Amplifier Settling Time via Pre-Charged High Gain Node
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Solution Overview
Problem
The settling time of amplifiers is prolonged due to the slow voltage increase on high gain nodes, which affects overall stability and frequency response, and existing solutions to reduce this time often lead to instability and oscillation.
Innovation Solution
A pre-set voltage is applied to the high gain node when the amplifier is disabled, using a startup circuit with a switch controlled by an enable signal, allowing the node to quickly reach the regulated voltage upon enabling, thus reducing settling time without compromising stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If capacitor 104 is made smaller to reduce settling time, then the settling time decreases, but the frequency response increases to the point that amplifier 100 begins to oscillate and becomes unstable
Solution Approach 1:
The patent applies preliminary action by pre-charging capacitor 104 to a voltage close to the final regulated voltage before the amplifier is enabled. This is achieved through a startup circuit that activates only during the initial settling phase, charging the capacitor in advance so that when the amplifier becomes operational, the high gain node is already near its target voltage, dramatically reducing settling time without requiring the capacitor to be smaller
2Loss of time
If the transconductance (gm) of OTA 101 is made higher to reduce settling time, then the settling time decreases, but the frequency response increases causing amplifier 100 to oscillate and become unstable
Solution Approach 1:
The patent uses preliminary action by pre-establishing the voltage condition on the high gain node through the startup circuit before the main amplifier operation begins. This allows the system to achieve fast settling without increasing the transconductance of OTA 101, thereby maintaining the original frequency response and stability characteristics while still reducing settling time
3Loss of time
If a startup circuit is introduced to pre-charge the high gain node, then the settling time is reduced, but the device complexity increases
Solution Approach 1:
The startup circuit implements preliminary action by providing a temporary charging path for capacitor 104 only during the initial enable transition. The circuit includes simple components (transistors and resistors) that automatically deactivate once the capacitor is charged to the target voltage, adding minimal complexity while achieving significant settling time reduction
Solution Approach 2:
The startup circuit is designed as a separate, extractable module that can be added to existing amplifier designs without modifying the core amplifier structure. The startup circuit components are isolated and can be independently designed and removed if needed, minimizing the impact on overall device complexity
Data Source
AI summary
A method of reducing the settling time of an amplifier includes providing a pre-set voltage on a high gain node of the amplifier when the amplifier is disabled. This pre-set voltage can be slightly less than the regulated voltage. In this manner, when the amplifier is enabled, the high gain node can quickly reach this regulated voltage. The pre-set voltage can be applied to the high gain node by operating a switch, e.g. if the amplifier is enabled (disabled), then the switch is open (closed). A startup circuit can generate the pre-set voltage. This startup circuit can include a startup current source and a transistor connected in series between VDD and VSS. The switch can be connected to the gate and drain of the transistor. Notably, the transistor can be the same type of device as the MOS device driven by the high gain node in the amplifier.