Patent Grant
8279156
1. Field of Invention
The disclosure relates to an output amplifier. More particularly, the disclosure relates to the output amplifier of a source driver for driving a display panel.
2. Description of Related Art
In general, a liquid crystal display usually includes a source driver for driving source lines (or column lines) in order to drive the LCD panel. The source driver provides source driving signals or input pixel signals to each of the source lines in order to indicate the color/data in the respective source lines, and displays a picture in an LCD.
Since most source drivers are used in portable devices such as the laptop computers or the notebooks, and these portable devices usually work in low analog power, therefore, reduction in power consumption becomes a very important issue. However, source driver having the output amplifier manufactured in high voltage process (ex. 12V) can hardly work in low analog power due to body effect of the transistor employed in the out amplifier.
The body effect is the threshold voltage variation raised by the change in the source-bulk (body) voltage, in other words, if the source and the body of the transistor are not connected together, the body effect problem appears, and the equivalent resistance of the transistor is raised. The body effect can be approximated by the following equation:
VTN=VT0+γ(√{square root over (VSB+2φ)}−√{square root over (2φ)}),
where VSB is the voltage drop between the source and the body of the transistor, VT0 is threshold voltage when VSB is zero, VTN is the result threshold voltage with substrate bias present, γ is the body effect parameter, and 2φ is the surface potential parameter. Because the body can be operated as a second gate, and is sometimes referred to as the “back gate”, the body effect is sometimes called the “back-gate effect”.
Due to the body effect, the driving ability of the output amplifier is weakened, and the driving ability of the output amplifier is even worse when the analog power is reduced, which might not be able to drive the pixel circuit.
Therefore, there is a need for a new output amplifier which can preserve the driving ability to drive the pixel circuit.
According to one embodiment of the present invention, an output amplifier of a source driver, driving a pixel circuit of a panel, includes an amplifier circuit, an output stage circuit, a first switch transistor, and a second switch transistor. The amplifier circuit has an inverting terminal for providing an inverted signal, and has a non-inverting terminal for providing a non-inverted signal, in which the amplifier circuit is used for amplifying an input pixel signal to generate the inverted signal and the non-inverted signal. The output stage circuit has a first output terminal for passing a supply voltage from a supply terminal or passing a ground voltage from a ground terminal to the pixel circuit according to the inverted signal and the non-inverted signal.
The first switch transistor has a source and a body both electrically connected to the supply terminal, and has a drain electrically connected to the output stage circuit, in which the first switch transistor passes or blocks the supply voltage according to a high impedance signal. The second switch transistor has a source and a body both electrically connected to the ground terminal, and has a drain electrically connected to the output stage circuit, in which the second switch transistor passes or blocks the ground voltage according to the inverted high impedance signal.
It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
The high impedance control switch employed in the output amplifier (also called as the transmission gate having a PMOS and a NMOS connected in parallel) in the following embodiments has been moved from somewhere between the pixel circuit and the source driver to the supply terminal (or the ground terminal), which reduces the body effect of the output amplifier. Therefore, driving ability of the output amplifier is improved.
The amplifier circuit 101, such as the operation amplifier, has an inverting terminal (−) for providing an inverted signal, and has a non-inverting terminal (+) for providing a non-inverted signal, in which the amplifier circuit 101 is used for amplifying an input pixel signal to generate the inverted signal and the non-inverted signal. The output stage circuit 105 has the first output terminal O1 for passing the supply voltage from the supply terminal, or passing a ground voltage from the ground terminal to the pixel circuit according to the inverted signal and the non-inverted signal.
The first switch transistor 147, such as a PMOS, has a source S1 and a body B1 both electrically connected to the supply terminal, and has a drain D1 electrically connected to the output stage circuit 105. The second switch transistor 145, such as a NMOS, has a source S2 and a body B2 both electrically connected to the ground terminal, and has a drain D2 electrically connected to the output stage circuit 105. In the output amplifier 100 of this embodiment, the first switch transistor 147 and the second switch transistor 145 do not receive any signal directly from the amplifier circuit 101.
The first switch transistor 147 passes or blocks the supply voltage according to a high impedance signal TP, and the second switch transistor 145 passes or blocks the ground voltage according to the inverted high impedance signal
As mentioned above, the first switch transistor 147 and the second switch transistor 145 have their bodies electrically connected to their sources, and the body effect of these transistors can be eliminated. As a result, the threshold voltage and the equivalent resistance of these transistors keep the same without increasing, and the driving ability of the output amplifier for driving the pixel circuit maintains the same even the output amplifier is operated by low voltage level.
The output stage circuit 105 includes a third transistor 137 and a fourth transistor 139. The third transistor 137, such as a PMOS, has a gate G3 electrically connected to the inverting terminal (−) of the amplifier circuit 101, and has a source S3 electrically connected to the drain D1 of the first switch transistor 147. The fourth transistor 139, such as a NMOS, has a gate G4 electrically connected to the non-inverting terminal (+) of the amplifier circuit 101, has a drain D4 electrically connected to the drain D3 of the third transistor 137, and has a source S4 electrically connected to the drain D2 of second switch transistor 145.
When the first switch transistor 147 and the second switch transistor 145 are turned on (conductive), the resistances of the first switch transistor 147 and the second switch transistor 145 are small (like a conducting wire), so the sources of the third transistor 137 and the fourth transistor 139 are substantially connected to the supply terminal and the ground terminal respectively. Therefore, the third transistor 137 can have both its body and source receiving the supply voltage, and the fourth transistor 139 can also have both its body and source receiving the ground voltage. Since the body and the source of the third transistor 137 or the fourth transistor 139 are connected together, the body effect of these transistors are eliminated, too.
The output amplifier 100 further includes the first capacitor 115 and the second capacitor 117 electrically connected to the output stage circuit 105. The first capacitor 115, electrically connected between the inverting terminal (−) and the negative input terminal (−) of the amplifier circuit 101 through the first output terminal O1, maintains the voltage drop between the inverting terminal (−) and the negative input terminal (−). The second capacitor 117, electrically connected between the non-inverting terminal (+) and the negative input terminal (−) of the amplifier circuit 101, maintains the voltage drop between the non-inverting terminal (+) and the negative input terminal (−).
The configuration and the operation of the output amplifier 200 are similar to the output amplifier 100 shown in
The driving stage circuit 111 providing a second output terminal E1 to drive the pixel circuit, in which the driving stage circuit 111 passes the supply voltage or the ground voltage to the pixel circuit. Therefore, the pixel circuit is driven by both the output stage circuit 105 and the driving stage circuit 111, which improves the driving ability of the output amplifier to drive the pixel circuit.
The negative input terminal (−) of the amplifier circuit 101 is electrically connected to the first output terminal O1 and the second output terminal E1, such that the voltage levels on the negative input terminal (−), the first output terminal O1 and the second output terminal E1 are the same. The driving stage circuit 111 includes a fifth transistor 141 and a sixth transistor 143. The fifth transistor 141 has a gate receiving the inverted signal, and has a source receiving the supply voltage. The sixth transistor 143 has a gate receiving non-inverted signal, has a drain electrically connected to the drain of the fifth transistor 141, and has a source receiving the ground voltage.
According to the above embodiments, the high impedance control switch, including a PMOS transistor and a NMOS transistor, has been moved from somewhere between the output amplifier and the pixel circuit to the supply/ground terminal, such that the body effect of the transistors employed in the high impedance control switch can be eliminated; meanwhile, the high impedance control switch can still put the pixel circuit in high impedance status according to the high impedance signal.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
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| Number | Date | Country | |
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| 20110084945 A1 | Apr 2011 | US |
