Two-Stage Operational Amplifier With Class AB Rail-to-Rail Output
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Solution Overview
Problem
Two-stage operational amplifiers (OP AMPs) used in source drivers for LCD panels have inferior falling characteristics, leading to signal deviations and degraded output voltage characteristics, making them unsuitable for reducing the size of output buffers 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 branches: a pull-up circuit with PMOSFET for rising edge control and a pull-down circuit with NMOSFET for falling edge control. Each branch operates independently to drive the output voltage to different rails, achieving rail-to-rail output swing without requiring a single large amplifier stage.
Solution Approach 2:
The output stage operates in class AB mode with dynamic switching between push and pull operations. The bias circuits dynamically adjust the operating points of the PMOSFET and NMOSFET based on the output voltage level, enabling efficient rail-to-rail operation while minimizing conduction losses and layout size.
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
Solution Approach 1:
The pull-down circuit is specifically optimized for falling edge performance by using an NMOSFET with its gate connected to the second output terminal through a bias circuit. This local optimization ensures that the falling characteristic matches the rising characteristic, achieving symmetric output voltage transitions despite the two-stage architecture.
Solution Approach 2:
Bias circuits provide feedback control to adjust the operating currents of the PMOSFET and NMOSFET based on the output voltage level. The first bias circuit controls the reference current in response to the second output terminal voltage, while the second bias circuit controls the mirror current in response to the first output terminal voltage, ensuring balanced rising and falling characteristics.
3Reliability
If the output stage is operated in class AB mode, then the rising and falling characteristics are improved, but the device complexity increases
Solution Approach 1:
The bias circuits are merged with the current mirror structure, where the first bias circuit controls the reference current branch and the second bias circuit controls the mirror current branch. This integration allows class AB operation to be achieved without adding separate complex biasing networks, reducing overall device complexity while maintaining improved rising and falling characteristics.
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.