Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-04-11
2002-01-15
Bruce, David V. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C348S308000
Reexamination Certificate
active
06339215
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-Y address type solid state image sensing device represented by a CMOS type image sensing device and an amplification type image sensing device and an image signal processing method of the same, and particularly to a current output type solid state image sensing device and a processing method for processing an image signal of the same.
2. Description of Related Art
A structure of a related art of a solid state image sensing device, for example, a CMOS type image sensing device which outputs an image signal as a current is illustrated in FIG.
8
.
In
FIG. 8
, a unit pixel
101
is formed of a photo-diode
102
, a transistor
103
for amplification, a transistor
104
for vertical selection and a transistor
105
for reset. This unit pixel
101
is arranged in the X direction (column direction) and Y direction (row direction). Here, in order to simplify the figure, only the pixels of m-row and n-column are illustrated.
In this unit pixel
101
, a vertical scanning pulse &phgr; Vm is impressed to the gate electrode of the transistor
104
for vertical selection via a vertical selection line
107
from a vertical scanning circuit
106
and a vertical reset pulse &phgr; V
R
m is impressed to the gate electrode of the transistor
105
for reset via a vertical reset line
108
from the vertical scanning circuit
106
. Moreover, signal charge photo-electrically converted by a photo-diode
102
is then converted to a signal current by the transistor
103
for amplification and is then output to a vertical signal line
109
via the transistor
104
for vertical selection.
Between the vertical signal line
109
and horizontal signal line
110
, a transistor
111
for horizontal selection is connected. To the gate electrode of the transistor
111
for horizontal selection, a horizontal scanning pulse &phgr; Hn is impressed to a horizontal scanning circuit
112
. Thereby, a signal current output to the vertical signal line
109
from the pixel
101
flows into the horizontal signal line
110
through the transistor
111
for horizontal selection.
At the end part of the horizontal signal line
110
, a current/voltage converting circuit
113
is provided. This current/voltage converting circuit
113
is composed of a differential amplifier
114
connected at its inverted (−) input terminal to the horizontal signal line
110
and a feedback resistor
115
connected between the inverted input terminal and an output terminal of the differential amplifier
114
. Moreover, to the non-inverted (+) input terminal of the differential amplifier
114
, a bias voltage Vbias is applied and a signal current input through the horizontal signal line
110
is converted to a signal voltage and is then output.
In a MOS type image sensing device of the related art of the structure explained above, when the on-chip structure is realized by fabricating a current/voltage converting circuit
113
on the same semiconductor substrate as pixel portion, a power source voltage VDD of the current/voltage converting circuit
113
usually becomes single power source voltage such as 5 V or 3 V. Therefore, when the circuit configuration formed of the differential amplifier
114
and feedback resistor
115
as explained above is used as the current/voltage converting circuit
113
, this circuit assures sufficient output voltage range only by setting the bias voltage Vbias to almost a half of the power source voltage VDD.
As explained above, when the bias voltage Vbias of the current/voltage converting circuit
113
is set to about a half of the power source voltage VDD, each potential of the vertical signal line
109
and horizontal signal line
110
to which a signal current is output from the pixel
101
becomes almost equal to the bias voltage Vbias, namely to about a half of the power source voltage VDD. Therefore, only a voltage (about a half of the power source voltage VDD) equal to the potential difference between the power source VDD and signal lines
109
,
110
is applied across the drain and source of the transistor
103
for amplification in the pixel
101
. Here, voltage drop of the transistor
104
for vertical selection and transistor
111
for horizontal selection is neglected here for discussion.
Therefore, when the power source voltage VDD is set, for example, to 3.0 V, only a voltage of 1.5 V is applied across the drain and source of the transistor
103
for amplification. Accordingly, the transistor
103
for amplification operates under the condition that its mutual conductance gm is rather lower. As a result, signal voltage of the photo-diode
102
cannot be converted to a signal current with a sufficient amplification factor and as a result, sensitivity of image sensing device is lowered.
Moreover, even in the same related art, it has also been proposed that each potential of the vertical signal line
109
and horizontal signal line
110
is lowered so that a higher voltage is applied across the drain and source of the transistor
103
for amplification in the unit pixel
101
in order to improve the sensitivity. For this purpose, the bias voltage Vbias of the current/voltage converting circuit
113
is lowered and the part, in which the input/output characteristic is not so good, of the differential amplifier
114
is used.
Drop of the bias voltage Vbias of the current/voltage converting circuit
113
corresponds to change of IN+=½*VDD to IN+=¼*VDD in the input/output characteristic of the differential amplifier illustrated in FIG.
4
. The dotted line of IN−=OUT written over the input/output characteristic indicates the operating point when the signal current is zero in the current/voltage converting circuit
113
and a thick solid line enclosed in the circle indicates the input/output characteristic (Rout) up to saturation from zero of the pixel signal current under the condition of IN+=¼*VDD.
Here, when attention is paid to the part of input/output characteristic enclosed by a circle, since the input/output characteristic is not indicated by a linear line, it can be understood that linearity is deteriorated and gain is low. Namely, this input/output characteristic indicates that as a result of compulsory adjustment of the bias voltage Vbias of the current/voltage converting circuit
113
in order to improve sensitivity of pixel, linearity and gain of the current/voltage converting circuit
113
are deteriorated.
SUMMARY OF THE INVENTION
The present invention is proposed considering the background explained above and it is therefore an object of the present invention to provide a solid state image sensing device which can improve amplification factor (sensitivity) of pixel while linearity of the current/voltage converting circuit is maintained and also provide a pixel signal processing method of the same.
In order to attain the object explained above, in the solid state image sensing device of the present invention in the structure that an image signal of each pixel is output to a signal line as the current and a signal current output to the signal line is converted to a signal voltage and it is then output, an input operation point of a current voltage converting means for converting a signal current to a signal voltage is set approximate to the ground level or power source level and a pixel signal can be derived by converting the signal current to the signal voltage with this current voltage converting means.
In a current output type solid state image sensing device, the potential of signal line can be lowered (or raised) by setting the input operation point of the current voltage converting means approximate to the ground level (or power source level). Thereby, a higher voltage can be applied to the transistor for amplification of the pixel. Sensitivity (amplification factor) of pixel can be raised by applying a higher voltage to the transistor for amplification.
Here, it is particularly preferable that the present invention
Shiono Koichi
Suzuki Ryoji
Ueno Takahisa
Yonemoto Kazuya
Bruce David V.
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
Thomas Courtney
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