# Patent

## Title
Analog buffer for LTPS amLCD

## Abstract

A buffer circuit for a liquid crystal display device that comprises a first transistor further comprising a gate connectable to an input signal, a first electrode coupled to a first power supply, and a second electrode connectable to a second power supply, a second transistor further comprising a gate coupled to the second electrode of the first transistor, a first electrode connectable to the first power supply, and a second electrode connectable to the second power supply, a first capacitor being connectable to the input signal storing a voltage of the input signal when connected to the input signal, and providing a first voltage to the gate of the first transistor when disconnected from the input signal, a second capacitor further comprising a terminal coupled to the second electrode of the first transistor and the gate of the second transistor providing a second voltage at the terminal when the first transistor is turned on, and a third capacitor coupled to the first electrode of the second transistor providing a third voltage when the second transistor is turned on, wherein the second voltage further comprises a first offset including a gate to source voltage of the first transistor, and the third voltage further comprises a second offset including a gate to source voltage of the second transistor.

## Background
DESCRIPTION OF THE INVENTION
1. Field of the Invention
This invention relates in general to a liquid crystal display (“LCD”) device and, more particularly, to an analog buffer circuit for an LCD device and a method of compensating an offset voltage in a buffer circuit for an LCD device.
2. Background of the Invention
An active matrix liquid crystal display (“LCD”) device generally includes a display panel and a drive circuit to drive the display panel. The drive circuit further includes gate drivers for selecting rows of gate lines and data drivers for providing pixel signals through data lines to pixels corresponding to selected gate lines. In a low temperature polycrystalline silicon (“LTPS”) LCD, drive circuits may be formed directly on a glass substrate. A data driver of an LTPS LCD typically employs source-follower analog buffers at its output stage. A buffer using a source-follower amplifier outputs a voltage produced by subtracting the gate to source voltage of a transistor from an input voltage through the source-follower amplifier. However, there is a problem that the output voltage of the buffer is susceptible to the variation in the characteristics of a device. There is therefore an increasing demand for a compact buffer not susceptible to the characteristics of a device and having simple circuitry.
An example of the source-follower techniques in the art is disclosed in U.S. Pat. No. 6,469,562 (hereinafter the '562 patent) to Shih et al., entitled “Source Follower with VGS Compensation.” The '562 patent discloses a source follower circuit including a constant current source. However, in an LTPS LCD, each data line may correspond to a buffer. For an increasing demand for higher resolution panels, the buffer circuit of the '562 patent may result in excessive power consumption. Furthermore, the constant current may be adversely affected by a drain to source voltage VDS of a transistor even though theoretically the constant current is proportional to (VGS−VT)2 when the transistor functions in a saturation region, where VGS is a gate to source voltage, and VT is a threshold voltage of the transistor. As a result, the square term (VGS−VT) is adversely affected, failing to properly provide linear compensation.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an analog buffer circuit and a method of compensating an offset voltage for an analog buffer that obviate one or more of the problems due to limitations and disadvantages of the related art.
To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and broadly described, there is provided a buffer circuit for a liquid crystal display device that comprises a first transistor further comprising a gate connectable to an input signal, a first electrode coupled to a first power supply, and a second electrode connectable to a second power supply, a second transistor further comprising a gate coupled to the second electrode of the first transistor, a first electrode connectable to the first power supply, and a second electrode connectable to the second power supply, a first capacitor being connectable to the input signal storing a voltage of the input signal when connected to the input signal, and providing a first voltage to the gate of the first transistor when disconnected from the input signal, a second capacitor further comprising a terminal coupled to the second electrode of the first transistor and the gate of the second transistor providing a second voltage at the terminal when the first transistor is turned on, and a third capacitor coupled to the first electrode of the second transistor providing a third voltage when the second transistor is turned on, wherein the second voltage further comprises a first offset including a gate to source voltage of the first transistor, and the third voltage further comprises a second offset including a gate to source voltage of the second transistor.
Also in accordance with the present invention, there is provided a buffer circuit for a liquid crystal display device that comprises a first transistor further comprising a gate connectable to an input signal, a second transistor further comprising a gate coupled to an electrode of the first transistor, a first capacitor being connectable to the input signal and the gate of the first transistor storing a voltage of the input signal when connected to the input signal, and providing the voltage of the input signal to the gate of the first transistor when disconnected from the input signal, a second capacitor coupled to the gate of the second transistor providing a voltage to the gate of the second transistor including a first offset component when the first transistor is turned on, and a third capacitor providing a voltage including a second offset component to neutralize the first offset component when the second transistor is turned on.
Still in accordance with the present invention, there is provided a buffer circuit for a liquid crystal display device that comprises a first capacitor being connectable to an input signal storing a reference voltage during a first period, and storing a voltage of the input signal during a second period after the first period, a second capacitor providing a voltage including a first offset during the first period, and providing a voltage including another first offset to neutralize the first offset during the second period, a third capacitor providing a voltage including a second offset during the first period, and providing a voltage including another second offset to neutralize the second offset during the second period, and a fourth capacitor storing the first and second offsets during the first period.
Further in accordance with the present invention, there is provided a method of compensating an offset voltage in a buffer circuit for a liquid crystal display device that comprises providing an input signal, charging a first capacitor with a voltage of the input signal, providing the voltage of the input signal to a first transistor, turning on the first transistor, storing a voltage including a first offset voltage in a second capacitor, the first offset voltage further comprising a gate to source voltage of the first transistor, turning on a second transistor, and storing a voltage including a second offset voltage in a third capacitor, the second offset further comprising a gate to source voltage of the second transistor.
Yet still in accordance with the present invention, there is provided a method of compensating an offset voltage in a buffer circuit for a liquid crystal display device that comprises providing a reference signal, determining a first offset for a first transistor, storing the first offset, determining a second offset for a second transistor, storing the second offset, providing an input signal different from the reference signal, determining another first offset for the first transistor, storing the other first offset, determining another second offset for the second transistor, storing the other second offset, and neutralizing the first and second offsets with the other first offset and the other second offset.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B and 1C are circuit diagrams of an analog buffer in accordance with one embodiment of the present invention; and
FIGS. 2A, 2B, 2C and 2D are circuit diagrams of an analog buffer in accordance with another embodiment of the present invention.



DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to the present embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
FIGS. 1A, 1B and 1C are circuit diagrams of an analog buffer 10 in accordance with one embodiment of the present invention. Analog buffer 10 functions to serve as a source follower wherein an output voltage VOUT follows an input voltage VIN. Analog buffer 10 includes a first transistor 12, a second transistor 14, a first capacitor C1, a second capacitor C2, and a third capacitor C3. Analog buffer 10 further includes a plurality of switches S1, S1, S2, S3, S3, S4 and S4, in which S1 and S1, S3 and S3, and S4 and S4 are switch pairs. A switch pair refers to a pair of switches operating in opposite switch conditions. For example, when switch S1 is closed, S1 is open, and vice versa.
First transistor 12 includes a gate (not numbered), a source (not numbered), and a drain (not numbered). The gate of first transistor 12 is coupled to input voltage VIN through switch pair S1 and S1, to first capacitor C1 through switch S1, and to second capacitor C2 and second transistor 14 through switch S2. The drain of first transistor 12 is coupled to a power supply line VDD. The source of first transistor 12 is coupled to second capacitor C2 and a gate of second transistor 14, and also coupled to a power supply line VSS2 through another switch S2. Second transistor 14 includes a gate (not numbered), a source (not numbered), and a drain (not numbered). The gate of second transistor 14 is coupled to the source of first transistor 12 and second capacitor C2. The drain of second transistor 14 is coupled to VSS2 through switch S3. The source of second transistor 14 is coupled to VDD through switch S3, and to third capacitor C3. Second capacitor C2 includes one end (not numbered) coupled to the source of first transistor 12 and the gate of second transistor 14, and the other end (not numbered) coupled to VSS2 through switch S4, and to a power supply line VSS1 through switch S4.
In one embodiment according to the invention, VDD is approximately 9 V (volts), VSS2 is approximately −6 V, VSS1 is greater than VSS2 or approximately 0 V, and VIN ranges approximately from 0 to 4 V.
Analog buffer 10 operates in three stages in sequence to provide output voltage VOUT. These stages are reset and sample, charge, and discharge and hold, which are illustrated in FIGS. 1A, 1B and 1C, respectively.
Referring to FIG. 1A, analog buffer 10 operates in the reset and sample stage. During this stage, switches S1, S2, S3 and S4 are closed, and switches S1, S3 and S4 are open. Input voltage VIN is stored in first capacitor C1 and isolated from the gate terminal of first transistor 12 because switch S1 is closed and switch S1 is open. A voltage VC1 at one end (not numbered) of first transistor C1 is approximately VIN. Since the gate terminal of first transistor 12 is biased at VSS2, first transistor 12 is turned off. Second transistor C2 is discharged to a power supply line VSS2 because switch S2 is closed. A voltage VC2 at one end (not numbered) of second capacitor C2 is pulled to VSS2. As a result, input voltage VIN is sampled and second capacitor C2 is reset in the reset and sample stage.
Referring to FIG. 1B, analog buffer 10 operates in the charge stage. During this stage, switches S1, S3 and S4 are closed, and switches S1, S2, S3 and S4 are open. First transistor 12 is turned on by the voltage VC1 provided by first capacitor C1 and may operate in a saturation region. A voltage at the source of first transistor 12, that is, VC2, is pulled to VC1−VGS1, where VGS1 is the gate to source voltage of first transistor 12. As a result, second capacitor C2 is charged to VC1−VGS1. On the other hand, since switch S3 is closed, third capacitor C3 is charged to VDD.
Referring to FIG. 1C, analog buffer 10 operates in the discharge and hold stage. During this stage, switches S1, S3 and S4 are closed, and switches S1, S2, S3 and S4 are open. Since switch S3 is open and switch S3 is clos transistor 14 is turned on and may operate in a saturation region. Third capacitor C3 is discharged through second transistor 14. The voltage VC3 at the source of second transistor 14 is discharged to approximately VC2+VSG2, that is, VC1−VGS1+VSG2 or VIN−VGS1+VSG2, where VSG2 is the source to gate voltage of second transistor 14. As a result, output voltage VOUT is held at the voltage level VIN−VGS1+VSG2.
After the discharge and hold stage, switch S4 is closed and switch S4 is open to turn off first transistor 12 and second transistor 14, resulting in a decrease of leakage current. The voltages VGS1 and VSG2 are substantially equal to the threshold voltages Vth1 and Vth2 of first transistor 12 and second transistor 14, respectively, when transistors 12 and 14 are turned off from a saturation region. The output voltage VOUT becomes approximately VIN−Vth1+|Vth2|, advantageously resulting in a linear compensation of input voltage VIN.
FIGS. 2A, 2B, 2C and 2D are circuit diagrams of an analog buffer 30 in accordance with another embodiment of the present invention. Analog buffer 30 includes a first transistor 32, a second transistor 34, a first capacitor CP1, a second capacitor CP2, a third capacitor CP3, and a fourth capacitor CP4. Analog buffer 30 further includes a plurality of switches SW1, SW2, SW3, SW3, SW4, SW4, SW5, SW5, SW6 and SW7, in which SW3 and SW3, SW4 and SW4, and SW5 and SW 5 are switch pairs.
First transistor 32 includes a gate (not numbered), a source (not numbered), and a drain (not numbered). The gate of first transistor 32 is coupled to input voltage VIN through switch SW1, to a ground level through switch SW7, and to one end (not numbered) of first capacitor CP1. The other end (not numbered) of first capacitor CP1 is coupled to one end (not numbered) of fourth capacitor CP4 through switch SW5, and to a ground level through switch SW6. The drain of first transistor 32 is coupled to a power supply line VDD. The source of first transistor 32 is coupled to second capacitor CP2 and a gate of second transistor 34, and also coupled to a power supply line VSS2 through switch SW2.
Second transistor 34 includes a gate (not numbered), a source (not numbered), and a drain (not numbered). The gate of second transistor 34 is coupled to the source of first transistor 32 and second capacitor CP2. The drain of second transistor 34 is coupled to VSS2 through switch SW3. The source of second transistor 34 is coupled to VDD through switch SW3, to third capacitor CP3, and to fourth capacitor CP4 through switch SW7.
Second capacitor CP2 includes one end (not numbered) coupled to the source of first transistor 32, the gate of second transistor 34, and to a power supply line VSS1 through switch SW4. The other end (not numbered) of second capacitor CP2 is coupled to VSS2 through switch SW4. Fourth capacitor CP4 includes one end (not numbered) coupled to the source of second transistor 34 through SW7, and to a ground level through SW5. The other end (not numbered) of second capacitor CP4 is coupled to first capacitor CP1 through another switch SW5, and to the ground level through SW5.
Analog buffer 30 operates in four stages in sequence to provide output voltage VOUT. These stages are first reset and sample, first discharge and hold, second reset and sample, and second discharge and hold, which are illustrated in FIGS. 2A, 2B, 2C and 2D, respectively. 
Referring to FIG. 2A, analog buffer 30 operates in the first reset and sample stage. During this stage, switches SW2, SW3, SW4, SW5 and SW7 are closed, and switches SW1, SW3, SW4, SW5 and SW6 are open. Input voltage VIN is isolated from first transistor 32 because switch SW1 is open. Since switch SW7 is closed, a voltage VCP1 at the one end of first capacitor CP1 is zero. Since switches SW2 and SW4 are closed, a voltage VCP2 at the one end of second capacitor CP2 is pulled to VSS2. First transistor 32 is turned on and may operate in a saturation mode. As a result, a zero voltage is sampled and second capacitor CP2 is reset. After switches SW7, SW2 and SW4 are closed, switches SW3 and SW5 are closed to charge third capacitor CP3 and fourth capacitor CP4. A voltage VCP3 at the one end of third capacitor CP3 and a voltage VCP4 at the one end of fourth capacitor CP4 are charged to VDD.
Referring to FIG. 2B, analog buffer 30 operates in the first discharge and hold stage. During this stage, switches SW4, SW3, SW5 and SW7 are closed, and switches SW1, SW2, SW3, SW4, SW5 and SW6 are open. Since switch SW2 is open, a voltage at the source of first transistor 32, that is, VCP2, is pulled to 0−VGS1 or −VGS1, where VGS1 is the gate to source voltage of first transistor 12. Since switch SW3 is open and switch SW3 is closed, second transistor 34 is turned on and may operate in a saturation region. Third capacitor CP3 and fourth capacitor CP4 are discharged through second transistor 34. The voltages VCP3 and VCP4 are discharged to −VGS1+VSG2, where VSG2 is the source to gate voltage of second transistor 34 at the time t0. As a result, an offset voltage −VGS1+VSG2 in response to an input level of zero is held in capacitor CP3. The offset voltage determined at the first and second stages will be used later to compensate for input signal VIN.
Referring to FIG. 2C, analog buffer 30 operates in the second reset and sample stage. During this stage, switches SW1, SW2, SW3, SW4, SW5 and SW6 are closed, and switches SW3, SW4, SW5 and SW7 are open. Since switches SW1 and SW6 are closed and switch SW7 are closed, VCP1 is charged to VIN. Since switches SW2 and SW4 are closed, VCP2 is pulled to VSS2. As a result, input voltage VIN is sampled and VCP2 is again reset. VCP3 is charged to VDD because switch SW3 is closed. The offset voltage, −VGS1+VSG2, is kept in fourth capacitor CP4 because switches SW5 and SW7 are open and switch SW5 is closed.
Referring to FIG. 2D, analog buffer 30 operates in the second discharge and hold stage. During this stage, switches SW4, SW3, SW5 are closed, and switches SW1, SW2, SW3, SW4, SW5, SW6 and SW7 are open. Since switch SW5 is closed, first capacitor CP1 and fourth capacitor CP4 are connected back to back. The voltage VCP1 is pulled to VIN−(−VGS1+VSG2). Since switch SW2 is open, VCP2 is pulled to VIN−(−VGS1+VSG2)−VGS1. When second transistor 34 is later turned on, VCP3 is discharged to VIN−(−VGS1+VSG2)−VGS1+VSG2, or VIN, which is then held at third capacitor CP3. As a result, the input signal VIN is compensated at the third and fourth stages by the offset voltage, that is, −VGS1+VSG2, obtained at the first and second stages.
The present invention also provides a method of compensating an offset voltage in a buffer circuit for a liquid crystal display device. An input signal VIN is provided. A first capacitor C1 is charged with a voltage of the input signal VIN. The voltage of the input signal VIN is provided to a first transistor 12. The first transistor 12 is turned on. A voltage VC1 including a first offset voltage VGS1 is stored in a second capacitor VC2. The first offset voltage VGS1 further comprises a gate to source voltage of first transistor 12. A second transistor 14 is turned on. A voltage VC3 including a second offset voltage VSG2 is stored in a third capacitor C3. The second offset VSG2 further comprises a gate to source voltage of second transistor 14.
In one embodiment, the first offset voltage further comprises a threshold voltage Vth1 of first transistor 12, and the second offset voltage further comprises a threshold voltage Vth2 of second transistor 14.
The present invention also provides another method of compensating an offset voltage in a buffer circuit for a liquid crystal display device. A reference signal is provided. A first offset VGS1 related to a first transistor 32 is determined. The first offset VGS1 is stored. A second offset VSG2 related to a second transistor 34 is determined. The second offset VSG2 is stored. An input signal VIN different from the reference signal is provided. Another first offset VGS1 related to first transistor 32 is determined. The other first offset VGS1 is stored. Another second offset VSG2 related to second transistor 34 is determined. The other second offset VSG2 is stored. The first and second offsets are neutralized with the other first and second offsets.
In one embodiment according to the invention, the first offset is stored in a second capacitor CP2, and the second offset is stored in a third capacitor CP3. In another embodiment, the first and second offsets are stored in a fourth capacitor CP4. In still another embodiment, the other first offset is stored in second capacitor CP2, and the other second offset is stored in third capacitor CP3. In another embodiment, the other first and other second offsets are stored in fourth capacitor CP4.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

## Claims

1. A buffer circuit for a liquid crystal display device comprising: a first transistor further comprising a gate connectable to an input signal, a first electrode coupled to a first power supply, and a second electrode connectable to a second power supply; a second transistor further comprising a gate coupled to the second electrode of the first transistor, a first electrode connectable to the first power supply, and a second electrode connectable to the second power supply; a first capacitor being connectable to the input signal storing a voltage of the input signal when connected to the input signal, and providing a first voltage to the gate of the first transistor when disconnected from the input signal; a second capacitor further comprising a terminal coupled to the second electrode of the first transistor and the gate of the second transistor providing a second voltage at the terminal when the first transistor is turned on; and a third capacitor coupled to the first electrode of the second transistor providing a third voltage when the second transistor is turned on; wherein the second voltage further comprises a first offset including a gate to source voltage of the first transistor, and the third voltage further comprises a second offset including a gate to source voltage of the second transistor.

2. The circuit of claim 1 further comprising a fourth capacitor including one terminal connectable to the second electrode of the second transistor, and another terminal connectable to the first capacitor.

3. The circuit of claim 1, the first voltage further comprising the voltage of the input signal.

4. The circuit of claim 1, the first voltage further comprising a reference voltage.

5. The circuit of claim 1, the first voltage further comprising the voltage of the input signal and offset voltages including a gate to source voltage each of the first transistor and the second transistor.

6. The circuit of claim 1, the second voltage further comprising the first voltage and an offset voltage including a gate to source voltage of the first transistor.

7. The circuit of claim 1, the third voltage being compensated by a threshold voltage each of the first transistor and the second transistor.

8. The circuit of claim 2, the fourth capacitor providing a fourth voltage when second transistor is turned on.

9. The circuit of claim 8, the fourth voltage further comprising offset voltages including a gate to source voltage each of the first and second transistors.

10. A buffer circuit for a liquid crystal display device comprising: a first transistor further comprising a gate connectable to an input signal; a second transistor further comprising a gate coupled to an electrode of the first transistor; a first capacitor being connectable to the input signal and the gate of the first transistor storing a voltage of the input signal when connected to the input signal, and providing the voltage of the input signal to the gate of the first transistor when disconnected from the input signal; a second capacitor coupled to the gate of the second transistor providing a voltage to the gate of the second transistor including a first offset component when the first transistor is turned on; and a third capacitor providing a voltage including a second offset component to neutralize the first offset component when the second transistor is turned on.

11. The circuit of claim 10, the first offset component further comprising a gate to source voltage of the first transistor.

12. The circuit of claim 10, the first offset component further comprising a threshold voltage of the first transistor.

13. The circuit of claim 10, the second offset component further comprising a gate to source voltage of the second transistor.

14. The circuit of claim 10, the second offset component further comprising a threshold voltage of the second transistor.

15. A buffer circuit for a liquid crystal display device comprising: a first capacitor being connectable to an input signal storing a reference voltage during a first period, and storing a voltage of the input signal during a second period after the first period; a second capacitor providing a voltage including a first offset during the first period, and providing a voltage including another first offset to neutralize the first offset during the second period; a third capacitor providing a voltage including a second offset during the first period, and providing a voltage including another second offset to neutralize the second offset during the second period; and a fourth capacitor storing the first and second offsets during the first period.

16. The circuit of claim 15 further comprising a first transistor and a second transistor.

17. The circuit of claim 16, the first and second offsets further comprising a gate to source voltage of the first transistor and the second transistor, respectively.

18. The circuit of claim 16, the other first and another second offsets further comprising a gate to source voltage of the first and second transistors, respectively.

19. The circuit of claim 15, the reference voltage further comprising a zero voltage.