AMOLED Pixel Circuit Offset Elimination Stage
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Solution Overview
Problem
In active-matrix organic light emitting diode (AMOLED) display devices, the inhomogeneity of light emission due to threshold voltage drifts in drive transistors is not accurately compensated, leading to inconsistent display brightness, as the hysteresis characteristic of drive transistors causes inaccurate threshold voltage sensing during compensation.
Innovation Solution
A pixel circuit with an additional offset elimination stage is introduced at the beginning of each image frame, where the drive transistor is switched off before the reset stage, allowing for accurate compensation of the threshold voltage by controlling the transistors using specific voltage and signal combinations to elevate the drive transistor's electrode voltage, ensuring the gate-source voltage is below the threshold voltage, thus eliminating offset and improving homogeneity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the drive transistor is kept in ON state during reset and sense stages for continuous operation, then productivity is maintained, but measurement precision of threshold voltage deteriorates due to hysteresis effect
Solution Approach 1:
The patent applies preliminary action by introducing an offset elimination stage before the reset and sense stages. In this preliminary stage, the drive transistor is switched off to eliminate hysteresis effects, and the source electrode voltage is elevated to a predetermined level. This preliminary preparation ensures that subsequent threshold voltage sensing occurs from a known, stable state, resolving the contradiction between continuous operation and accurate measurement.
2Measurement precision
If the drive transistor is switched off before reset stage to eliminate hysteresis offset, then measurement precision improves, but device complexity increases due to additional control stages
Solution Approach 1:
The patent applies universality by designing the control circuit to perform multiple functions using the same hardware components. The control circuit sequentially executes offset elimination, reset, and sense operations by adjusting control signals timing and levels. This multi-functional approach achieves improved measurement precision without proportionally increasing device complexity, as the same transistors and capacitors serve multiple purposes across different operational stages.
3Manufacturing precision
If additional offset elimination stage is added to switch off drive transistor before reset, then manufacturing precision of display brightness improves, but loss of time increases due to extra operation stage
Solution Approach 1:
The patent applies periodic action by structuring the pixel circuit operation as a sequence of distinct stages (offset elimination, reset, sense, data writing, light emission) that repeat each image frame. The offset elimination stage is integrated into this periodic cycle with optimized timing, where the drive transistor is switched off only during the brief offset elimination period and then restored to ON state. This periodic structuring minimizes the time penalty while ensuring brightness homogeneity through consistent threshold voltage compensation in each frame cycle.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach accurately compensates for the threshold voltage drifts, enhancing the homogeneity of light emission in AMOLED display devices by avoiding the inaccuracies caused by the hysteresis effect, resulting in consistent display brightness across the screen.
Implementation Method 1
the voltage of the first electrode of the drive transistor is elevated by charging a parasitic capacitor of the sense line
Data Source
AI summary
A pixel circuit, a driving method and a display device. The pixel circuit includes a light emitting unit; a drive transistor configured to drive the light emitting unit to emit light to display an image frame; and a control circuit configured to control the drive transistor according to signals on control lines. The method of controlling includes: resetting, in a reset stage of an image frame, a voltage of the first electrode of the drive transistor by applying a reset voltage, and sensing, in a sense stage, a threshold voltage of the drive transistor by applying a reference voltage. The control circuit is further configured to control, in an offset elimination stage at the beginning of the image frame and prior to the reset stage, the drive transistor to be switched off, which was in an ON state at the end of a previous image frame.