Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-03-10
2003-06-17
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG, C348S294000
Reexamination Certificate
active
06580063
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a solid state imaging device or a solid state image pickup device, and more particularly to a solid state imaging device which uses so-called active pixels and in which an electrical signal after photoelectric conversion can be outputted almost without attenuation.
BACKGROUND OF THE INVENTION
In an active pixel in which an electrical signal after photoelectric conversion is amplified by a transistor disposed in each of the pixels, MOSFET's are generally used as the amplification means and switches. A plurality of pixels each composed of such an active pixel are disposed in a matrix, and a solid state imaging device is constituted thereof.
FIG. 8
is a circuit diagram showing a structure of a conventional solid state imaging device which comprises the above-mentioned active cells. In
FIG. 8
, an example of a circuit structure in which 2×2 pixels are disposed in a matrix is shown for the sake of simplicity.
Each pixel which constitutes an active pixel comprises a photodiode
801
, a reset FET
802
, an FET for amplification or an amplification FET
803
, a row selection FET
804
. An output portion or node of the photodiode
801
is reset by the reset FET
802
. Also, the output portion of the photodiode
801
is coupled to the gate of the FET for amplification
803
. The source of the FET for amplification
803
is connected to each of column signal line
808
via the row selection FET
804
, and the drain of the FET for amplification
803
is connected to a power supply line
807
.
For each column, there are provided a current source FET
809
, and a column selecting FET
812
. Each of the column signal line
808
is coupled to the ground via the current source FET
809
. A bias line
815
biases the gate voltage of the current source FET
809
to an appropriate value such that the current source FET
809
performs a constant current operation. Each of the column signal lines
808
is connected to a signal line
813
via a column selecting FET
812
. The signal line
813
is coupled to an input of an output buffer
814
from which an output signal of the solid state imaging device is outputted. There is also provided a row scanning circuit
810
which accesses each row, by turning on and off the row selection FET
804
, and by turning on and off the reset switches
802
every row. Also, a column scanning circuit
811
is provided which accesses each column, by sequentially turning on and off the column selecting FET's
811
.
In operation, the reset line
806
of a particular row is raised to a high potential level by the row scanning circuit
810
. Thereby, the particular row is accessed, and the reset FET's
802
of all pixels in that row are turned on. Therefore, electric charges are injected into an output portion or node of each of the photodiodes
801
from a power supply line
807
, and the potential of the output portion of each of the photodiodes
801
as photoelectric conversion means becomes approximately equal to a power supply voltage. Subsequently, the reset FET's
802
are turned off and an exposure time period begins to start. Thereafter, a current corresponding to an intensity of light irradiated onto each pixel flows through a photodiode
801
of a corresponding pixel, and electric charges existing at the output portion of each of the photodiodes
801
are gradually extracted to the ground.
After a predetermined time period has elapsed, a row selection line
805
is raised to a high potential level by the row scanning circuit
810
, and the row selection FET's
804
in respective pixels are turned on simultaneously. Thereby, output voltages of the photodiodes
801
appear at respective column signal lines
808
. That is, in each of the pixels, the FET for amplification or the amplification FET
803
and the current source FET
809
constitute a source follower circuit, and therefore a voltage near a gate voltage of the FET for amplification
803
, that is, the output voltage of the photodiode
801
, appears at the column signal line
808
. Here, the bias line
815
supplies a bias voltage of appropriate potential to the gate of each of the current source FET's
809
such that the current source FET's
809
perform constant current operation. In this way, signals of a particular row are outputted and, subsequently, the column selection FET's
812
are sequentially turned on by the column scanning circuit
811
, thereby the signals are readout via the signal line
813
and the output buffer
814
.
The above-mentioned conventional active pixel type solid state imaging device has the following disadvantages. First, a gain of each source follower circuit becomes a value much lower than the ideal value of 100 percent, for example, a typical value of about 75 percent of the ideal value, due to the body effect or backgate effect. Thereby, a signal voltage at the output portion of each photodiode
801
and a signal voltage on the column signal line
808
are not equal to each other, and it is only possible to output an attenuated signal having a small amplitude. In other words, if the output signal of the photodiode ranges from 0 to 3 V, it is only possible to obtain a small signal typically ranging from 0 through 2.3 V on the column signal line
808
. Therefore, the conventional solid state imaging device has a disadvantage that it is vulnerable to external noises.
Also, the conventional active pixel type solid state imaging device has a disadvantage that it is not suitable for use with a low power supply voltage. The reason for this is as follows. In general, a full-scale range of an output signal of a photodiode varies depending on a power supply voltage. That is, if the power supply voltage is high, the full-scale range becomes large, and if the power supply voltage is low, the full-scale range becomes small. This is because, the reset voltage of a photodiode is approximately equal to the power supply voltage, and an output voltage of the photodiode after exposure is equal to or smaller than the reset voltage. Therefore, in case of a conventional solid state imaging device having a source follower configuration, an amplitude of an output signal is small and the amplitude of the output signal becomes smaller as the power supply voltage becomes lower. Thus, usable voltage range of a power supply voltage in which the solid state imaging device is operable as a device is limited.
Further, in general, an AD converter is coupled to a rear stage of a solid state imaging device. However, since the amplitude of an output signal of a conventional solid state imaging device is small, the AD converter must have a high resolution, so that a high performance AD converter is required. Therefore, an overall cost of an imaging apparatus or system becomes disadvantageously high.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an active pixel type solid state imaging device in which a signal after photoelectric conversion can be outputted at approximately a unity gain and deterioration of an amplitude of the signal can be suppressed.
It is another object of the present invention to provide an active pixel type solid state imaging device which is robust against noises.
It is still another object of the present invention to provide an active pixel type solid state imaging device which is suitable for use with a low power supply voltage.
It is still another object of the present invention to provide an active pixel type solid state imaging device in which an increase in the total cost of an imaging system can be suppressed.
According to an aspect of the present invention, there is provided a solid state imaging device comprising: a plurality of active pixels each of which has at least a photodiode and a first amplification transistor for amplifying an output of said photodiode and which are disposed in a matrix having rows and columns; a plurality of row selecting lines disposed corresponding to rows of said plurality of active pixels respectivel
Choate Hall & Stewart
Pyo Kevin
Sohn Seung C
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