilizes the differential operation. The summing circuit also performs dual functions by combining amplifier outputs and generating the common mode component for feedback. This multi-functionality reduces the need for separate dedicated components for each function
Solution Approach 2:
The common mode gain is made adjustable through the compensation circuit, allowing optimization of the common mode rejection ratio (CMRR) parameter. By changing the gain parameter of the compensation circuit, the system can achieve better common mode rejection without adding more physical components
3Stability of the object's composition
If conventional differential amplifiers are used, then the circuit is compact, but output stability and sensitivity to load matching increase
Solution Approach 1:
A feedback mechanism is implemented where the summing circuit monitors the combined output of both amplifying circuits, extracts the common mode component, and feeds it back through the compensation circuit to both amplifiers. This feedback loop actively stabilizes the output by correcting common mode variations and reducing sensitivity to load impedance changes
Solution Approach 2:
The compensation circuit ensures that both amplifying circuits operate at equal potential levels by providing identical common mode feedback signals to both circuits. This equipotential operation reduces differential errors and improves output stability by maintaining balanced operation
Data Source
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AI summary
A fully differential amplifier (500) is provided. A first amplifying circuit (502) receives a first input voltage signal and provides a first output voltage signal. A second amplifying circuit (504) to receive a second voltage signal and to provide a second output voltage signal. A summing circuit (509) to provide a common mode component of the first input voltage signal and the second input voltage signal. A compensation circuit (540) to amplify the common mode component of the first input voltage signal and the second input voltage signal and output an injection signal. A common gain setting network (505) including a plurality of resistors to receive the injection signal and to interface with the first amplifying circuit (502), the second amplifying circuit (504), and the compensation circuit (540) to prevent the common mode component from being present in the first output voltage signal and the second output voltage signal.