Class AB Two-Stage Operational Amplifier for Compact Rail-to-Rail Output
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Solution Overview
Problem
The existing two-stage operational amplifiers (OP AMPs) used in source drivers for LCD panels have a large layout size and inferior falling characteristics, leading to signal deviations and degraded output voltage characteristics, making them unsuitable for reducing the size of the output buffer while maintaining image quality.
Innovation Solution
A two-stage OP AMP design with a differential amplifier, current mirror, bias circuits, and capacitors is implemented, utilizing NMOSFET and PMOSFET transistors to improve falling and rising characteristics, allowing for class AB operation similar to rail-to-rail amplifiers, thereby reducing layout size and enhancing signal fidelity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a rail-to-rail OP AMP is used to achieve full output voltage swing, then the output voltage range is improved, but the layout size increases significantly
Solution Approach 1:
The output stage is divided into two separate circuits: a pull-up circuit using PMOSFET and a pull-down circuit using NMOSFET. Each circuit is optimized independently to achieve rail-to-rail output swing while minimizing the area occupied by each component, thereby reducing the overall layout size compared to a conventional rail-to-rail OP AMP.
Solution Approach 2:
The output stage is designed to perform multiple functions: it provides full rail-to-rail output voltage swing, operates in class AB mode for both pull-up and pull-down operations, and maintains small layout size. This multi-functionality is achieved through the complementary CMOS structure where PMOSFET handles positive swings and NMOSFET handles negative swings.
2Area of stationary object
If a two-stage OP AMP is used to reduce layout size, then the area is reduced, but the falling characteristic deteriorates causing signal deviations
Solution Approach 1:
The pull-down circuit is specifically optimized with NMOSFET configured as a common source amplifier to enhance the falling characteristic. This local optimization ensures that the transistor responsible for pulling down the output voltage has high transconductance and low output impedance, thereby improving the falling edge response and eliminating signal deviations without increasing overall layout size.
Solution Approach 2:
The output stage operates in class AB mode with dynamic biasing, allowing the pull-up and pull-down circuits to be actively controlled based on the output voltage level. This dynamic operation ensures optimal performance during both rising and falling transitions, maintaining reliability while keeping the layout compact.
3Strength
If the output stage is operated in class AB mode, then the output voltage characteristic is improved, but the device complexity increases
Solution Approach 1:
The class AB operation is achieved by merging the pull-up circuit (PMOSFET) and pull-down circuit (NMOSFET) into a single complementary output stage. Both circuits share common biasing networks and control signals, allowing them to operate cooperatively in class AB mode. This integration reduces the overall complexity compared to having separate class A circuits or using more complex class B push-pull configurations with additional components.
Data Source
AI summary
An operational amplifier includes a differential amplifier including an active load, a current mirror including a first branch and a second branch, a first switch connected between a first power source and an output node and switched in response to a voltage of a first output terminal of the differential amplifier, a first bias circuit to control an amount of a reference current flowing in the first branch in response to a voltage of a second output terminal of the differential amplifier, a second bias circuit to control a voltage of the second branch in which a mirror current flows, in response to a voltage of the first output terminal, a second switch connected between the output node and a second power source and switched in response to a voltage of the second branch, and a capacitor connected between the output node and the first output terminal.