Television – Camera – system and detail – Combined image signal generator and general image signal...
Reexamination Certificate
1997-01-21
2003-03-25
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Combined image signal generator and general image signal...
C348S250000, C348S302000
Reexamination Certificate
active
06538693
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a photoelectric converting apparatus and, more particularly, to a one-dimensional or two-dimensional photoelectric converting apparatus for executing a reading operation of, for example, a facsimile, a digital copying apparatus, an X-ray image pickup apparatus, or the like. More specifically, the invention relates to a removal of a random noise which is caused by a resetting operation of a photosensor.
2. Related Background Art
Hitherto, a CCD has mainly been used as an image pickup device of an image reading system such as facsimile, digital copying apparatus, or the like, a video camera, a digital camera, or the like. In recent years, however, a development of what is called an amplifying type photoelectric converting apparatus having an amplifying function of an MOS transistor or a bipolar transistor on a pixel unit basis is also vigorously being executed.
In an amplifying type photoelectric converting apparatus, in order to realize a high sensitivity, a removal of a noise becomes important. With respect to the noise removal, however, several methods have conventionally been proposed.
FIGS. 6A and 6B
are circuit diagrams of a one-dimensional photoelectric converting apparatus having a transistor as a photosensor for each pixel and a timing chart for such a circuit (refer to the magazine of The Institute of Television Engineers of Japan, Vol. 47, No. 9, pp. 1180, 1993).
The operation of such a photoelectric converting apparatus will now be described hereinbelow. When a start pulse (SP) is inputted, accumulating capacitors C
TS
and C
TN
are reset and a photosignal including a sensor noise is subsequently transferred to the capacitor C
TS
.
After that, a resetting operation of a photosensor is executed and an output in a dark state including the sensor noise is transferred to the capacitor C
TN
. The sensor is again reset and an accumulating operation is started. At the same time, a shift register of the first chip starts to scan and data of C
TS
and C
TN
is respectively capacitance divisionally outputted to C
HS
and C
HN
as common output lines of the first chip. The outputted signals are respectively inputted to a differential amplifier through voltage followers, so that a signal without any sensor noise is derived as an output of an IC.
“Sensor reset noise” used here denotes an FPN (Fixed Pattern Noise) which is caused due to a variation in h
FE
of a transistor serving as a photosensor of each pixel or a variation in capacitance C
bc
, between a base and a collector. Namely, an emitter potential after an emitter was reset by &phgr;ERS varies due to the variations in h
FE
and C
bc
, every pixel. Therefore, it appears as an offset in the ordinary reading operation. However, since the offset can be removed by such a method, the FPN can be reduced.
As another prior art, s solid-state image pickup apparatus as shown in
FIGS. 7 and 8
has been proposed in Japanese Patent Application Laid-Open No. 1-154678.
In the diagram, signal charges which were caused by an incident light and accumulated in each photodiode
101
are outputted by the following procedure. At the start of a horizontal blanking period of an output of the apparatus, when the photodiodes
101
of one line in the horizontal direction to be read out next are selected, a reset line
106
corresponding to such a line is turned on or off. After a resetting operation was performed by a reset switch
103
, when a drain line
107
is subsequently turned on, each pixel amplifier
104
of such a line operates as a driver transistor of a source follower. An output of each source follower in this instance is an amplifier output when there is no signal charge. By turning on/off a gate line
116
, the output voltage is stored into an accumulating capacitor
111
through a gate switch
109
.
Subsequently, when a vertical gate line
105
corresponding to such a line in the horizontal direction is turned on or off and signal charges are supplied to a gate of each pixel amplifier
104
, the output of each source follower has a value corresponding to an amount of signal charges. By turning on or off a gate line
117
, the output voltage is stored into an accumulating capacitor
112
through a gate switch
110
.
The operation in the horizontal blanking period is executed as mentioned above. In a horizontal scanning output period of time, a horizontal register
122
sequentially on/off scans horizontal gate switches
113
and
114
corresponding to each pixel, so that the source-follower output charges accumulated in the accumulating capacitors
111
and
112
are sequentially outputted from a horizontal signal line
120
.
The output charges accumulated in the capacitors
111
and
112
are obtained by time-sequencing outputs in both of the case of resetting and the case of inputting the signal charges with regard to one pixel amplifier
104
. Further, by obtaining a difference between both of those outputs, a noise which is caused by a variation in input offsets of a plurality of source followers and a 1/f noise of the source followers can be easily suppressed.
However, the prior art mentioned above has a problem to be solved such that the reset noise which is caused upon resetting of a photoelectric conversion unit cannot be removed.
Each time the photosensor is reset, an electric potential which was reset fluctuates and a reset noise appears as a random noise.
For example, in a photodiode having a pn junction, when a light production carrier Q
p
is accumulated into a capacitor C
PD
of a photodiode unit and converted into a voltage, a photosignal voltage V
p
by the light production carrier is
V
P
=Q
P
/C
PD
(1)
On the other hand, since a reset noise V
N
is
V
N
=(kT/C
PD
) (2)
where, k: Boltzmann's constant
T: Temperature (° K.)
an S/N ratio is
V
P
/V
N
=Q
PD
·(1/(kTC
PD
)) (3)
Therefore, from the equation (3), in order to raise the S/N ratio, although it is desirable to reduce the accumulated capacitance C
PD
of the photosensor as small as possible, there is practically a limitation. On the other hand, since there is a tendency such that the signal charges Q
p
decrease in accordance with the realization of a high fineness and a high speed of the photoelectric converting apparatus, the removal of the reset noise becomes an important point when realizing a high S/N ratio of the photoelectric converting apparatus.
However, in the prior art
1
, as shown in the timing chart of
FIGS. 6A and 6B
, the sensor is reset twice and the photosignal and noise signal to be read out are based on the different sensor resetting operations. Namely, after completion of the second sensor resetting operation, the accumulating operation and the photosignal reading operation are executed. A sensor noise N which is generated by the first sensor resetting operation is subtracted from a read-out signal (S+N′) including a sensor noise N′ generated by the second sensor resetting operation, thereby removing the sensor noise. Therefore, a random noise that is 2 times as large as the sensor reset noise is generated.
In the prior art
2
as well, since the resetting operation by the reset switch
103
is executed in a closed state of the gate switch
109
, the resetting operation of the photodiode
101
and the resetting operation by the reset switch
103
are different. Therefore, a random noise which is caused due to the resetting operation cannot be perfectly removed in a manner similar to the prior art
1
.
In the prior art
2
, although an example in which the gate switch
109
is not provided is also disclosed, there is a disclosure such that there is a problem such that the random noise due to the sensor resetting operation is also generated even in such a case.
Namely, in the conventional techniques, although the FPN can be improved, the random noise due to the resetting operation of the photosensor is not improved yet.
It is an object of the invention to reduce a random noise due to a res
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