System and methods for extraction of threshold and mobility parameters in AMOLED displays
A system reads a desired circuit parameter from a pixel circuit that includes a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal. One embodiment of the extraction system turns off the drive device and supplies a predetermined voltage from an external source to the light emitting device, discharges the light emitting device until the light emitting device turns off, and then reads the voltage on the light emitting device while that device is turned off. The voltages on the light emitting devices in a plurality of pixel circuits may be read via the same external line, at different times.
1. A method of extracting a circuit parameter from a pixel circuit including a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal, the method comprising:
turning on the drive device so that the voltage of the light emitting device rises to a level higher than its turn-on voltage,
turning off the drive device so that the voltage on the light emitting device is discharged through the light emitting device until the light emitting device turns off, and
reading the voltage on the light emitting device while that device is turned off.
2. A method of extracting a circuit parameter from a pixel circuit including a light emitting device, a drive transistor having gate, source and drain terminals to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal, the method comprising:
turning on the drive device and measuring the current and voltage of the drive transistor while changing the voltage between the gate and the source or drain of the drive transistor to operate the drive transistor in the linear regime during one time interval and in the saturated regime during a second time interval, and
extracting a parameter of the light emitting device from the relationship of the currents and voltages measured with the drive transistor operating in the two regimes.
3. A system for extracting a circuit parameter from a pixel circuit including a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal, the system comprising
a controller coupled to the pixel circuit and supplying controlling input signals to the pixel circuit in a predetermined sequence to produce an output voltage value which is a function of a parameter of the pixel circuit, the sequence including
turning off the drive device and supplying a predetermined voltage from an external source to said light emitting device,
discharging said light emitting device until the light emitting device turns off, and
reading the voltage on the light emitting device while that device is turned off.
4. The system of claim 3 which includes a plurality of pixel circuits ccoupled to said external source via a common external line, and said reading of voltage on the light emitting devices in said plurality of pixel circuits is effected via said common external line at different times.
5. The system of claim 4 which includes
a charge-pump amplifier having a current input and a voltage output, the charge-pump amplifier including an operational amplifier in negative feedback configuration, wherein the feedback is provided by a capacitor connected between the output and the inverting input of the operational amplifier, a common-mode voltage source to drive the non-inverting input of the operational amplifier, and an electronic switch coupled across the capacitor to reset the capacitor; and
a switch module including the input coupled to the output of the pixel circuit and an output coupled to the input of the charge-pump amplifier, the switch module including a plurality of electronic switches to steer current in and out of the pixel circuit, provide a discharge path between the pixel circuit and the charge-pump amplifier, and isolate the charge-pump amplifier from the pixel circuit.
6. A system for extracting a circuit parameter from a pixel circuit including a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal, the system comprising:
a controller coupled to the pixel circuit and supplying controlling input signals to the pixel circuit in a predetermined sequence to produce an output voltage value which is a function of a parameter of the pixel circuit, the sequence including
turning on the drive device so that the voltage of the light emitting device rises to a level higher than its turn-on voltage,
turning off the drive device so that the voltage on the light emitting device is discharged through the light emitting device until the light emitting device turns off, and
reading the voltage on the light emitting device while that device is turned off.
7. A system for extracting a circuit parameter from a pixel circuit including a light emitting device, a drive transistor having gate, source and drain terminals to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal, the system comprising:
a controller coupled to the pixel circuit and supplying controlling input signals to the pixel circuit in a predetermined sequence to produce an output voltage value which is a function of a parameter of the pixel circuit, the sequence including
turning on the drive device and measuring the current and voltage of the drive transistor while changing the voltage between the gate and the source or drain of the drive transistor to operate the drive transistor in the linear regime during one time interval and in the saturated regime during a second time interval, and
extracting a parameter of the light emitting device from the relationship of the currents and voltages measured with the drive transistor operating in the two regimes.