Dynamic Cascaded OTA for Faster Transient Response and Stability
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Solution Overview
Problem
Conventional operational transconductance amplifiers suffer from limited additional current supply and slow response speed due to the number of current mirrors, which inadequately enhance loop bandwidth and stability, and the introduction of a gain stage introduces additional stability issues.
Innovation Solution
A parallel input and dynamic cascaded operational transconductance amplifier (OTA) is designed with multiple sub-OTAs and cascading capacitors to generate transient bias currents, enhancing loop bandwidth and response speed during transient states while reducing bias current during steady states, and incorporating a DC bias voltage load and MOS diodes to improve phase margin and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If additional current is supplied via current mirrors to increase common mode bias current in transient state, then loop bandwidth and response speed are enhanced, but response speed becomes slow due to the number of current mirrors
Solution Approach 1:
The amplifier is divided into multiple operational stages (first operational amplification pair, second operational amplification pair, third operational amplification pair) with distinct functions. Each stage processes signals at different levels, allowing the circuit to achieve high response speed without requiring excessive current mirrors in a single stage. The segmentation distributes the amplification task across multiple simpler stages.
Solution Approach 2:
A current sharing circuit is introduced as an intermediary component between the input stage and output stage. This circuit includes current sharing transistors that dynamically allocate bias current to different operational pairs based on signal conditions, enabling fast transient response without directly coupling multiple current mirrors together, thus reducing overall complexity.
2Speed
If a gain stage is added to increase common mode bias current, then loop bandwidth is enhanced, but additional stability issues are introduced
Solution Approach 1:
Negative feedback paths are implemented through carefully designed feedback networks that include capacitive elements. The feedback circuit monitors the output and adjusts the bias current distribution in real-time, automatically compensating for stability issues that arise during transient states. This dynamic feedback mechanism allows the gain stage to operate at high bandwidth while maintaining stability through continuous correction.
Solution Approach 2:
The circuit dynamically changes operating parameters (bias current levels, transconductance values) based on signal conditions. During transient states, the circuit switches to high-gain modes with increased bias current to maximize loop bandwidth. During steady-state operation, parameters are adjusted to lower-gain modes that prioritize stability, thus resolving the contradiction between bandwidth and stability.
3Speed
If common mode bias current is increased during transient state, then loop bandwidth and response speed are enhanced, but stability is reduced during transient state
Solution Approach 1:
The bias current distribution is made dynamic rather than static. Current sharing transistors and switching mechanisms automatically adjust the amount of bias current supplied to each operational pair based on the instantaneous signal conditions. During transient events, additional current is dynamically allocated to enhance response speed, while during steady-state operation, current distribution is optimized for stability, thus resolving the contradiction through time-varying parameter adjustment.
Solution Approach 2:
The circuit employs periodic sampling and adjustment of bias current levels through oscillating control signals that modulate the current sharing circuit. This periodic action allows the system to alternately prioritize speed and stability in a controlled manner, with the oscillation frequency designed to be much higher than the signal bandwidth, effectively averaging out the stability issues while maintaining high transient response capability.
Data Source
AI summary
A parallel input and dynamic cascaded OTA (operational transconductance amplifier includes: plural sub-OTAs which generate corresponding plural transconductance output currents according to corresponding plural differential input voltages; and at least one cascading capacitor which is cascaded between a first sub-OTA and a second sub-OTA. A second transconductance output current generated by the second sub-OTA is coupled through the cascading capacitor to generate a transient bias current on a common mode bias node of the first sub-OTA, thus providing the transient bias current to a differential pair circuit of the first sub-OTA in a case when a transient variation occurs in the differential input voltage corresponding to the first sub-OTA, so that a loop bandwidth and a response speed during a transient state are enhanced.