Startup Bias Circuit for Multi-Stage OTAs Against Latching
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Solution Overview
Problem
Existing multistage operational transconductance amplifier (OTA) circuits face issues with undesired stable bias points, which can lead to latching in inappropriate conditions and shutdown due to common mode voltage perturbations, especially during start-up and normal operations, exacerbated by process-voltage-temperature variations and non-idealities like noise and mismatch.
Innovation Solution
A startup circuit for a multi-stage amplifier circuit is introduced, featuring a cascade of differential stages with a startup differential stage and current mirror circuitry that senses common mode voltage drops and compensates them, ensuring robust operation against PVT variations and noise, while maintaining performance and reducing area occupancy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a multi-stage OTA circuit is designed with cascade differential stages, then amplification capability is improved, but the circuit becomes susceptible to undesired stable bias points and latching issues
Solution Approach 1:
The startup circuit activates before the main amplifier operation to establish correct initial bias conditions. It forces the differential pair nodes to appropriate voltage levels during power-up, preventing the circuit from settling into undesired stable bias points or latching states before the main operational amplifiers are fully operational.
Solution Approach 2:
The startup circuit acts as an intermediary between the power supply and the main amplifier stages. It temporarily provides bias control during the transition period, mediating the startup process to ensure stable operation before the main feedback loops become active.
2Reliability
If feedback circuit branches are added to control bias points, then bias stability is improved, but circuit complexity increases
Solution Approach 1:
The startup function is extracted as a separate, dedicated circuit block rather than being integrated into the main feedback paths. This allows bias control functionality to be added without complicating the primary signal processing feedback loops, maintaining clarity in the main amplifier design while providing necessary startup control.
Solution Approach 2:
The startup circuit is designed to be self-activating based on power-up conditions, automatically establishing correct bias points without requiring external control signals or complex coordination with other circuit blocks. Once startup is complete, the circuit self-deactivates, leaving the main amplifier to operate independently.
3Reliability
If startup phase is extended to prevent latching, then reliability is improved, but startup time increases
Solution Approach 1:
The startup circuit operates in a time-limited manner, being active only during the critical power-up transition period. Once the main amplifier stages are operational and their feedback loops are established, the startup circuit automatically deactivates, providing reliable latching prevention without extending the operational startup time of the main amplifier.
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
The solution effectively stabilizes the amplifier circuit, preventing latching and shutdown, enhancing robustness and accuracy by compensating common mode voltage drops and maintaining performance across various conditions without performance loss, all while being compact and area-efficient.
Implementation Method 1
The startup differential stage is configured to sense a common mode voltage drop at the first differential stage of the multi-stage amplifier circuit
Implementation Method 2
The current mirror circuitry is configured to perform current mirroring of a current variation at the common source of the startup differential stage, and compensating the sensed common mode voltage drop
Data Source
AI summary
A circuit for startup of a multi-stage amplifier circuit includes a pair of input nodes and at least two output nodes configured to be coupled to a multi-stage amplifier circuit. A startup differential stage includes a differential pair of transistors having respective control terminals coupled to the pair of input nodes, and each transistor in the differential pair of transistors has a respective current path therethrough between a respective output node and a common source terminal. The startup differential stage is configured to sense a common mode voltage drop at a first differential stage of the multi-stage amplifier circuit. Current mirror circuitry includes a plurality of transistors coupled to the common terminal of the differential pair of transistors and coupled to two output nodes of the at least two output nodes.