Active matrix organic electroluminescent display device

1. An active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:

an electroluminescent display element;

a drive transistor for driving a current through the display element, a drive voltage being provided to the gate of the drive transistor;

and a storage capacitor for storing a pixel voltage, said storage capacitor being connected between an input to the pixel and the gate of the drive transistor,

wherein a driver circuitry provides a stepped voltage waveform to the input of the pixel, the stepped voltage waveform being voltage-shifted by the storage capacitor before application to the gate of the drive transistor, and wherein a height of steps in the stepped voltage waveform is greater than a voltage width of a linear operating region of the drive transistor so that the linear operating region of the drive transistor is avoided, the pixel voltage being selected so that a voltage range of the linear operating region corresponds to voltage levels between a transition of two steps of the stepped voltage waveform.

2. The device as claimed in claim 1 , wherein the height of the steps in the stepped voltage waveform is sufficient to include the linear operating region voltages of the drive transistors of all pixels of the display.

3. The device as claimed in claim 1 , wherein each pixel further comprises an address transistor, connected between a power supply line and the gate of the drive transistor.

4. The device as claimed in claim 3 , wherein each pixel further comprises means for disabling the driving of current by the drive transistor through the display element.

5. The device as claimed in claim 4 , wherein the means for disabling comprises an isolating transistor in series with the drive transistor and the display element.

6. The device as claimed in claim 3 , wherein the device further comprises disabling means comprising a switch for switching the voltage on one terminal of the display elements of the array of pixels.

7. The device as claimed in claim 1 , wherein the device is operable in at least two sequential phases, one phase providing coarse resolution pulse width modulation and the other, shorter phase, providing fine resolution pulse width modulation.

8. The device of claim 1 , further comprising an address transistor having a gate terminal for receiving an address signal to turn the address transistor on and off, and source and drain terminals that are directly connected between the power supply line and the gate of the drive transistor.

9. An active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:

an electroluminescent display element;

a drive transistor for driving a current through the display element, a drive voltage waveform being provided to the gate of the drive transistor;

an address transistor, connected between a power supply line and the gate of the drive transistor;

means for disabling the driving of the current by the drive transistor through the display element; and

a storage capacitor for storing a pixel voltage, said storage capacitor being connected between an input to the pixel and the gate of the drive transistor,

wherein a driver circuitry provides a stepped voltage waveform to the input of the pixel, the stepped voltage waveform being voltage-shifted by the storage capacitor before application to the gate of the drive transistor to form the drive voltage waveform, and wherein a height of steps in the stepped voltage waveform is greater than a voltage width of a linear operating region of the drive transistor, the pixel voltage being selected so that a voltage range of the linear operating region corresponds to voltage levels between a transition of two steps of the stepped voltage waveform, and

wherein the device is operable in two modes:

a first mode in which the pixel voltage is applied to the input of the pixel, the address transistor is turned on, the disabling means is turned on to turn off the display element and the storage capacitor is charged to the pixel voltage; and

a second mode in which the address transistor is turned off, the disabling means is turned off and the stepped voltage waveform is applied to the input of the pixel.

10. The device of claim 9 , wherein the address transistor has a gate terminal for receiving an address signal to turn the address transistor on and off, and source and drain terminals that are directly connected between the power supply line and the gate of the drive transistor.

11. A method of addressing an active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising an electroluminescent display element, a drive transistor for driving a current through the display element, and a storage capacitor connected between an input to the pixel and a gate of the drive transistor, the method comprising the acts of:

storing a pixel voltage on the storage capacitor;

providing a stepped voltage waveform to the input of the pixel, the stepped voltage waveform being voltage-shifted by the pixel voltage stored on the storage capacitor before application to the gate of the drive transistor, such that for a first set of voltage steps of the stepped voltage waveform applied to the gate of the drive transistor, the drive transistor is turned on, and for a second set of the voltage steps applied to the gate of the drive transistor, the drive transistor is turned off, the first and second sets being determined by the stored pixel voltage; and

avoiding a linear operating region of the drive transistor by having a height of the voltage steps of the first set and the second set being greater than a voltage width of a linear operating region of the drive transistor, the pixel voltage being selected so that a voltage range of the linear operating region corresponds to voltage levels between a transition of two steps of the stepped voltage waveform.

12. The method as claimed in claim 11 , wherein the height of the steps in the stepped voltage waveform is greater than the voltage width of the overlaid linear operating region voltages of the drive transistors of all pixels of the display.

13. The method as claimed in claim 11 , wherein the act of storing a pixel voltage on the storage capacitor comprises turning on an address transistor connected between a power supply line and the gate of the drive transistor and charging the storage capacitor using the address transistor.

14. The method as claimed in claim 11 , wherein the device is operable in at least two sequential phases, one phase providing coarse resolution pulse width modulation and the other, shorter phase, providing fine resolution pulse width modulation.

15. The method as claimed in claim 14 , wherein the stepped voltage waveform to the input of the pixel has the same voltage levels in the two phases, and the shorter phase has shorter step durations.

16. The method of claim 11 , wherein the pixel voltage stored on the storage capacitor determines a duty cycle of a current profile for operating the electroluminescent display element.

17. A method of addressing an active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising an electroluminescent display element, a drive transistor for driving a current through the display element, and a storage capacitor connected between an input to the pixel and the gate of the drive transistor, the method comprising the acts of:

storing a pixel voltage on the storage capacitor;

providing a stepped voltage waveform to the input of the pixel, the stepped voltage waveform being voltage-shifted by the pixel voltage stored on the storage capacitor before application to a gate of the drive transistor, such that for a first set of the voltage steps applied to the gate of the drive transistor, the drive transistor is turned on, and for a second set of the voltage steps applied to the gate of the drive transistor, the drive transistor is turned off, the first and second sets being determined by the stored pixel voltage;

disabling the driving of the current by the drive transistor through the display element during the storing of the pixel voltage on the storage capacitor;

in a first mode in which the pixel voltage is applied to the input to the pixel, turning on the address transistor, turning off the display element, and charging the storage capacitor to a level derived from the drive voltage; and

in a second mode in which the address transistor is turned off, applying the stepped voltage waveform to the input of the pixel;

wherein a height of steps in the stepped voltage waveform is greater than a voltage width of a linear operating region of the drive transistor so that the linear operating region of the drive transistor is avoided, and

wherein the pixel voltage is selected so that a voltage range of the linear operating region corresponds to voltage levels between a transition of two steps of the stepped voltage waveform.

18. The method of claim 17 , wherein the pixel voltage stored on the storage capacitor determines a duty cycle of a current profile for operating the electroluminescent display element.