Electroluminescent display with efficiency compensation

1. A method of providing a drive signal to a gate electrode of a drive transistor in an electroluminescent (EL) subpixel, the method comprising:

(a) providing the EL subpixel comprising the drive transistor, an EL emitter, and a readout transistor, the drive transistor comprising: a first electrode, a second electrode, and the gate electrode;

(b) providing a first voltage source and a first switch for selectively connecting the first voltage source to the first electrode of the drive transistor;

(c) connecting the EL emitter to the second electrode of the drive transistor;

(d) providing a second voltage source connected to the EL emitter;

(e) connecting the first electrode of the readout transistor to the second electrode of the drive transistor;

(f) providing a current source and a third switch for selectively connecting the current source to the second electrode of the readout transistor;

(g) providing a voltage measurement circuit connected to the second electrode of the readout transistor;

(h) opening the first switch, closing the third switch, and using the voltage measurement circuit to measure the voltage at the second electrode of the readout transistor to provide a first emitter-voltage signal;

(i) using the first emitter-voltage signal to provide an aging signal representative of the efficiency of the EL emitter;

(j) receiving an input signal;

(k) using the aging signal and the input signal to produce a compensated drive signal;

(l) providing the compensated drive signal to the gate electrode of the drive transistor to compensate for changes in efficiency of the EL emitter; and

providing a second switch for selectively connecting the EL emitter to the second voltage source,

wherein step (h) includes closing the second switch.

2. The method of claim 1 , wherein step (h) further includes:

(1) measuring the voltage at the second electrode of the readout transistor at a first time to provide the first emitter-voltage signal;

(2) storing the first emitter-voltage signal;

(3) measuring a second emitter-voltage signal at a second time, the second time being different from the first time; and

(4) storing the second emitter-voltage signal.

3. The method of claim 2 , wherein step (1) further includes comparing the stored first and second emitter-voltage signals to provide the aging signal.

4. The method of claim 1 , wherein the voltage measurement circuit includes an analog-to-digital converter.

5. The method of claim 4 , wherein the voltage measurement circuit further includes a low-pass filter.

6. The method of claim 1 , further comprising:

providing a plurality of EL subpixels,

wherein steps (h) and (i) are performed for each EL subpixel to produce a plurality of corresponding aging signals, and

wherein steps (j) through (l) are performed for each of the plurality of subpixels using the corresponding aging signals.

7. The method of claim 6 , wherein step (h) is performed for a predetermined number of such EL subpixels during which the predetermined number of subpixels are driven simultaneously.

8. The method of claim 6 , wherein the EL subpixels are arranged in rows and columns, the method further comprising providing a plurality of row select lines connected to the gate electrodes of corresponding select transistors and a plurality of readout lines connected to the second electrodes of corresponding readout transistors.

9. The method of claim 8 , further comprising using a multiplexer connected to the plurality of readout lines for sequentially measuring each of the predetermined number of EL subpixels to provide corresponding first emitter-voltage signals.

10. The method of claim 1 , further comprising:

providing a select transistor connected to the gate electrode of the drive transistor,

wherein the gate electrode of the select transistor is connected to the gate electrode of the readout transistor.

11. The method of claim 1 , wherein:

each EL emitter comprises an OLED emitter; and

each EL subpixel comprises an OLED subpixel.

12. The method of claim 1 , wherein step (1) further includes:

providing a source driver; and

using the source driver to provide the compensated drive signal to the gate electrode of the drive transistor.

13. The method of claim 12 , wherein the source driver comprises a digital-to-analog converter.