Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2002-02-14
2004-06-08
Luu, Thanh X. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C330S308000, C348S300000
Reexamination Certificate
active
06747264
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable gain amplifier capable of adjusting a gain according to a level of an input signal, to a solid-state imaging device for converting an optical signal into a digital signal and outputting the digital signal, which is used for a video camera, an electronic camera, an image input camera, a scanner, a facsimile or the like, and to an optical signal reading method.
2. Description of the Related Art
A semiconductor image sensors such as a CCD image sensor and a MOS image sensor are excellent in mass production, and thus such sensors have been applied to many image input devices along with an advance in a fine patterning technology.
Especially, in recent years, a CMOS image sensor has been a main focus of attention because of its advantages in contrast with the CCD image sensor, i.e., smaller power consumption, and capability of fabricating a sensor device and a peripheral circuit device by the same CMOS technology.
Such a CMOS image sensor is described in U.S. Pat. No. 6,128,039. The CMOS image sensor disclosed in U.S. Pat. No. 6,128,039 is called an active pixel sensor. As shown in
FIG. 1
transferred from this US patent, in combination with an active load such as a constant current source M
4
or the like, a signal voltage is outputted by a source follower.
In the CMOS image sensor of U.S. Pat. No. 6,128,039, a load capacitor C
1
for storing signal charges and a gate-source capacitor Cgs of the reading transistor M
2
are connected in series to a gate of M
2
. These capacitors are set parallel in a fixed capacitor for charge/voltage conversion, and the fixed capacitor for charge/voltage conversion is seemingly changed in the capacitance. Charging of the capacitor C
1
connected to the source of M
2
greatly changes a source potential during signal reading. As this potential change is returned through the capacitor Cgs to the gate, an input potential is also changed, causing a considerable deterioration of linearity of a transmission characteristic. Consequently, in this CMOS image sensor, the constant current source M
4
as a load of a reading transistor M
2
was inevitable.
In addition, in recent years, a column type analog/digital converter of an integration system (hereinafter referred to as a column type ADC) has been mounted on the image sensor. In such a column type ADC, an analog electric signal (photoelectric signal) into which an optical signal is converted is compared with a comparison lamp voltage having a predetermined gradient by a comparator, and is converted into a pulse count corresponding to an amplitude of the photoelectric signal by a pulse counter.
In such a case, when the analog signal is small, a gradient of the comparison lamp voltage is also reduced to secure a dynamic range.
Furthermore, in a solid-state imaging device including a number of unit pixels arrayed in horizontal and vertical directions, sampling pixels are thinned to output only detecting signals of remaining pixels not thinned during moving image reproduction, thereby increasing a frame rate.
However, in the case of the image sensor including the constant current source M
4
for each column, since there are variances in characteristics among the respective constant current sources M
4
, variances are generated in gains among columns. This variance appears as an offset difference between columns, and when seen on a screen, the variance appears as a so-called vertical fixed pattern noise.
When a signal voltage outputted from the conventional image sensor is inputted to the column type ADC, a gradient of a comparison lamp voltage is reduced corresponding to a small analog signal. In this case, a limitation is placed on an S/N ratio because of linearity of a lamp signal or comparison accuracy of the comparator, and because of an influence of an offset voltage generated in a photoelectric conversion device or the like.
Furthermore, if sampling pixels are thinned in the conventional image sensor, a modulation transfer function (MTF: resolution) is deteriorated, forming an image of much moire. In addition, when a reduction is made twice or lower as large as a sampling frequency proportional to an inverse number of a sampling pixel interval because of the thinning of the sampling pixels, a folding noise (aliasing noise) may be thus generated. Also, as it is necessary to operate the image sensor at a high speed according to the number of pixels, power consumption is increased.
SUMMARY OF THE INVENTION
This present invention is performed considering the foregoing drawbacks.
An object of the present invention is to provide a variable gain amplifier, a solid-state imaging device, and an optical signal reading method being capable of improving an S/N ratio while enhancing a dynamic range when a photoelectric signal is digitized.
Another object thereof is to provide a solid-state imaging device and an optical signal reading method being capable of reducing fixed pattern noises and further suppressing a reduction in resolution and generation of folding noises while maintaining a low power consumption operation by thinning sampling pixels.
A variable gain amplifier of the present invention is characterized in that it converts a first signal voltage and a second signal voltage into charges by sequentially inputting the first and second signal voltages, generates a difference signal between the first and second signal voltages, amplifies the difference signal by a gain so as to set the difference signal within a requested range of a digital encoding analog input level, and outputs the difference signal.
A variable gain amplifier comprises a so-called chopper type switched capacitor integrating circuit. The chopper type switched capacitor integrating circuit includes, as shown in
FIG. 2
as the example,: an operational amplifier
31
having a positive input terminal (+) to which a reference voltage Vref is applied and a negative input terminal (−), and an output terminal; an input capacitor Ci (C
1
) provided in a signal path extending from the input terminal of the variable gain amplifier
105
a
to the negative input terminal (−) of the operational amplifier
31
; a feedback capacitor Cf composed of a plurality of capacitors (C
2
, C
3
, C
4
, . . . etc.) provided between the negative input terminal (−) and output terminal of the operational amplifier
31
; first switch devices SW
1
and SW
2
for connecting/disconnecting a signal path extending from the input terminal of the variable gain amplifier
105
a
to the other end of the input capacitor Ci; a second switch device SW
3
for turning ON/OFF (connecting/disconnecting) an input of the reference voltage Vref to the other end of the input capacitor Ci; and a third switch device SW
4
for connecting/ disconnecting a signal path between the negative input terminal (−) and output terminal of the operational amplifier
31
.
An amplifying gain of the operational amplifier
31
can be adjusted as follows.
That is, the capacitors C
2
, C
3
, C
4
. . . etc. constituting the feedback capacitor Cf are connected to switch devices (SW
5
, SW
6
. . . etc.) for controlling connection/disconnection of the respective capacitors between the input and output terminals of the operational amplifier
31
. Accordingly, by selectively connecting/disconnecting the switch devices (SW
5
, SW
6
. . . etc.), a proper capacitor is selected and connected between the input and output terminals of the operational amplifier
31
. Therefore, by increasing/reducing a capacitance of the feedback capacitor Cf, a ratio (Ci/Cf) of the input capacitor Ci with respect to the feedback capacitor Cf is adjusted. With this, it is possible to adjust the gain of the operational amplifier
31
.
A solid-state imaging device comprises photoelectric conversion devices arrayed in rows and columns, the foregoing variable gain amplifier provided for each of the columns and connected to outputs of the photoelectric conversion devices of each column, and an analog/digital conversio
Innotech Corporation
Lorusso, Loud & Kelly
Luu Thanh X.
Yam Stephen
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