Translinear Slew Boost Circuit for Op-Amp CMRR Stability
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Solution Overview
Problem
Operational amplifiers face a trade-off between increasing slew rate and maintaining common mode rejection ratio (CMRR), noise, and voltage offset stability, as existing solutions degrade these parameters when enhancing slew rate.
Innovation Solution
A circuit with a slew boost controller and tail current regulator that provides a variable tail current proportional to differential input voltage, maintaining a minimum constant current for common mode signals and increasing tail current only when differential input exceeds a threshold, thus enhancing slew rate without degrading CMRR or stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a larger tail current is provided to increase slew rate, then the slew rate is improved, but stability is degraded
Solution Approach 1:
The patent applies dynamics by making the tail current variable rather than fixed. The tail current dynamically adjusts based on the differential input voltage: it remains at a low regulated minimum for common mode signals (maintaining stability) and increases automatically when a large differential voltage is detected (improving slew rate). This dynamic adaptation resolves the contradiction between stability and slew rate.
2Speed
If a larger tail current is provided to increase slew rate, then the slew rate is improved, but noise is degraded
Solution Approach 1:
The dynamic tail current regulation ensures that high current (which improves slew rate) is only activated when needed during large signal transitions. During normal operation with common mode signals, the tail current remains at a low level, minimizing noise generation. This temporal and conditional separation resolves the noise-slew rate tradeoff.
3Speed
If a larger tail current is provided to increase slew rate, then the slew rate is improved, but voltage offset drift is degraded
Solution Approach 1:
The regulated minimum tail current provides a stable baseline that prevents voltage offset drift during normal operation. The current only increases dynamically when a large differential input voltage is detected, at which point the differential signal dominates and makes offset drift insignificant. This dynamic approach maintains stability while enabling high slew rate when needed.
4Speed
If the tail current is increased to improve slew rate, then the full power bandwidth is improved, but common mode rejection ratio is degraded
Solution Approach 1:
The patent maintains a low regulated minimum tail current during common mode operation, which preserves common mode rejection ratio. When a large differential voltage is detected, the tail current dynamically increases to improve slew rate and full power bandwidth. Since the current increase is conditional on differential signal presence, common mode rejection is not degraded.
Data Source
AI summary
A method of improving the slew rate of an amplifier is described where a differential pair of transistors receives a differential first control signal and second control signal. The tail current for the transistors is provided by a tail current regulator. The same control signals are applied to a slew boost controller, whose output increases as the differential between the control signals increase. The tail current regulator generates a bias signal that sets a minimum tail current. The tail current is controlled to be the minimum tail current until the slew boost output signal exceeds a threshold, whereupon the tail current increases in response to an increasing differential between the control signals. Common mode rejection is not adversely affected by the slew boost controller generating a slightly varying current under common mode conditions due to the minimum tail current.