Stacked Transistor Circuit Biasing for Intermediate Node Stress Control
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Solution Overview
Problem
In stacked transistor configurations, the intermediate node can float during operation, leading to a negative voltage that causes the drain-to-source voltage (VDS) of a transistor to exceed the supply voltage, resulting in non-conductive stress and potential transistor degradation or failure.
Innovation Solution
Incorporating a third transistor or a capacitor connected to the intermediate node to provide a direct current (DC) bias, ensuring the node remains at a stable potential, thereby preventing excessive VDS and sub-threshold operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the intermediate node is left floating during transistor operation, then the circuit structure remains simple, but the drain-to-source voltage exceeds the supply voltage causing transistor stress and potential failure
Solution Approach 1:
A third transistor is introduced as an intermediary component connected to the intermediate node. This mediator transistor actively regulates the voltage at the intermediate node, preventing it from floating and ensuring the drain-to-source voltage remains within safe limits, thus protecting the transistors from stress and failure
Solution Approach 2:
The biasing transistor is configured to preemptively maintain the intermediate node at a safe voltage level before excessive voltage conditions can develop. By continuously monitoring and adjusting the intermediate node voltage, the system prevents harmful voltage excursions before they cause transistor stress or failure
2Reliability
If a third transistor is added to bias the intermediate node, then transistor reliability improves by preventing excessive VDS, but device complexity increases
Solution Approach 1:
The third transistor serves as a dedicated intermediary biasing element that simplifies the overall circuit design by providing a straightforward voltage regulation mechanism at the intermediate node, making the reliability improvement achievable with minimal additional complexity
Solution Approach 2:
The biasing transistor is specifically targeted at the intermediate node where voltage regulation is most critical. This localized approach applies complexity only where needed to prevent transistor stress, rather than redesigning the entire circuit, thus minimizing the increase in overall device complexity
Data Source
AI summary
A circuit includes a first transistor having first and second current terminals and a first control input, and a second transistor having third and fourth current terminals and a second control input. The third current terminal is coupled to the second current terminal at an intermediate node. In some cases, a third transistor is connected to the intermediate node to bias the intermediate rather than letting the intermediate node float. In other cases, a capacitor is connected to the intermediate node to reduce a negative voltage that might otherwise be present on the intermediate node.