N-Type Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
In organic electroluminescent display apparatuses, the threshold voltage of driving transistors and the second power voltage applied at the cathode electrode of OLEDs affect the driving current, leading to non-uniform image brightness and sensitivity to temperature and OLED deterioration, especially in larger displays.
Innovation Solution
A pixel circuit using N-type transistors with specific configurations, including a driving transistor, capacitors, and additional transistors for signal control, is designed to reduce the influence of threshold voltage and second power voltage on driving current, ensuring consistent brightness and image quality by compensating for threshold voltage deviations and stabilizing voltage across the OLED.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional pixel circuits are used with N-type transistors, then the circuit can be implemented with available transistor technology, but the driving current is significantly affected by threshold voltage variations and second power voltage, leading to non-uniform image brightness
Solution Approach 1:
The pixel circuit is divided into multiple functional blocks: a driving transistor for current control, a storage capacitor for voltage holding, a compensation circuit for threshold voltage correction, and switching transistors for signal routing. This segmentation allows each component to address specific issues independently, with the compensation circuit specifically targeting threshold voltage variations to improve brightness uniformity
Solution Approach 2:
A compensation transistor is introduced as an intermediary element that specifically counteracts the threshold voltage effects. This compensation transistor works in conjunction with the storage capacitor to create a feedback mechanism that cancels out threshold voltage variations, thereby reducing their harmful influence on the driving current and image brightness uniformity
2Reliability
If the second power voltage at the OLED cathode varies due to parasitic resistance, then the driving current changes, but this causes sensitivity to temperature and OLED deterioration
Solution Approach 1:
The pixel circuit implements a feedback mechanism where the storage capacitor continuously holds and updates the gate voltage of the driving transistor based on the actual voltage conditions. This feedback loop compensates for second power voltage variations by adjusting the driving transistor's operating point, thereby stabilizing the driving current against temperature changes and OLED deterioration
Solution Approach 2:
The storage capacitor performs preliminary action by pre-charging and holding the gate voltage at the appropriate level before the OLED operates. This preliminary voltage establishment ensures that the driving transistor starts with the correct operating conditions, compensating for anticipated second power voltage drops and preventing their harmful effects on driving current stability
3Reliability
If additional compensation circuits and transistors are added to reduce threshold voltage influence, then image brightness uniformity improves, but the device complexity increases
Solution Approach 1:
The compensation function is merged with the existing driving transistor and storage capacitor rather than being implemented as a completely separate circuit. The compensation transistor shares the same power supply nodes and control signals as the driving transistor, allowing threshold voltage compensation to be achieved while minimizing additional circuit elements and maintaining relatively simple circuit structure
Solution Approach 2:
The storage capacitor serves multiple functions: it holds the gate voltage for the driving transistor, stores the compensation voltage for threshold voltage correction, and maintains the operating point during OLED operation. This multi-functionality reduces the need for separate dedicated components, thereby limiting the increase in device complexity while still achieving improved brightness uniformity
Data Source
AI summary
An improved pixel circuit including N-type transistors is provided. The pixel circuit includes a light emitting device driven by a driving current according to a gate voltage of a driving transistor. The pixel circuit also includes a first capacitor, a second transistor for transferring a data signal to a first terminal of the first capacitor in response to a scan control signal, a third transistor for diode-connecting the driving transistor in response to the scan control signal, a fourth transistor for applying a first power voltage to a first electrode of the driving transistor in response to an emission control signal, a fifth transistor for applying a sustain voltage to the first terminal of the first capacitor in response to the emission control signal, and a sixth transistor for applying the first power voltage to a second terminal of the first capacitor in response to an initialization control signal.