Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-10-12
2004-05-11
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S045000, C345S046000, C345S080000
Reexamination Certificate
active
06734836
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a current driving circuit which drives a current running through a load such as an organic electroluminescence device.
2. Description of the Related Art
Among loads which are required to drive a current running therethrough, a typical one is an organic electroluminescence device.
Though an organic electroluminescence device has to be developed in respect of enhancement in a brightness, a longer lifetime, a sealing structure of a module including an organic electroluminescence device and other parts, and so on, an organic electroluminescence device has many advantages: (a) it can be driven with a dc current at a low voltage; (b) it can accomplish a high brightness with a high efficiency; (c) it has a quicker response than a liquid crystal display device; (d) it has superior temperature characteristic at low temperatures; (e) it presents superior visibility; and (f) it emits a light from itself, and hence, it does not need to have a backlight device unlike a liquid crystal display device, and makes it possible to fabricate an image display device thinner than conventional ones when an image display device is designed to include an organic electroluminescence device. Because of the above-mentioned advantages, there is an eager need to be able to accomplish mass-production of an organic electroluminescence device.
As a circuit for driving an organic electroluminescence device applied to an image display device, an active matrix type driving circuit has been studied long, because it can present a high light-emitting efficiency and high image quality. The active matrix type driving circuit includes an amorphous or polysilicon thin film transistor (hereinafter, referred to simply as “TFT”) as an active device.
Japanese Unexamined Patent Publication No. 11-282419, for instance, has suggested an active matrix type current driving circuit including TFT for driving an organic electroluminescence device.
FIG. 1
is a circuit diagram of the suggested active matrix type current driving circuit.
The active matrix type current driving circuit is comprised of a first terminal
1
electrically connected to a voltage source, a second terminal
2
grounded, a signal line
3
through which a signal current runs, a selection line
4
, an organic electroluminescence device
31
as a load through which a current required to be driven runs, a driving transistor
32
, a transistor
33
, a storage capacity
34
, a first switch
35
, and a second switch
36
.
The organic electroluminescence device
31
is electrically connected at one end to the first terminal
1
, and at the other end to the second terminal
2
through the driving transistor
32
.
The driving transistor
32
controls a drive current in accordance with a voltage applied to a gate electrode thereof, and supplies the thus controlled drive current to the organic electroluminescence device
31
.
The storage capacity
34
for keeping a constant voltage is electrically connected between a gate of the driving transistor
32
and the second terminal
2
.
The second switch
36
is electrically connected at one of terminals to both the storage capacity
34
and the driving transistor
32
, and at the other terminal to the transistor
33
which converts a current into a voltage.
The transistor
33
is designed to have the same polarity as that of the driving transistor
32
. The transistor
33
has a drain and a gate both of which are electrically connected to each other, and hence, has a diode structure.
In addition, the transistor
33
and the driving transistor
32
cooperate with each other to define a current-mirror circuit through the second switch
36
.
The transistor
33
is electrically connected to the signal line
3
through the first switch
35
.
Each of the first and second switches
35
and
36
has a control terminal electrically connected to the selection line
4
.
When a control signal is input into the selection line
4
to thereby turn both the first and second switches
35
and
36
on, a signal current running through the signal line
3
is input into the transistor
33
through the first switch
35
, and is converted into a voltage in the transistor
33
. The signal current further charges the storage capacity
34
through the second switch
36
. Thus, a voltage associated with the signal current is stored in the storage capacity
34
.
Since the transistor
33
and the driving transistor
32
define a current-mirror circuit through the second switch
36
, a signal current running through the signal line
3
is supplied to the organic electroluminescence device
31
through the current-mirror circuit.
Even if a control signal is stopped to be input into the selection line
4
, and accordingly, the first and second switches
35
and
36
are turned off, the voltage associated with a signal current running through the signal line
3
is stored in the storage capacity
34
. Accordingly, the voltage stored in the storage capacity
34
is kept applied to a gate of the driving transistor
32
. This ensures that a current which is the same as the signal current running through the signal line
3
is kept supplied to the organic electroluminescence device
31
.
The conventional current driving circuit illustrated in
FIG. 1
is accompanied with the following problems.
The first problem is that a signal current running is supplied to the organic electroluminescence device
31
from the signal line
3
with low accuracy.
The reason is as follows. A thin film transistor composed of an amorphous or polysilicon sometimes has variance in an order of hundred-millivolts with respect to a threshold voltage because of existence of grain boundary, unlike a semiconductor device composed of mono-crystal. As a result, in the current driving circuit illustrated in
FIG. 1
including the transistor
33
and the driving transistor
32
both comprised of TFT, even if the transistors
33
and
32
are arranged adjacent to each other, it would be quite difficult or almost impossible to eliminate the above-mentioned variance in threshold voltages of the transistors
33
and
32
, and match the transistors
33
and
32
with each other.
As a result, if the conventional current driving circuit illustrated in
FIG. 1
were comprised of TFT, a resultant circuit could be fabricated only in a low fabrication yield and further in remarkably high fabrication costs.
In order to eliminate the above-mentioned variance in a threshold voltage of TFT, there has been suggested a method of processing signals not in an analog form, but in a digital form, for instance, in “a patent has been issued to a circuit for enhancing accuracy in an organic electroluminescence panel”, Nikkei Electronics, Apr. 24, 2000, No. 768.
However, the suggested circuit is unavoidably complex in a structure and large in a scale, resulting in an increase in fabrication costs.
The second problem is that the conventional current driving circuit illustrated in
FIG. 1
consumes much electric power.
The reason is that though the signal current having run through the signal line
3
is supplied to the transistor
33
defining a current-mirror circuit together with the driving transistor
32
, the current having run through the transistor
33
does not run directly through the organic electroluminescence device
31
.
Japanese Patent No. 2953465 has suggested a current driving circuit including an input terminal, a first transistor having a drain electrically connected to the input terminal and a source grounded, a switching transistor electrically connected between a gate and a drain of the first transistor, a control terminal electrically connected to a gate of the switching transistor and receiving a signal in accordance with which the switching transistor is turned on or off, a second transistor having a gate electrically connected to the switching transistor and a source grounded, and defining a current-mirror circuit together with the first transistor, and a capacity device electrically connected at one of electrodes th
Alphonse Fritz
McGinn & Gibb PLLC
NEC Corporation
Saras Steven
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