Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-06-21
2003-11-25
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S257000, C330S261000
Reexamination Certificate
active
06653900
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a driving method of an operational transconductance amplifier, and more particularly, to a driving method for improving power efficiency of an operational transconductance amplifier.
2. Description of the Prior Art
Advantages of the liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination. Thus, the LCD has been widely applied to several portable information products such as notebooks, PDAs, etc. The LCD is gradually replacing the CRT monitors of conventional desktop computers. Incident light will produce different polarization or refraction when alignments of these liquid crystal molecules are different. The LCD utilizes the characteristics of the liquid crystal molecules to control the light transmittance and produce gorgeous images.
Please refer to FIG.
1
.
FIG. 1
is a schematic diagram of a conventional thin film transistor (TFT) liquid crystal display (LCD) monitor
10
. The LCD monitor
10
comprises an LCD panel
12
, a controller
14
, a first driving circuit I
6
, a second driving circuit I
8
, a first voltage generator
20
, and a second voltage generator
22
. The LCD panel
12
comprises two substrates. An LCD layer is filled in the space between these two substrates. One substrate is disposed with a plurality of first data lines
24
, a plurality of second data lines
26
which are perpendicular to the first data lines
24
, and a plurality of thin film transistors
28
. The other substrate is disposed with a common electrode (not shown) for providing a stable voltage (Vcom) by the first voltage generator
20
. For convenience, only four thin film transistors
28
are illustrated in FIG.
1
. In fact, the thin film transistors
28
are disposed on the LCD panel
12
in a matrix format. That is, each of the thin film transistors
28
is disposed on the intersection of each of the first data lines
24
and each of the second data lines
26
. Each first data line
24
corresponds to a column of the LCD panel
12
, each second data line
24
corresponds to a row of the LCD panel
12
, and each thin film transistor
28
corresponds to a pixel. Additionally, the circuit characteristic formed by the substrates can be deemed an equivalent capacitor
30
.
A driving principle for the conventional LCD monitor
10
is described as follows. When the controller
14
receives horizontal synchronization signals or vertical synchronization signals, the controller
14
provides corresponding control signals respectively to the first driving circuit I
6
and to the second driving circuit I
8
. Then the first driving circuit I
6
and the second driving circuit I
8
provide input signals to the first data lines
24
and the second data lines
26
by determining the control signals. Next, the input signals received by the first data lines
24
and the second data lines
26
change the states of the thin film transistors
28
and the voltage of the equivalent capacitor
30
. Finally, the alignment of the liquid crystal molecules and the light transmittance are changed. Therefore, changing the voltage of the input signals provided from the first driving circuit I
6
and from the second driving circuit I
8
can change the gray level of the corresponding pixel. For example, if the second driving circuit
26
transmits a pulse to the second data lines
18
to turn on the thin film transistor
28
, the first driving circuit I
6
can transmit signals to the equivalent capacitor
30
through the first data lines
24
and the thin film transistors
28
to control the gray level of a corresponding pixel. Additionally, the signals, transmitted from the first driving circuit I
6
, of the first data lines
24
are generated from the second voltage generator
22
.
Please refer to FIG.
2
.
FIG. 2
is a schematic diagram illustrating an operational amplifier buffer (op buffer)
40
circuit of the conventional LCD monitor
10
shown in FIG.
1
. The op buffer
40
is a class-A driver amplifier. The op buffer
40
is used to drive the LCD monitor
10
so that each pixel on the LCD monitor
10
can reach a predetermined gray level. When a voltage Vin turns on a transistor
41
and a bias voltage Vb turns on transistors
42
,
43
, a first stage circuit
44
of the op buffer
40
will drive a second stage circuit
45
of the op buffer
40
to generate a corresponding output voltage Vout with current I
3
. The voltage Vout is used to drive the LCD monitor
10
. Because the op buffer
40
is a class-A driver amplifier, it bears a high power efficiency. That is, most power-consumption of the op buffer
40
is used to drive the LCD monitor
10
. For example, the sum of currents I
1
, and I
2
is assumed to be 10 uA and the current I
3
derived from the op buffer
40
might be 100 uA. That is, the current I
3
is much greater than the currents I
1
, and I
2
. In other words, most electric power consumed by the op buffer
40
is used for driving the LCD monitor
10
.
Concerning a dot inversion driving applied on the LCD monitor
10
, a positive driving buffer is used for pulling up voltage of a pixel from a negative polarity to a positive polarity, and a negative driving buffer is used for pushing down voltage of the pixel from the positive polarity to the negative polarity. Therefore, each of the positive driving buffer and the negative driving buffer is only responsible for driving pixels toward a positive or a negative polarity according to the dot inversion driving. The class-A operational amplifier with small bias current is generally adopted to be the required positive or negative driving buffer owing to great power efficiency on driving single polarity. Although the op buffer
40
, which is a class-A operational amplifier, bears high power efficiency, yet it still needs a compensating capacitor
46
and an output resistor
47
to control the output slew rate of the op buffer
40
. Thus, a bigger layout area and a higher manufacturing cost of the op buffer
40
are inevitable.
Please refer to FIG.
3
.
FIG. 3
is a schematic diagram illustrating a conventional operational transconductance amplifier (OTA)
50
circuit. A voltage Vin turns on a transistor
51
. A bias voltage Vb turns on a transistor
52
and keeps the transistor
52
in a saturation state. Because the voltage at node D is not large enough to turn on a transistor
53
in the beginning, the transistor
53
is cut-off and current I
5
equals current I
4
. Although the OTA
50
bears many advantages such as a smaller size, a simpler structure, and a good slew rate (no extra compensating capacitors or output resistors are necessary), yet the power efficiency of the OTA
50
is not high. As described previously, since the current I
5
is equal to the current I
6
before the voltage at node D is equal to the voltage Vin to turn on the transistor
53
, the power efficiency of the OTA is only 50% (power efficiency=I
6
/(I
5
+I
6
)).
In conclusion, contrary to the op buffer
40
, the OTA
50
bears advantages of smaller size and simpler structure. However, the low power efficiency for the OTA
50
prevents it from being applied to the LCD monitor
10
.
SUMMARY OF INVENTION
It is therefore a primary objective of the claimed invention to provide an operational transconductance amplifier with simpler structure, smaller size, but higher power efficiency to solve the above-mentioned problems.
The claimed invention provides a driving method for improving power efficiency of an operational transconductance amplifier. The operational transconductance amplifier comprises a first current route and a second current route symmetrical to the first current route. Both of the first current route and the second current route comprise a plurality of transistors. Each of the transistors of the first current route has a smaller width/length ratio than the corresponding transistors of the second current route. The driving method comprises turning on the transistors of the first current route for outputting a ref
Himax Technologies Inc.
Hsu Winston
Mottola Steven J.
LandOfFree
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