Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-06-28
2004-05-04
Saras, Steven (Department: 2675)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S262000, C345S096000
Reexamination Certificate
active
06731170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a source drive amplifier of a liquid crystal display and, more particularly, to a source drive amplifier used in, for example, the driving circuit of a thin film transistor liquid crystal display.
2. Description of Related Art
The thin film transistor liquid crystal display (TFT LCD) is known as an active array type display. The array is composed of a plurality of pixels (or dots), each having a driving electrode and a common electrode commonly used with the other pixels. The LCD is driven by an AC (alternative current) signal. That is, if the voltage applied to the driving electrode is positive with respect to that of the common electrode when the first frame is displayed, the voltage applied to the driving electrode is negative with respect to that of the common electrode in the next frame.
Under the consideration of the difference of the common electrodes and the image quality, there are two well-known driving methods provided: dot inversion driving and row inversion driving. In the dot inversion driving system, if the odd dots of the odd lines of the first frame are driven by a positive voltage with respect to the common electrode, the even dots of the odd lines of the first frame are driven by a negative voltage with respect to the common electrode. The odd dots of the even lines of the first frame are driven by a negative voltage with respect to the common electrode, and the even dots are driven by a positive voltage with respect to the common electrode.
Then, the odd dots of the odd lines of the second frame are driven by a negative voltage with respect to the common electrode, and the even dots are driven by a positive voltage with respect to the common electrode. Meanwhile, the odd points of the even lines of the second frame are driven by a positive voltage with respect to the common electrode, and the even points are driven by a negative voltage with respect to the common electrode.
In the row inversion system, if all dots of the odd lines of the first frame are driven by a positive voltage with respect to the common electrode, all the dots of the even lines of the first frame will be driven by a negative voltage with respect to the common electrode. Then, all dots of the odd lines of the second frame are driven by a negative voltage with respect to the common electrode, and all dots of the even lines of the second frame are driven by a positive voltage with respect to the common electrode.
FIG. 5
is a schematic view showing the driving structure of an active thin film liquid crystal display with K columns by L rows. As shown in the figure, If there are K pixels
901
in the horizontal direction, K channels of source drive units (SDUs) are required for driving. In the vertical direction, a gate driver
903
is employed to drive the voltages of the pixels
901
on each scanning line
904
sequentially for being sampled and hold on the driving electrode of each pixel
901
.
FIG. 6
is a circuit diagram of the source drive unit
902
of an active thin film liquid crystal display, which has a multiplex (MUX)
911
controlled by a polarity switching signal PN for switching the output of a positive digital to analog converter
912
(P-DAC) or negative digital to analog converter
913
(N-DAC) to a voltage follower formed by an operational amplifier
914
, thereby amplifying the driving ability to generate a driving output DRVO. The driving output DRVO is then entered to a CMOS transmission gate
915
controlled by an output enable signal (OE) to output a driving voltage VLCD to the column of the panel of a thin film transistor liquid crystal display. The operating waveforms are illustrated in
FIG. 7
, wherein the P-DAC
912
and N-DAC
913
are controlled by an input digital data so as to generate a driving voltage required by a respective illumination. The outputs of the P-DAC
912
and N-DAC
913
are similar, but symmetric with respect to the common electrode, so as to satisfy the AC driving requirement.
To save power, the output voltages of the P-DAC
912
and N− DAC
913
are generally in the range from VSS+0.1V to VDD−0.1V. Therefore, the operational amplifier used in the source drive unit
902
must have the capability of fill rail-to-rail. Moreover, when the output is higher than the voltage of the common electrode, a large current source out is required so that the load capacitor (primarily the layout strayed capacitor on the panel of the thin film transistor liquid crystal display) is charged rapidly to a high voltage. Moreover, when the output is lower than the voltage of the common electrode, a large current sink capability is required for discharging the high voltage of the load capacitor of the thin film transistor liquid crystal display to a driving low voltage.
To match this requirement, the circuit of an operational amplifier used in a conventional source drive unit is disclosed as shown in
FIG. 8
, which is a full rail-to-rail AB class operational amplifier (a detailed description of such can be found in U.S. Pat. No. 6,100,762). The operational amplifier includes a first differential amplifier formed by an NMOS pair (N1, N2) and a second differential amplifier formed by a PMOS pair (P1, P2). The two differential amplifiers are connected in parallel for being used as an input. The output currents of the two differential amplifiers are summed via a current mirror circuit (N
5
_N
6
, N
7
_N
8
, P
5
_P
6
), and outputted at node A to drive the AB class amplifier formed by transistors N
9
, N
10
, N
12
, N
13
, N
14
, P
10
, P
11
, and P
12
) for being used as the output of the operational amplifier, so as to acquire a large current source out and sink in capabilities.
The aforesaid conventional operational amplifier suffers a disadvantage in having a very large DC offset. Such a disadvantage is encountered because the threshold voltages (V
TH
) of different MOS devices may be varied from ± several mV to ± several tens of mV, in the CMOS manufacturing process. Moreover, in the full rail-to-rail AB class amplifier, the DC offset caused by V
TH
is particularly serious, which is analyzed as follows:
when
V
i
n
<
V
TH_N1
,
V
OS_L
=
gm
P1
Δ
V
TH_P1P2
+
gm
N5
Δ
V
TH_N5N6
+
gm
N7
Δ
V
TH_N7N8
+
gm
P5_L
Δ
V
TH_P5P6
gm
P1
when
V
TH_N1
<
V
i
n
<
(
V
DD
-
V
TH_P1
)
,
V
OS_M
=
gm
P1
Δ
V
TH_P1P2
+
gm
N1
Δ
V
TH_N1N2
+
gm
N5
Δ
V
TH_N5N6
+
gm
N7
Δ
V
TH_N7N8
+
gm
P5_M
Δ
V
TH_P5P6
gm
P1
+
gm
N1
when
(
V
DD
-
V
TH_P1
)
<
V
i
n
,
V
OS_H
=
gm
N1
Δ
V
TH_N1N2
+
gm
P5_H
Δ
V
TH_P5P6
gm
N1
;
wherein gm
Pt
, gm
Nj
represent the transfer-conductance of PMOS transistor (Pi, i=1, 2, 3 . . . ), and the transfer-conductance of NMOS transistor (Nj, j=1, 2, 3 . . . ); the gm
P5
—
H
gm
P5
—
M
, gm
P5
—
L
are different from each other due to conducting current; &Dgr;V
TH—
N1N2
represents the difference of the voltage threshold between the NMOS differential pair N
1
and N
2
. Other differential pairs or current mirror pairs are represented by same symbols.
In practical, in the middle voltage section V
TH
—
N1
<V
in
<(V
DD
−V
TH
—
PI)
; this AB class operational amplifier generally has a DC offset as high as ±15 mV, or even ±20 mV, and when in a low voltage, V
in
<V
TH
—
N1
, the DC offset is as high as ±40 mV.
An active thin film transistor liquid crystal display may use several thousand channels of source drive units. If such a large DC offset is existed in each channel, it implies that the voltage driven to each pixel has different constant error, which will c
Anyaso Uchendu O.
Bacon & Thomas PLLC
Saras Steven
Sunplus Technology Co. Ltd.
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