Television – Camera – system and detail – Combined image signal generator and general image signal...
Reexamination Certificate
1999-07-28
2003-12-23
Garber, Wendy R. (Department: 2712)
Television
Camera, system and detail
Combined image signal generator and general image signal...
C348S302000, C348S241000, C250S208100
Reexamination Certificate
active
06667767
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image sensor and more particularly to a MOS (Metal Oxide Semiconductor) type image sensor in which the dispersion in a threshold voltage of a transistor constituting a source follower to output a photoelectric conversion voltage of a pixel to the outside is compensated.
2. Description of the Related Art
An image sensor is a sensor to convert optical image information to an electrical signal in a TV camera and the like. A MOS-type image sensor in particular is so configured as to have a photodiode as a photoelectric conversion device and its peripheral devices composed of a MOS-type FET (Field Effect Transistor), which is featured by low power consumption, low costs, etc.
FIG. 5
is a block diagram illustrating configurations of a conventional MOS-type image sensor. Configurations and operations of the MOS-type image sensor are hereafter described referring to FIG.
5
.
As depicted in
FIG. 5
, a conventional MOS-type image sensor is composed of a pixel array
31
in which many unit pixels (i.e., picture elements)
30
are arrayed in a matrix-like manner in horizontal (low) and vertical (column) directions, an address decoder
32
, a vertical shift register
33
, a vertical driver
34
, a clock control circuit
35
, a noise control circuit
36
and a horizontal shift register
37
.
The unit pixel
30
consists of a photodiode
301
being a photoelectric conversion device, a transistor
302
to reset the photodiode to a power supply voltage Vcc, a transistor to amplify a photoelectric conversion voltage of the photodiode
301
and a read-out transistor to connect a transistor
303
for amplification to a bit line BL corresponding with activation of a word line WL.
An address signal read out from a control circuit not shown is divided, via the address decoder
32
, into two signals, one going in the vertical direction and the other in the horizontal direction. The vertical shift register
33
, when receiving the address signal in the vertical direction, generates a read-out control signal which is shifted sequentially in the vertical direction. The control signal is sequentially fed to each word line WL so that a pixel in the pixel array
31
for each word line can be read out. On the other hand, the horizontal shift register
37
, when receiving the address signal in the horizontal direction, selects sequentially the bit line BL to connect it to the noise control circuit
36
and reads out the photoelectric conversion voltage of the photodiode
301
for every unit pixel at a cross point of the word line WL and the bit line BL and then, after noise control, generates an image signal output. At this point, the noise control circuit
36
is adapted to remove noises caused by switching operations of each gate transistors which are produced in a manner overlapping the photoelectric conversion output fed from the photodiode
301
.
There are conventionally a variety of types of circuits for reading outputs and removing noises in such a MOS-type image sensor.
FIG. 6
is a block diagram illustrating a conventional image sensor and
FIG. 7
is a timing chart showing operations of the circuit shown in FIG.
7
.
As shown in
FIG. 6
, the conventional image sensor is composed of unit pixels
41
and
42
located on an arbitrary n-th (n=0,1,2 . . . ) line and in a n-th+1st string being neighboring to an arbitrary n-th string in a pixel array constituting the image sensor, noise control circuits
43
and
44
on the n-th string and n-th+1st string respectively, a current source
45
constituting a source follower together with transistors
203
of the noise control circuit
43
and the like, a P channel transistor
46
constituting the source follower, a current source
47
and a P channel transistor
48
. The number of lines and strings constituting the pixel array is configured arbitrarily.
The pixel
41
consists of a photodiode
101
being a photoelectric conversion device, N channel transistors
102
,
103
and
104
, and a current source
105
.
The photodiode
101
is adapted to convert incident light at the unit pixel to an electrical signal. The transistor
102
resets an initial voltage of the photodiode
101
to a power supply voltage Vcc in accordance with a reset control signal RSTn on the n-th line. The transistor
103
constitutes a source follower to amplify the photoelectric conversion voltage of the photodiode
101
together with the current source
105
. The transistor
104
serves to connect the transistor
103
through a data output line DATAn on the n-th string to the current source
105
in accordance with a word line read-out control signal WLn on the n-th line. The current source
105
feeds a constant current to the transistor
103
connected through a gate composed of a transistor
104
. Configurations and functions of a pixel
42
are the same as the pixel
41
.
The noise control circuit
43
is composed of N channel transistors
201
,
203
and
204
, a coupling capacitor
202
and a P channel transistor
205
.
The transistor
201
connects the data output line DATAn to the coupling capacitor
202
in accordance with a signal voltage read-out control signal SHS. The coupling capacitor acts to transmit a change of an output voltage of the data output line DATAn to a node S/Hn. The transistor
203
constitutes a source follower together with the current source
45
and outputs a voltage of the node S/Hn to a transistor
46
. The transistor
204
connects the transistor
203
to the current source
45
in accordance with a bit line read-out control signal YSWn. The transistor
205
connects the node S/Hn to a power supply source OCVn in accordance with a clamp control signal OCI. Configurations and functions of a noise control circuit
44
and other noise control circuits not shown are the same as the noise control circuit
43
.
A current source
45
feeds a constant current to the transistors
203
and the like. The transistor
46
constitutes a source follower together with a current source
47
and outputs an output voltage Vout in accordance with a voltage of a gate. The transistor
48
connects the transistor
46
to the current source
47
in accordance with an output enable signal OE.
Operations of the conventional image sensor shown in
FIG. 6
are described by referring to FIG.
7
. Operations of the pixel
41
and the noise control circuit
43
are described primarily, however, operations of the pixel
42
and other pixels not shown and of the noise control circuit
44
and other noise control circuits not shown are the same as above. By making LOW a vertical read-out control signal IRASB, a vertical address of the pixel array is sequentially designated. Moreover, by making HIGH a signal voltage read-out control signal SHS, a gate composed of the transistor
201
is turned ON, causing the data output line DATAn to be connected to the coupling capacitor
202
. Then, in accordance with the designated address, the word line read-out control signal WLn is made HIGH and a clamp control signal OCI is made LOW.
When a reset control signal RSTn is made HIGH by a previous resetting operation and the gate composed of the transistor
102
is turned ON, if the photodiode
101
is exposed to light for a definite time while it is charged to the power supply voltage Vcc, its voltage drops responsive to an incident light level. At this point, since the word read-out control signal WLN is made HIGH, a gate composed of the transistor
104
is turned ON, causing a photoelectric conversion voltage generated when the photodiode is exposed to light to be outputted to the data output line DATAn through a source follower composed of the transistor
103
.
In this state, because the signal voltage read-out control signal SHS is made HIGH and a gate composed of the transistor
201
is turned ON, a voltage of the data output line DATAn is applied to the coupling capacitor
202
. At this point, since the clamping control signal OCI is made LOW, a gate composed of the transistor
20
Kurosawa Susumu
Muramatsu Yoshinori
Garber Wendy R.
McGinn & Gibb PLLC
NEC Electronics Corporation
Villecco John M.
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