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Analog active matrix emissive display
| Details |
Inventors: Stewart, Roger G.; Ipri, Alfred C.;
Assignee: Sarnoff Corporation (Princeton, NJ)
Primary Examiner: Hjerpe; Richard
Assistant Examiner: Nguyen; Kevin M.
Attorney, Agent or Firm: Burke; William J.
An emissive display device such as an active matrix electroluminescent display (AMEL display) has an improved method of operation. The AMEL display produces gray scale operation comprising an array of pixels, each pixel including a first transistor having its gate connected to a select line, its source connected to a data line, and its drain connected to the gate of a second transistor. The second transistor has its source adapted to receive a ramped voltage level, and its drain connected to a first electrode of an electroluminescent cell. The electroluminescent cell has a second electrode connected to an alternating current high voltage power source, wherein the electroluminescent cell is illuminated, when the ramp voltage level is less than a voltage level on the gate of the second transistor. The ramp voltage level is increased linearly during a frame duration, and the alternating current high voltage power source is on continuously during the same frame duration. The alternating current high voltage power source may also be varied in amplitude from a minimum peak-to-peak value to a maximum peak-to-peak value during the frame duration. |
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DETAILED DESCRIPTION OF THE INVENTION In order to overcome the high power dissipation and the minimum number of gray levels, a new pixel structure and method for operating the same has been achieved, and is disclosed herein.
The new pixel operates with analog input data, eliminates the need for external frame store circuitry, and operates at a much lower frequency.
The new pixel may also be loaded with data at any time during the frame time (period), eliminating the need for a separate non-illuminating load cycle.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
FIG.
2 shows an analog AMEL display 200 which includes a ramp signal for greyscale generation.
For simplicity, FIG.
2 depicts only one of these AMEL display pixels 202.
In accordance with a preferred embodiment of the present invention, pixel 202 contains elements previously described with reference to FIG.
1.
Also shown in FIG.
2 is a ramp generator 204 connected to transistor 112.
A pair of capacitors 206 and 208 have been added, as shown, connected in series with EL 108.
The EL cell is shown in series with two capacitors, which are blocking capacitors formed as part of the structure of the EL cell.
Analog data levels are loaded by way of data line 116 into each individual pixel during the 60 Hz frame time using, for example, sample-and-hold data scanner circuitry (not shown).
This circuitry permits the data to be transmitted to the display at lower voltages than needed by the display.
The data is loaded into the array, one row at a time, on a continual basis during illumination.
The illumination is continuous in order to be able to achieve high brightness.
Furthermore, this loading technique eliminates the need for a frame store memory in the external system electronics.
Ramp generator 204 generates a ramp signal that increases from a minimum voltage (e.
g.
ground) at the beginning of the 60 Hz frame time and reaches a maximum voltage at the end of the frame
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LCD 
