AMOLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Conventional voltage compensation type pixel circuits for AMOLED display devices face issues with detecting both positive and negative threshold voltages of driving TFTs, leading to non-uniform luminance and rapid TFT degradation due to operation in linear regions during light emission, which affects contrast and longevity.
Innovation Solution
The proposed pixel circuit uses a driving transistor connected to the light emitting element between high and low-potential power lines, with a storage capacitor and program transistors to compensate threshold voltage, allowing the driving TFT to operate in the saturation region during light emission, and utilizes the OLED as a capacitor to accurately detect threshold voltages without a diode structure, ensuring operation in the saturation region and preventing unnecessary light emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional voltage compensation type pixel circuit uses a diode structure for the driving TFT to detect threshold voltage, then the circuit can detect threshold voltage, but it cannot detect negative threshold voltage (for n-type TFTs) or positive threshold voltage (for p-type TFTs)
Solution Approach 1:
The patent inverts the conventional diode structure connection by connecting the source to the anode and drain to the cathode of the OLED, rather than the conventional gate-to-anode and drain-to-cathode connection. This inversion allows the driving TFT to operate in saturation region during light emission while enabling detection of both positive and negative threshold voltages through the reverse-structured diode configuration.
Solution Approach 2:
The patent changes the operational parameters of the driving TFT by ensuring it operates in the saturation region during light emission through the inverted diode structure. This parameter change from linear region to saturation region operation enables accurate threshold voltage detection for both polarities while maintaining stable light emission control.
2Device complexity
If the driving TFT operates in linear region during light emission, then the circuit structure is simpler, but the TFT degrades rapidly due to bias stress
Solution Approach 1:
The patent inverts the conventional operation mode by forcing the driving TFT to operate in saturation region during light emission through the inverted diode structure, rather than the conventional linear region operation. This inversion maintains circuit simplicity while dramatically improving TFT reliability by reducing bias stress degradation.
3Ease of operation
If a light emitting control TFT is serially connected between the driving TFT and light emitting element to control light emission, then light emission can be controlled, but the control TFT operates in linear region and degrades faster than the driving TFT
Solution Approach 1:
The patent extracts the light emission control function from a separate control TFT and integrates it into the inverted diode structure of the driving TFT itself. By removing the need for a separate control TFT, the patent eliminates the reliability issue of control TFT degradation while maintaining full light emission control capability through the inverted diode configuration.
4Measurement precision
If the gate and drain of the driving TFT are connected to detect threshold voltage, then threshold voltage detection is enabled, but the gate-drain voltage is 0V and detection is limited to threshold voltages of 0V or higher
Solution Approach 1:
The patent inverts the conventional gate-to-drain connection by connecting source to anode and drain to cathode, creating a reverse-structured diode configuration. This inversion allows the detection of both positive and negative threshold voltages by reversing the polarity constraints of the conventional connection method.
Data Source
AI summary
A voltage compensation type pixel circuit of an AMOLED display device includes a driving transistor serially connected to a light emitting element between high-potential and low-potential power lines to drive the light emitting element in response to a voltage supplied to a first node, a first program transistor for supplying a data voltage of a data line to a second node in response to a scan signal of a scan line, a second program transistor for supplying a reference voltage from a reference voltage supply line to the first node in response to the scan signal of the scan line, a merge transistor for connecting the first and second nodes in response to a merge signal of a merge line, a storage capacitor connected between a third node and the second node interposed between the driving transistor and the light emitting element to store a voltage which corresponds to the data voltage in which the threshold voltage is compensated, and first and second reset transistors for initializing at least two of the first, second, and third nodes to an initialization voltage of an initialization voltage line in response to a reset signal of a reset line.