Active matrix display devices
View Patent ↗An active matrix EL display device has pixels comprising an EL display element, a drive transistor and a storage capacitor for storing a voltage to be used for controlling the addressing of the drive transistor. Each pixel also has a light-dependent device for controlling discharge of the storage capacitor, thereby to alter the control of the drive transistor in dependence on the light output of the display element. A circuit is associated with the drive transistor for increasing the rate of discharge of the storage capacitor when the storage capacitor is discharged in response to the light dependent device output. This improvement in the rate of capacitor discharge ensures that good correction for differential aging is obtained at all grey levels.
1. An active matrix display device comprising an array of display pixels, each pixel comprising:
a current-driven light emitting display element ( 2 );
a drive transistor ( 22 ) for driving a current through the display element;
a storage capacitor ( 24 ) for storing a voltage to be used for controlling the addressing of the drive transistor ( 22 ; 40 );
a light-dependent device ( 27 ) for controlling discharge of the storage capacitor ( 24 ), thereby to alter the control of the drive transistor in dependence on the light output of the display element; and
a circuit ( 60 ) associated with the drive transistor for increasing the rate of discharge of the storage capacitor ( 24 ) when the storage capacitor is discharged in response to the light dependent device ( 27 ) output, wherein the circuit associated with the drive transistor comprises a clocked inverter circuit ( 36 , 140 ), the output of the inverter circuit controlling the drive transistor.
2. A device as claimed in claim 1 , further comprising a discharge transistor ( 36 ) for discharging the storage capacitor ( 24 ) thereby to switch off the drive transistor ( 22 ).
3. A device as claimed in claim 2 , wherein the light-dependent device controls the timing of the switching of the discharge transistor ( 36 ) from an off to an on state.
4. A device as claimed in claim 2 , wherein a discharge capacitor ( 40 ) is provided between the gate of the discharge transistor ( 36 ) and a constant voltage line, and the light dependent device is for charging or discharging the discharge capacitor ( 40 ).
5. A device as claimed in claim 1 , wherein the circuit associated with the drive transistor comprises a second transistor ( 130 ) in series with the drive transistor ( 22 ), such that the second transistor ( 130 ) and the drive transistor ( 22 ) form an inverter circuit, the output of the inverter circuit driving the display element.
6. A device as claimed in claim 1 , wherein the circuit associated with the drive transistor comprises a feedback transistor (T feedback ), wherein the discharge transistor (T discharge ) connects to the gate of the feedback transistor and the feedback transistor connects to the gate of the discharge transistor.
7. A device as claimed in claim 1 , wherein the light dependent device ( 27 ) comprises a discharge photodiode.
8. A device as claimed in claim 1 , wherein each pixel further comprises an address transistor ( 16 ) connected between a data signal line and an input to the pixel.
9. A device as claimed in claim 1 , wherein the drive transistor is connected between a power supply line and the display element.
10. A device as claimed in claim 9 , wherein the storage capacitor ( 24 ) is connected between the gate and source of the drive transistor ( 22 ).