Amplifiers – With semiconductor amplifying device – Including plural amplifier channels
Reexamination Certificate
2000-06-14
2001-05-01
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including plural amplifier channels
C330S310000, C330S311000, C330S277000
Reexamination Certificate
active
06225866
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a sampling circuit for sampling analog signals such as video signals, a signal amplifier f or amplifying analog signals, and an image display.
BACKGROUND OF THE INVENTION
In recent years, sampling circuits for sampling analog signals have been utilized in various fields, and they are modified into various systems that are suitable for the respective fields. In particular, in image displays such as liquid crystal displays, sampling circuits for sampling video signals are adopted in data-signal-line driving circuits, which will be described below.
For example, in a liquid crystal display of the active-matrix driving system, multiple scanning signal lines and multiple data signal lines are installed in an intersecting manner, and a pixel is disposed in each area enclosed by the adjacent scanning signal lines and the adjacent data signal lines. Thus, multiple pixels are disposed in the form of matrix.
Each pixel is provided with a switching device consisting of an FET (Field Effect Transistor) of the MOS type, and a pixel capacity. The switching device is conducted by a signal given to the scanning signal line so that it receives data (video signal) given to the data signal line, and supplies the data to the pixel capacity.
The data signal lines receive video signals that have been sampled by a data-signal-line driving circuit, and the scanning signal lines are successively selected by the scanning-signal-line driving circuit. Through the selections of the scanning signal lines, the video signals, which are given to the respective data signal lines, are written to the pixels, and held therein.
This writing process of data to the data signal lines is carried out using the point-sequential driving method or the line-sequential driving method.
In the point-sequential driving method, the inputted video signals are written to the data signal lines by opening and closing the sampling switch installed in the sampling circuit in synchronism with pulses that are released from a plurality of outputs of a shift register. In this method, supposing that the number of data lines in the horizontal direction is n, the time that allows the video signals to be written to the data signal lines is only as short as a 1
of an effective horizontal scanning period (approximately 80% of the horizontal scanning period). For this reason, when the time constant (product of capacity and resistance) of the data signal lines becomes greater upon adopting large screens to meet the current demands, it is difficult to maintain a sufficient writing process, thereby causing adverse effects on the quality of displayed images.
In particular, this problem is aggravated when the sampling switch is constituted of transistors with low driving performance, such as multi-crystal thin-film transistors which will be described later. Therefore, in conventional arrangements, the channel width of the transistors constituting the sampling switch is set to be greater in order to maintain a sufficient writing process.
In the line-sequential driving method, the sampling switch is opened and closed in synchronism with pulses that are released from a plurality of outputs of a shift register, in the same manner as the point-sequential driving method. Moreover, in the line-sequential driving method, the inputted video signals are temporarily stored in sampling capacities, and then released to the data signal lines through a buffer amplifier during the next horizontal scanning period.
In general, since the sampling capacities are smaller than the capacities of the data signal lines, the line-sequential driving method makes it possible to shorten the time during which the video signals are inputted from the video signal line and are written to the data signal lines. Further, the writing process to the data signal lines, which requires a greater load, is carried out during the horizontal scanning period; this allows a sufficient writing process to the data signal lines. As described above, the line-sequential driving method has less problems than the point-sequential driving method.
However, the disadvantage of the line-sequential driving method is that the electric charge held in the sampling capacity decreases due to leakage current in the sampling switch as time elapses and it also decreases due to capacity divisions upon transferring data to the buffer amplifier. In order to suppress these adverse effects, it is proposed that the sampling capacity be increased; however, this might cause an insufficient writing process in the same manner as caused in the point-sequential driving method. Therefore, in this case, also, it is necessary to increase the channel width of the transistors constituting the sampling switch in order to maintain a sufficient writing process.
Here, for example, as shown in
FIG. 47
, the above-mentioned sampling circuit is provided with a group of inverters
202
consisting of a plurality of stages of series connected inverters
201
and a sampling switch
203
consisting of only n-channel transistors. In this sampling circuit, when the video signal from the video signal line VL is written to the data signal line SL, an output signal from the shift register
204
, which forms a timing signal, is amplified by the group of the inverters
202
, and is inputted to the gate electrode of the sampling switch
203
.
As shown in
FIG. 48
, the inverter
201
has a construction wherein an n-channel transistor
201
a
and a p-channel transistor
201
b
are connected in series with each other.
During the writing process, the sampling switch
203
requires a high-level signal V
H
that is sufficient to write the video signal on the high-voltage side in its conducted state, while it requires a low-level signal V
L
that allows the holding of the video signal on the low-voltage side in its cutoff state. Therefore, during the writing process, it is necessary to substantially increase the amplitude of the signal to be given to the gate electrode of the sampling switch
203
.
More specifically, supposing that the amplitude of the video signal is V
sig
, the threshold voltage of the sampling switch
203
is V
tn
, and the on-margin and off-margin of the sampling switch
203
are V
on
and V
off
respectively, signals V
H
and V
L
are indicated as follows:
V
H
=V
sig
+V
tn
+V
on
(1)
V
L
=−V
sig
+V
tn
−V
off
(2)
Here, the on-margin represents a voltage to be added to the threshold voltage of the sampling switch
203
in order to provide a sufficient writing process, and the off-margin represents a voltage to be subtracted from the threshold voltage of the sampling switch
203
in order to substantially reduce the leakage current. For example, typical values for the respective voltages are: V
sig
=5(V), V
tn
=2(V), V
on
=4(V), and V
off
=5(V). In accordance with equations (1) and (2), signals V
H
and V
L
are represented as follows based on these values.
V
H
=5+2+4=11(V)
V
L
=−5+2−5=−8(V)
Therefore, it is necessary to provide a source voltage of 19V that is the voltage difference between V
H
and V
L
. Accordingly, the elements need to have a withstand voltage of up to 19V.
Moreover,
FIGS. 49 and 50
show other sampling circuits, each of which is provided with a group of inverters
205
(
206
) consisting of a plurality of steps of inverters
201
, and a sampling switch
207
. The group of inverters
205
(
206
) is branched into two signal paths from the inverter
201
at the second stage from the shift register
204
, and a plurality of steps of inverters
201
are installed in each signal path.
The sampling switch
207
has a CMOS construction wherein n-channel and p-channel type transistors
207
a
and
207
b
are connected in parallel with each other. In this sampling switch
207
, the video signal on the low-voltage side is written by the n-channel transistor
207
a
and the video signal on the high-voltage side is writte
Kubota Yasushi
Shiraki Ichiro
Nixon & Vanderhye PC
Sharp Kabushiki Kaisha
Shingleton Michael B
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