Differential Amplifier Startup Circuit for Common-Mode Voltage Stability
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Solution Overview
Problem
Fully differential amplification devices face challenges in stabilizing the output common voltage at the desired reference voltage during startup and mode switching, often settling at undesired voltages like ground or power supply voltage, leading to impaired amplification due to incorrect common mode feedback operation.
Innovation Solution
A fully differential amplification device with a small-current, small-area startup circuit that uses an auxiliary current supply to stabilize the output common voltage, eliminating the need for additional comparators and minimizing parasitic capacitance, ensuring the output common voltage remains at the desired level even when the input DC voltage is outside the dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional common mode feedback circuit is used, then the circuit structure is simple, but the output common voltage cannot be stabilized at the desired reference voltage during startup and mode switching
Solution Approach 1:
The startup circuit activates before the main amplification circuit to pre-establish the correct common mode voltage level. The auxiliary current source M6 and M7 are enabled first to charge the parasitic capacitances at the gates of M10 and M11, ensuring that when the main circuit activates, the output common voltage is already at the correct reference level, preventing the circuit from settling at incorrect voltages during startup
Solution Approach 2:
The patent introduces an intermediary startup circuit that mediates between the power supply and the main amplification circuit. This intermediate circuit (comprising M6, M7, and associated resistors) acts as a bridge to gradually establish the correct voltage levels and charge distribution before the main circuit operates, avoiding direct connection issues that would cause voltage settling problems
2Reliability
If additional comparators are added to stabilize output common voltage, then the stability improves, but the chip area and parasitic capacitance increase
Solution Approach 1:
The patent extracts the startup function from the main common mode feedback circuit, separating the voltage stabilization task into two distinct phases: startup phase handled by the auxiliary current sources M6 and M7, and normal operation phase handled by the standard common mode feedback circuit. This extraction eliminates the need for additional comparators while maintaining stability, as the startup circuit directly charges the critical parasitic capacitances without requiring voltage comparison and control logic
Solution Approach 2:
The startup circuit uses the existing circuit elements (auxiliary current sources M6 and M7, and parasitic capacitances of M10 and M11 gates) to self-establish the correct operating conditions. The circuit leverages its own internal resources to charge the parasitic capacitances and set the initial voltage levels, eliminating the need for external comparators or additional control circuits
3Reliability
If additional comparators and control circuits are added, then the voltage stabilization improves, but the current consumption increases
Solution Approach 1:
The startup circuit operates periodically rather than continuously - it activates only during startup and mode switching events to charge the parasitic capacitances, then disables itself during normal operation. The auxiliary current sources M6 and M7 are controlled by the control signal line to provide current only when needed, avoiding continuous current consumption while maintaining voltage stability during critical transitions
Data Source
AI summary
An auxiliary current source is provided that includes MOS transistors feeding startup auxiliary current to the input portion of an output common mode voltage generation circuit amplifying the output of a differential amplifier and generating an output common voltage, and a correction current source is provided that includes MOS transistors feeding correction current corresponding to the auxiliary current to a common mode feedback comparator. Thereby, a control loop that controls the output common voltage to a predetermined voltage even when the input of the differential amplifier is outside the dynamic range at the beginning is correctly started up, thereby stabilizing the output common voltage at a desired voltage.