Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-05-04
2001-02-20
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S297000
Reexamination Certificate
active
06191409
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image sensor being a photographing apparatus, and more specifically to a semiconductor image sensor.
2. Description of the Related Art
There are conventionally known as photographing devices CCD (Charge Coupled Device) image sensors and MOS (Metal Oxide Semiconductor) image sensors. The CCD image sensors are prevailing at present. Compared with the CCD image sensors, however, the MOS image sensors have the following advantages: the MOS image sensors require less power consumption, and includes an optical detection section and its peripheral circuit which can be integrated on the same chip.
As illustrated in
FIG. 6
, the aforesaid MOS image sensor includes a vertical scanning section
101
, a horizontal scanning section
102
, and a cell array
103
. The vertical scanning section
101
outputs DC voltage V
1
and a readout signal S
2
to a cell array
103
, and the horizontal scanning section
102
outputs a readout signal S
2
to the cell array
103
.
The cell array
103
converts incident light to an electric signal, and outputs a signal representing a picture image (hereinafter, referred to as a picture image signal ),and includes m×n conversion cells
111
ij
(i=1 to m, j=1 to n), and regulated power supplies
112
1
to
112
m
, as illustrated in FIG.
6
.
The regulated current supplies
112
1
to
112
m
, supply a regulated current to signal lines
113
1
to
113
m
, respectively. The conversion cells
111
ij
arranged in a matrix include transistors each of which is an enhancement N channel MOSFET (Field Effect Transistor) and a photodiode
124
for converting incident light to an electric signal, as illustrated in FIG.
7
.
The vertical scanning section
101
vertically scans the cell array
103
. The vertical scanning section
101
outputs DC voltage V
1
and a high level reset signal S
1
to the conversion cells
111
ij
to
111
mj
that are an object of vertical scanning. Hereby, a photodiode
124
of the conversion cell
111
ij
is initialized. More specifically, the transistor
121
is switched on with the high level reset signal S
1
, and the DC voltage V
1
is applied to the photodiode
124
.
Consequently, the photodiode
124
stores electric charges thereon, and a voltage at a connection point (hereinafter, referred to a node M) between the photodiode
124
and the transistor
122
becomes the DC voltage V
1
. The photodiode
124
is therefore initialized.
After completion of initialization of the photodiode
124
, when a light is incident on the photodiode
124
, electric charges stored in the photodiode
124
are reduced in response to the intensity of the incident light to a lower voltage at the node M. The voltage at the node M is a conversion signal that is a result of the conversion of the incident light.
Thereafter, the horizontal scanning section
102
horizontally scans the cell array
103
. At this time, a high level readout signal S
2
is inputted in order into the conversion cells
111
ij
to
111
jm
that are the object of the horizontal scanning. Hereby, the conversion signal amplified by the transistor
122
is outputted to the signal line
113
i
after passage through the source follower transistor
123
.
When the vertical/horizontal scanning for the conversion cells
111
11
to
111
m1
and the conversion cells
111
1n
to
111
mn
is finished, the horizontal scanning section
102
outputs a picture image signal.
The prior art technique suffers from the following difficulty. Each conversion cell
111
ij
of the cell array
103
outputs the amplified conversion signal to the signal line
113
i
. The amplification is achieved with the transistor
122
of the conversion cell
111
ij
.
In contrast, the transistor
122
has a threshold, and further has an amplifying function for input voltage above the threshold. The threshold is however varied depending upon fine differences among dozes of doped impurities, and slight differences of gate configurations produced upon manufacturing. In other words, the transistors
122
have different thresholds which provide variations of the threshold of the transistor
122
. Thus, noise of a fixed pattern due to the transistor
122
is generated on the aforesaid picture image signal.
SUMMARY OF THE INVENTION
The present invention has been made to solve the aforesaid problems of the prior art, and has an object to provide an image sensor capable of reducing an influence of variations of a threshold of an amplifying transistor included in a conversion cell.
To achieve the aforesaid object, an image sensor according to one aspect of the present invention is featured by comprising: a plurality of photoelectric conversion cells arranged in a matrix for generating conversion voltage in response to incident light; and a processing section for performing a processing of deriving the conversion voltage from each the photoelectric conversion cell, each the photoelectric conversion cell comprising: a conversion section for changing, after the predetermined voltage is set with initial setting of the processing section, the predetermined voltage in response to incident light and outputting the predetermined voltage as the conversion voltage; an amplifying device for amplifying the conversion voltage from the conversion section; an outputting section for outputting the conversion voltage amplified by the amplifying device to the processing section; and setting section for adding the voltage corresponding to the threshold of the amplifying device to the predetermined voltage set to the conversion section.
In the foregoing, the conversion section comprises a photodiode for changing, after the predetermined voltage is set with the initial setting, the predetermined voltage in response to the incident light and outputting the predetermined voltage as the conversion voltage, and a first switching device which is made conductive with a first signal outputted from the processing section upon the aforesaid initial setting to add the supply voltage supplied by the processing section to the photodiode as the predetermined voltage.
Further, in the foregoing it is preferable that the amplifying device is a field effect transistor in which the conversion voltage from the photodiode is applied to a gate thereof, and the supply voltage is applied to a source thereof to amplify the conversion voltage. The thus amplified conversion voltage is outputted from a drain thereof.
The processing section, after outputting the first signal, outputs a second signal and supplies the supply voltage while lowering the same, and the set means is a second switching device which is made conductive with the second signal from the aforesaid processing section to connect the gate of the field effect transistor to the drain.
The processing section supplies voltage above the threshold of the field effect transistor from the supply voltage while lowering the voltage above the threshold.
The second switching device is a field effect transistor which is made conductive with the second signal applied to the gate of the second switching device from the processing section.
The gate width of the second switching device is smaller than those of the other field effect transistors.
In accordance with the present invention, since the threshold of the amplifying device is applied to the predetermined voltage of the conversion section upon initial setting, an influence of variations of the threshold of the amplifying device can be removed from the conversion voltage.
Circuit construction of the setting section is simplified by using a switching device such as a field effect transistor as the setting section, whereby an influence of variations of the threshold of the amplifying device can be removed from the conversion voltage.
Since a switching device is used as in the setting section in a manner such that switching of conduction of the switching device is achieved with the second signal from the processing section, severe alteration of the setting section is prevented,
Le Que T.
McGinn & Gibb PLLC
NEC Corporation
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