Pixel circuit, display, and method
Active Matrix Organic Light Emitting Diode (AMOLED) displays, novel pixel circuits therefor, and methods of programming the pixel circuit and measuring the current of the pixel circuit and OLED thereof are disclosed. One pixel circuit includes four TFT transistors, a storage capacitor and an OLED device and is programmed with use of voltage supplied through a data line. One method measures currents of the OLED and the pixel circuit through the data line by a readout circuit.
1. A display system including an array of pixel circuits arranged in rows and columns, each including a drive transistor, a storage capacitor, and a light emitting device, the display system comprising:
a controller for driving each pixel circuit in a plurality of operation states for the pixel circuit including a programming state for programming the storage capacitor of the pixel circuit with use of a data voltage provided over a data line of the display system, and at least two measurement states for measuring current from the pixel circuit over the data line; and
a pixel circuit of the array of pixel circuits including:
a drive transistor including a first terminal coupleable to the data line;
a storage capacitor coupled across a gate terminal and the first terminal of the drive transistor;
a light emitting device coupleable to the first terminal of the drive transistor; and
a first transistor coupled to the first terminal of the drive transistor and coupled between the drive transistor and the light emitting device, for coupling the light emitting device to the first terminal of the drive transistor during a first measurement state of the at least two measurement states and for decoupling the first terminal of the drive transistor from the light emitting device during a second measurement state of the at least two measurement states.
2. The display system of claim 1 , further comprising a readout circuit coupleable to the data line for measuring the current from the pixel circuit over the data line.
3. The display system of claim 2 wherein the readout circuit comprises an integrator for integrating said current from the pixel during said measuring and generating an output voltage corresponding to said integrated current, and an analog to digital converter for converting said output voltage into a digital code output.
4. The display system of claim 2 , wherein the readout circuit is not coupleable to the pixel circuit via a signal line different from the data line for measuring the current from the pixel circuit.
5. The display system of claim 1 , wherein the first measurement state for measuring a current from the pixel circuit comprises an organic light emitting diode (OLED) measurement state for measuring an OLED current from the pixel circuit passing through said light emitting device.
6. The display system of claim 5 , wherein the pixel circuit further comprises a reference line coupleable to a gate terminal of the drive transistor, and wherein the controller, during the OLED measurement state, couples the gate terminal of the drive transistor to the reference line and provides a reference voltage over the reference line sufficient to completely turn off the drive transistor, couples the light emitting device to the data line and provides a data voltage over the data line sufficient to turn on the light emitting device.
7. The display system of claim 6 further comprising a readout circuit coupleable to the data line for measuring the current from the pixel circuit over the data line, the readout circuit comprising an integrator for integrating said OLED current from the pixel during said measuring and generating a corresponding output voltage, and an analog to digital converter for converting said output voltage into a digital code output, wherein the controller couples the gate terminal of the drive transistor to the reference line with use of a second transistor in the pixel circuit, and couples the light emitting device to the data line with use of a third transistor coupled between the first terminal of the drive transistor and the data line.
8. The display system of claim 1 , wherein the second measurement state for measuring a current from the pixel circuit comprises a pixel circuit measurement state for measuring a pixel circuit current from the pixel circuit passing through said drive transistor, said pixel circuit measurement state subsequent to the programming state.
9. The display system of claim 8 , wherein the controller, during the pixel circuit measurement state, decouples the light emitting device from the first terminal of the drive transistor with use of the first transistor and couples the first terminal of the drive transistor to the data line.
10. The display system of claim 9 further comprising a readout circuit coupleable to the data line for measuring the current from the pixel circuit over the data line, the readout circuit comprising an integrator for integrating said pixel circuit current from the pixel circuit during said measuring and generating a corresponding output voltage and an analog to digital converter for converting said output voltage into a digital code output, wherein the pixel circuit further comprises a reference line coupleable to a gate terminal of the drive transistor, wherein the controller decouples the reference line from the gate terminal of the drive transistor to maintain a voltage difference across the storage capacitor, and wherein the controller couples the first terminal of the drive transistor to the data line with use of a third transistor coupled between the first terminal of the drive transistor and the data line.
11. The display system of claim 1 , wherein the pixel circuit comprises transistors which are only n-type thin film transistors (TFTs), and wherein said light emitting device is an OLED.
12. The method of claim 11 , further comprising turning on the first transistor during the first measurement state, wherein measuring the current from the pixel circuit comprises measuring an OLED current from the pixel circuit passing through said light emitting device during an OLED measurement state.
13. The method of claim 12 , wherein the pixel circuit further comprises a reference line coupleable to a gate terminal of the drive transistor, and wherein measuring the OLED current during the OLED measurement state comprises, coupling the gate terminal of the drive transistor to the reference line, providing a reference voltage over the reference line sufficient to completely turn off the drive transistor, coupling the light emitting device to the data line, and providing a data voltage over the data line sufficient to turn on the light emitting device.
14. The method of claim 13 wherein measuring the OLED current during the OLED measurement state comprises:
coupling the gate terminal of the drive transistor to the reference line with use of a second transistor in the pixel circuit;
coupling the light emitting device to the data line with use of a third transistor coupled between the first terminal of the drive transistor and the data line; and
coupling a readout circuit to the data line and measuring said current from the pixel circuit with use of said readout circuit, including, integrating said OLED current from the pixel circuit, generating an output voltage corresponding to the integrated current, and converting said output voltage into a digital code output.
15. A method of driving a display system, the display system including an array of pixel circuits arranged in rows and columns, a pixel circuit of the array of pixel circuits including: a drive transistor including a first terminal coupleable to a data line of the display system; a storage capacitor coupled across a gate terminal and the first terminal of the drive transistor; a light emitting device coupleable to the first terminal of the drive transistor; and a first transistor coupled to the first terminal of the drive transistor and coupled between the drive transistor and the light emitting device, the method comprising:
driving the pixel circuit in a plurality of operation states for the pixel circuit including:
programming the storage capacitor of the pixel circuit with use of a data voltage provided over the data line and turning off the first transistor during a programming state, and
measuring a current from the pixel circuit over the data line during at least two measurement states, including controlling the first transistor for coupling the light emitting device to the first terminal of the drive transistor during a first measurement state of the at least two measurement states and controlling the first transistor for decoupling the first terminal of the drive transistor from the light emitting device during a second measurement state of the at least two measurement states.
16. The method of claim 15 , wherein measuring the current from the pixel circuit comprises coupling a readout circuit to the data line and measuring said current from the pixel circuit with use of said readout circuit.
17. The method of claim 16 wherein measuring said current from the pixel circuit with use of said readout circuit comprises integrating said current from the pixel circuit, generating a corresponding output voltage, and converting said output voltage into a digital code output.
18. The method of claim 16 , wherein the readout circuit is not coupleable to the pixel circuit via a signal line different from the data line for measuring the current from the pixel circuit.
19. The method of claim 15 , further comprising turning off the first transistor during the second measurement state, wherein measuring said current from the pixel circuit comprises measuring a pixel circuit current from the pixel circuit passing through said drive transistor, during a pixel circuit measurement state subsequent to the programming state.
20. The method of claim 19 , wherein measuring the pixel current during the pixel circuit measurement state comprises decoupling the light emitting device from the first terminal of the drive transistor with use of the first and coupling the first terminal of the drive transistor to the data line.
21. The method of claim 20 wherein measuring the pixel circuit current during the pixel circuit measurement state comprises:
decoupling a reference line from the gate terminal of the drive transistor to maintain a voltage difference across the storage capacitor;
coupling the first terminal of the drive transistor to the data line with use of a third transistor coupled between the first terminal of the drive transistor and the data line; and
coupling a readout circuit to the data line and measuring said current from the pixel circuit with use of said readout circuit, including, integrating said pixel circuit current from the pixel circuit, generating an output voltage corresponding to the integrated current, and converting said output voltage into a digital code output.
22. The method of claim 15 , wherein the pixel circuit comprises transistors which are only n-type TFTs, and wherein said light emitting device is an OLED.