Class AB OTA Biasing for High Slew Rate and DC-Gain
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Solution Overview
Problem
Existing class AB operational amplifiers face challenges in low-power applications due to uncontrolled maximum output current and limited DC-gain, particularly when driving large capacitive loads in low-voltage systems.
Innovation Solution
A fully symmetrical operational transconductance amplifier (OTA) with an adaptive bias block and positive feedback network is designed, incorporating a current subtractor and mirror blocks to enhance DC-gain and control power consumption, allowing for efficient operation with a boosted tail current and improved slew rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If class AB operational amplifier with push-pull output stage is used, then slew rate is improved and large signal tracking is enhanced, but maximum output current is not well controlled
Solution Approach 1:
The patent implements a feedback mechanism where the output current is sensed and fed back to control the tail current of the differential pair. This feedback loop dynamically adjusts the bias current based on the output current demand, ensuring that the maximum output current is well-controlled while maintaining high slew rate performance during large signal transitions.
2Speed
If adaptive bias mechanism is implemented to boost tail current, then large signal operation is improved, but DC-gain is limited due to single pole at high impedance output node
Solution Approach 1:
The patent segments the amplification function into two distinct stages: a first stage with high impedance output node that provides high DC-gain, and a second stage with push-pull output that provides high slew rate and drives the capacitive load. This segmentation allows each stage to be optimized for its specific function, resolving the contradiction between DC-gain and large signal response.
3Use of energy by moving object
If low power architecture is used, then power consumption is reduced, but ability to drive large capacitive loads is compromised
Solution Approach 1:
The patent employs dynamic biasing where the tail current is adjusted according to the operating conditions. During large signal transitions, the tail current is increased to provide high slew rate for charging large capacitive loads. During quiescent operation, the tail current is reduced to minimize power consumption. This dynamic adaptation resolves the contradiction between low power consumption and large signal driving capability.
Data Source
AI summary
A class AB operational amplifier is provided that includes first and second input transistors respectively coupled between first and second internal nodes and a first common node, first and second input stage load transistors diode connected and respectively coupled between a first voltage reference and the first and second internal nodes, first and second output transistors coupled in series between the first voltage reference and a second voltage reference, a tail current generator coupled between the first common node and the second voltage reference, an adaptive bias block coupled between the first and second voltage references and coupled to the first common node, and a positive feedback network coupled between the first voltage reference and the first and second internal nodes. Also provided is an integrated circuit having at least one such operational amplifier.