System and methods for extraction of threshold and mobility parameters in AMOLED displays
A system to improve the extraction of transistor and OLED parameters in an AMOLED display includes a pixel circuit having an organic light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input to provide the programming signal, and a storage device to store the programming signal. A charge-pump amplifier has a current input and a voltage output. The charge-pump amplifier includes an operational amplifier in negative feedback configuration. The feedback is provided by a capacitor connected between the output and the inverting input of the operational amplifier. A common-mode voltage source drives the non-inverting input of the operational amplifier. An electronic switch is coupled across the capacitor to reset the capacitor. A switch module including the input is coupled to the output of the pixel circuit and an output is coupled to the input of the charge-pump amplifier.
1. A data extraction system for an organic light emitting device (OLED) based display, the system comprising:
a pixel circuit including a drive transistor, an organic light emitting device, and a programming input coupled to the gate of the drive transistor, the drive transistor having a source or a drain coupled to the OLED;
a charge-pump amplifier having an input and an integrated voltage output;
a switch module including an 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 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; and
a controller coupled to the pixel circuit, charge-pump amplifier and the switch module, the controller controlling voltage inputs to the pixel circuit, charge-pump amplifier and switch module in a predetermined sequence to produce an output voltage value which is a function of a parameter of the pixel circuit, the sequence including providing a program voltage to the programming input to either pre-charge a capacitance of the pixel circuit to a charge level and transfer the charge to the charge-pump amplifier via the switch module to generate the output voltage value, or provide a current from the pixel circuit to the charge-pump amplifier via the switch module to produce the output voltage value by integration;
a select transistor coupled between the programming input and the gate of the drive transistor; and
an output transistor having a source or drain coupled to the source or drain of the drive transistor, a source or drain coupled to said charge-pump amplifier, and a gate coupled to a read signal line.
2. The system of claim 1 , wherein the charge-pump amplifier includes:
an operational amplifier having a negative input and a positive input coupled to a common mode voltage source;
a feedback capacitor coupled in a feed-back loop from the negative input to the output of the amplifier; and
wherein the output of the switch module is coupled to the negative input of the operational amplifier.
3. The system of claim 1 further comprising:
a select transistor coupled between the programming input and the gate of the drive transistor; and
an output transistor having a gate coupled between the source or drain of the drive transistor, and a source or drain coupled to the select transistor and a monitor output, the output transistor enabled via a select signal to the drive transistor and the select transistor.
4. The system of claim 1 , wherein the drive transistor is an n-type transistor or a p-type transistor.
5. The system of claim 1 , wherein the parameter is at least one of the threshold voltage and mobility of the drive transistor, the program voltage is set to a proper level to turn the drive transistor on, the switch module steers the current of the drive transistor into the charge-pump amplifier to be directly integrated for a certain amount of time, and the amplifier output is a value of at least one of the threshold voltage and mobility as a function of the amplifier's feedback capacitor, length of integration time, and the program voltage.
6. The system of claim 1 , wherein the parameter is the turn-on voltage of the OLED, the program voltage set to a proper level to operate the drive transistor as a switch, the switch module steers the current of the OLED into the charge-pump amplifier to be directly integrated for a certain amount of time, and the amplifier output is a value of the OLED's turn-on voltage as function of the amplifier's feedback capacitor, length of integration time, and the common-mode voltage set at the non-inverting input of the amplifier.