Folded Cascode Amplifier with Slew-Triggered Branch Current Boost
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Solution Overview
Problem
Folded-cascode amplifiers face a trade-off between slew rate and recovery time, noise, and offset performance, as increasing current to improve slew rate degrades noise and offset performance, while reducing current improves noise but worsens slew rate and recovery time.
Innovation Solution
A boost circuit is introduced to increase the cascode branch current only during slewing conditions, using a transconductance amplifier to detect slewing and supply additional current to the cascode load transistors, thereby enhancing slew performance without degrading other characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If cascode branch current is increased to improve slew rate, then slew performance is improved, but noise and offset performance are degraded
Solution Approach 1:
The cascode branch current is made dynamic rather than fixed. A detection circuit monitors the differential pair currents and automatically increases the cascode branch current only when slewing is detected (when one transistor carries significantly more current than the other). During normal operation, the current remains at its lower value to maintain low noise and offset. This dynamic adjustment resolves the contradiction by providing high current only when needed for slew rate while maintaining low current during normal operation to preserve noise and offset performance.
2Object-generated harmful factors
If cascode branch current is reduced to improve noise performance, then noise and offset are improved, but slew rate and recovery time are worsened
Solution Approach 1:
The system uses dynamic current adjustment where the cascode branch current is lowered during normal operation to minimize noise and offset, then rapidly increased when slewing is detected through the differential pair current monitoring circuit. This ensures optimal noise performance during steady-state operation while maintaining the capability for high slew rate when needed.
Solution Approach 2:
The detection circuit continuously monitors the differential pair currents in advance to detect slewing conditions before they fully develop. When an imbalance is detected (indicating the amplifier is entering a slewing condition), the circuit proactively increases the cascode branch current to support the upcoming large signal transition, ensuring optimal slew performance without maintaining high current continuously.
3Loss of time
If cascode branch current is increased to improve recovery time, then recovery speed is improved, but noise performance is degraded
Solution Approach 1:
The cascode branch current is dynamically adjusted based on operational conditions. During normal operation with low current, noise performance is optimized. When slewing is detected through differential pair current monitoring, the current is increased to enable fast recovery. This dynamic approach ensures fast recovery time only when needed, while maintaining low noise during normal operation.
Data Source
AI summary
A folded cascode amplifier having improved slew comprises an input differential transistor pair circuit, a cascode branch circuit coupled to the differential pair circuit, and a boost circuit for increasing branch current when the amplifier is in a slew condition. The additional current increases slew of the amplifier without negatively affecting amplifier characteristics.