Differential Slew Boost Circuit for Op-Amp Slew Rate Limits
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Solution Overview
Problem
Increasing the slew rate of operational amplifiers while maintaining circuit stability and minimizing noise is challenging, as enhancing the bias current of the input stage leads to increased bandwidth and compensation capacitance, which in turn reduces slew rate and increases noise.
Innovation Solution
Incorporating differential slew boost circuits that temporarily increase the current through the compensation capacitor during input voltage changes, providing additional current paths for charge and discharge, thereby enhancing the rate of change of the output voltage without increasing quiescent current during steady-state operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the bias current of the input stage is increased to increase the slew rate, then the slew rate is improved, but the bandwidth increases which requires increased compensation capacitance that in turn decreases the slew rate and increases noise
Solution Approach 1:
The patent segments the current path by introducing separate slew boost circuits that provide dedicated current paths for charging the compensation capacitor during large signal transitions. This separates the slew rate enhancement function from the regular bias current, allowing the input stage to operate at low bias current (low noise) while still achieving high slew rate when needed through the boost circuits.
Solution Approach 2:
The slew boost circuits are pre-configured with transistors and current mirrors that can rapidly activate to provide additional current to the compensation capacitor when a large signal transition is detected. This preliminary arrangement of current paths allows immediate slew rate enhancement without requiring continuous high bias current, thus minimizing noise while maintaining high slew rate capability.
2Speed
If the bias current of the input stage is increased to increase the slew rate, then the slew rate is improved, but the compensation capacitance must be increased which results in a decrease of the slew rate
Solution Approach 1:
The patent segments the current delivery function by creating separate slew boost circuits that independently provide current to the compensation capacitor during large signal transitions. This segmentation allows the use of a smaller, fixed compensation capacitance value while achieving high slew rate through the dedicated boost current paths, avoiding the need to increase capacitance to maintain stability.
Solution Approach 2:
The slew boost circuits dynamically activate to provide additional current only when large signal transitions occur. During normal operation, the circuits remain inactive, allowing the compensation capacitor to be optimized for stability with a smaller value. The dynamic current boosting provides high slew rate without requiring a larger capacitance that would slow down the response.
Data Source
AI summary
A differential input stage of a circuit includes a first transistor, a second transistor, a third transistor, and a fourth transistor. Drains of the first and third transistors couple together at a first node, and drains of the second and fourth transistors couple together at a second node. A first slew boost circuit includes a fifth transistor and a first current mirror. A gate of the fifth transistor couples to the second node. A source of the fifth transistor couples to the first node. The first current mirror couples to the fifth transistor and to the second node. A second slew boost circuit includes a sixth transistor and a second current mirror. A gate of the sixth transistor couples to the first node. A source of the sixth transistor couples to the second node. The second current mirror couples to the sixth transistor and to the first node.