Differential Op-Amp Common-Mode Feedback With Start-Up Bias Control
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Solution Overview
Problem
Current fully-differential operational amplifiers face challenges in reducing current consumption while maintaining high gain-bandwidth and stability, as the common-mode feedback circuit represents an additional source of current consumption and can lead to undesired bias conditions, especially in low power configurations.
Innovation Solution
A fully differential operational amplifier with a start-up circuit that detects the operating condition of the differential pair transistors and only turns on the bias circuit when the desired condition is reached, reducing unnecessary current consumption and maintaining stability even at low currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the common-mode feedback circuit is continuously active to maintain stability, then the operational amplifier remains stable, but current consumption increases
Solution Approach 1:
The common-mode feedback circuit operates periodically rather than continuously. The start-up circuit activates the bias circuit only when needed (during startup or when instability is detected), allowing the feedback circuit to remain inactive during stable operation, thus reducing average current consumption while maintaining stability when required
Solution Approach 2:
The start-up circuit automatically detects the operational state of the differential pair and autonomously controls the activation of the bias circuit. This self-monitoring and self-regulation mechanism ensures the feedback circuit is activated only when necessary, eliminating the need for external control and reducing unnecessary power consumption
2Reliability
If the bias circuit is continuously on to ensure proper operating conditions, then the differential pair maintains correct bias, but power consumption increases
Solution Approach 1:
The start-up circuit performs preliminary detection of the differential pair's operating condition before activating the bias circuit. This preliminary action ensures the bias circuit is activated only when the differential pair is ready and needs biasing, preventing unnecessary power consumption during already-stable operation
Solution Approach 2:
The bias circuit is activated in periodic bursts rather than continuously. The start-up circuit monitors the differential pair state and activates the bias circuit only during periods when biasing is needed, reducing overall power consumption while maintaining proper bias conditions
Data Source
AI summary
A fully differential operational amplifier includes a differential input stage, at least one output stage and a common-mode feedback circuit connected with the input stage. The differential input stage includes a differential pair of transistors and a bias circuit for the differential pair of transistors. A start-up circuit operates to detect an operating condition of the differential pair of transistors of the input stage and in response thereto turn on the bias circuit.