Display Substrate Current Source Circuit for Stable AMOLED Bias
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Solution Overview
Problem
Current display technologies face challenges in achieving improved display resolution and manufacturing yield while enabling normal sequential scan programming, particularly due to limitations in thin-film transistor (TFT) types and the need for better control over output current variability in current sources and sinks for AMOLED circuits.
Innovation Solution
A circuit design that shares a switch transistor between multiple pixel circuits, using a reference voltage and bias current to control programming and emission in a current-biased, voltage-programmed scheme, allowing for reduced transistor count and improved spatial and temporal uniformity in OLED displays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a p-type TFT is used in conventional current mirror and current sources with source terminal fixed to VDD, then the output current remains constant despite line voltage changes, but the output resistance is high which is undesirable
Solution Approach 1:
The patent inverts the conventional current source configuration by using n-type TFTs instead of p-type TFTs, and connecting the source terminal to ground rather than VDD. This inversion allows the current source to achieve low output resistance while maintaining stable output current, directly resolving the technical contradiction.
2Adaptability or versatility
If a p-type TFT is used for current sink with source connected to output line, then the circuit can be implemented, but source voltage variability directly affects gate-source voltage causing output current to vary with different loads
Solution Approach 1:
The patent introduces a compensation capacitor as an intermediary element that couples the source voltage variability to the gate, creating a feedback mechanism that compensates for the voltage changes. This allows the current sink to maintain constant output current despite load variations, resolving the contradiction between implementability and current stability.
3Productivity
If the number of transistors in each pixel is reduced to improve display resolution, then aperture ratio and manufacturing yield improve, but the ability to integrate useful circuitry and maintain normal sequential scan programming becomes limited
Solution Approach 1:
The patent makes the switch transistor universal by enabling it to perform multiple functions: serving as the pixel switch for sequential scan programming, acting as the emission control transistor, and functioning as part of the current source circuitry. This multi-functionality allows reduced transistor count while maintaining full circuit capability and normal programming operation.
4Stability of the object's composition
If spatial and temporal uniformity of OLED display is improved, then display quality increases, but circuit complexity increases requiring more transistors per pixel
Solution Approach 1:
The patent merges the switch transistor and emission control transistor into a single shared transistor. This combination reduces the transistor count per pixel while maintaining the circuit's ability to control both pixel selection and emission timing, thereby improving uniformity without significantly increasing complexity.
Data Source
AI summary
A technique to implement a stable and high impedance current sink or source onto a display substrate using a single device. The high output current source or sink circuit includes an input that receives a fixed reference current and provides the reference current to a node in the current source or sink circuit during a calibration operation of the current source or sink circuit. The circuit further includes a first transistor and a second transistor series-connected to the node such that the reference current adjusts the voltage at the node to allow the reference current to pass through the series-connected transistors during the calibration operation. The circuit includes one or more storage devices connected to the node, and an output transistor connected to the node to source or sink an output current from current stored in the one or more storage devices to a drive an active matrix display with a bias current corresponding to the output current.