MOS Differential Pair RF Filtering at the Common Node
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Solution Overview
Problem
Existing MOS differential pairs in operational amplifiers are sensitive to high frequency electromagnetic fields, leading to accuracy issues due to unbalanced RF filtering and parasitic capacitances, which result in offset currents and gain degradation.
Innovation Solution
Incorporating a resistive element with a resistance value greater than 100 Ohms, preferably a MOS transistor, in series with the current source in the third branch of the differential pair, which couples the common node to the power supply, thereby reducing the impact of high frequency electromagnetic fields and achieving symmetrical operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional MOS differential pair is used, then the circuit is simple and easy to manufacture, but the accuracy is degraded due to sensitivity to high frequency electromagnetic fields
Solution Approach 1:
A resistive element is introduced as an intermediary component in the third branch between the current source and the common node. This resistor acts as a mediator to provide RF filtering and balance the parasitic capacitances, thereby improving accuracy without fundamentally changing the differential pair structure
Solution Approach 2:
The resistance value of the resistive element is specifically chosen to be greater than 100 Ohms to optimize the filtering effect. By changing the resistance parameter, the circuit achieves better RF rejection and balanced parasitic effects, improving measurement precision while maintaining practical manufacturability
2Productivity
If the differential pair operates at high frequencies, then the productivity increases, but the harmful effects of electromagnetic fields and parasitic capacitances increase
Solution Approach 1:
The resistive element converts the harmful high frequency electromagnetic interference into a beneficial filtering effect. By introducing the resistor in the third branch, the circuit creates an RC filter that actively rejects RF signals, turning potential harm into improved performance at high frequencies
Solution Approach 2:
The resistive element provides preliminary anti-action by preemptively filtering RF signals before they can affect the differential pair operation. The resistor is positioned to counteract the harmful effects of parasitic capacitances and electromagnetic fields before they degrade the signal accuracy
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration reduces RF-induced offset and improves power supply rejection ratio (PSRR) by providing efficient filtering at the common node, minimizing gain degradation and additional noise, and is particularly effective in high frequency applications.
Implementation Method 1
Incorporating a resistive element with a resistance value greater than 100 Ohms, preferably a MOS transistor, in series with the current source in the third branch of the differential pair, which couples the common node to the power supply, thereby reducing the impact of high frequency electromagnetic fields
Implementation Method 2
The circuit further includes a capacitor having a first terminal coupled to a gate of the at least one transistor of the second branch and a second terminal coupled to ground
Data Source
AI summary
A differential pair circuit includes a first branch and a second branch having a common first node. Each of the first and second branches includes at least one transistor having a conduction node directly connected to the common first node. A third branch couples the common first node to a power supply node. The third branch includes a current source in series with a resistive element.