JPH0646761B2 - Image sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor of a type in which a plurality of photodiodes are sequentially scanned based on a sawtooth voltage.

[Problems to be Solved by the Related Art and Invention] An image sensor selects a plurality of photoelectric conversion elements for exchanging optical information with electric signals and a plurality of photoelectric conversion elements that are electrically scanned to select an electric signal. And an analog switch for obtaining the same. The analog switch is composed of a field effect transistor (FET) as disclosed in JP-A-63-2377, for example, and is arranged in the vicinity of a plurality of photoelectric conversion elements.

By the way, in an image sensor having an integrated circuit configuration,
The width of one photoelectric conversion element, that is, one pixel (for example, 125
One field effect transistor must be arranged so that it falls within the range of (μm). However, it is not easy to form a field effect transistor so as to fit in such an extremely narrow width. Further, when the three wiring conductor layers for the drain, the source and the gate of the field effect transistor are provided within a predetermined width on the substrate, the width of the three wiring conductor layers is inevitably narrowed. Yield of manufacturing was low.

In order to solve this kind of problem, a method of scanning a photodiode using a series circuit of diodes is disclosed in Japanese Patent Application No. 63-190848 and Japanese Patent Application No. 1-198279, which is the domestic priority claim application. It is disclosed. However, no specific technique for accurately obtaining the output is disclosed.

Therefore, an object of the present invention is to provide an image sensor capable of accurately obtaining a scan output.

[Means for Solving the Problems] The present invention for achieving the above object will be described with reference to the reference numerals of the drawings showing the embodiments. A sawtooth wave that continuously or stepwise increases or decreases with time is supplied. Is a circuit in which a plurality of first diodes Da1 to Da3 each having a first electrode and a second electrode are connected in series, and one end of which is connected to the voltage source 1. And each of the first diodes Da1 to Da3 has a directivity such that the forward current of each of the first diodes Da1 to Da3 flows based on the sawtooth wave, and each of the first diodes Da1 to Da3. To Da3, a first series circuit in which the first electrodes are arranged on the side of the voltage source 1, a first resistor Ra1 to Ra3 and a second diode Db1 respectively.
To Db3 connected in series, each connected between the second electrode of each of the first diodes Da1 to Da3 and the other end of the voltage source 1, and each second diode. A plurality of second series circuits in which the respective second diodes Db1 to Db3 have a directivity such that the forward current of Db1 to Db3 flows based on the sawtooth wave, and the respective first diodes Da1. To Da3, a plurality of second resistors Rb1 to Rb3 connected between the second electrodes and the other end of the voltage source 1, respectively, and the first resistor R.
a1 to Ra3 and the second diodes Db1 to Db3 connecting points P1 to P3 and the common current output line 3 are reverse biased and have a plurality of photodiodes S1 to S1 respectively connected to each other with directivity. In the image sensor including S3 and the current-voltage conversion circuit 2 connected between the common current output line 3 and the other end of the voltage source 1, the equivalent capacitance of the first diodes Da1 to Da3 is Ca. , Cb is the equivalent capacitance of the second diodes Db1 to Db3, and the first resistors Ra1 to R
When the value of a3 is Ra and the values of the second resistors Rb1 to Rb3 are Rb, all of the time constants represented by τ1 = Ca · Rb τ2 = {CaCb / (Ca + Cb)} · Ra τ3 = Cb · Ra The image sensor is characterized in that the first diodes Da1 to Da3 are smaller than twice the mutual time interval in which the first diodes Da1 to Da3 are sequentially turned on in response to the sawtooth wave.

[Operation] When the time constant is set in accordance with the present invention, the rising speed of the potential at the connection points P1 to P3 of the photodiodes S1 to S3 becomes faster, and the plurality of photodiodes S1 to S3 obtained on the common current output line 3 are obtained. Separation of the current component of is improved.

[Embodiment] The one-dimensional image sensor according to the embodiment of the present invention shown in FIG. 1 includes a voltage source 1, four unit circuits K0, K1, K2, K3 corresponding to four pixels or bits, and a current. −
The voltage conversion circuit 2 is included. This one-dimensional image sensor is configured to be able to detect a number of pixels greater than four. However, since it is difficult to show the entire structure of this one-dimensional image sensor in the drawing, only a part thereof is shown in FIG.

The three unit circuits K1, K2, and K3 that are identical to each other are
Diodes Da1, Da2, Da3 of the second diode D
b1, Db2, Db3, first resistors Ra1, Ra2, Ra3, second resistors Rb1, Rb2, Rb3, photodetection photodiodes S1, S2, S3, and blocking diode Dc.
It consists of 1, Dc2 and Dc3. Another unit circuit K0 arranged between the voltage source 1 and the unit circuit K1 includes a second diode Db0, a first resistor Ra0, a photodiode S0, and a blocking diode Dc0. The unit circuit K0 is the first of the other unit circuits K1, K2, K3.
Of the diodes Da1, Da2, Da3, and the second resistor Rb1,
It does not have one corresponding to Rb2 and Rb3. However, the unit circuit K0 can also be connected to the unit diodes K1, K2, and K3 corresponding to the first diode and the second resistor. Further, the unit circuit K0 of the first stage can be omitted from the image sensor of FIG. 1 if necessary.

One end (left end) of a circuit (first series circuit) in which three first diodes Da1, Da2, Da3 having an anode (first electrode) and a cathode (second electrode) are connected in series is It is connected to one end of the voltage source 1. The first diodes Da1, Da2, Da3 have a directionality in which they are forward biased by the voltage of the voltage source 1. That is, the anodes (first electrodes) of the first diodes Da1 to Da3 are arranged on the voltage source 1 side. When the upper terminal of the voltage source 1 is negative, the cathodes of the first diodes Da1 to Da3 are arranged on the voltage source 1 side.

Between the cathodes (second electrodes) of the first diodes Da1, Da2, Da3 and the other end (ground) of the voltage source 1, the first resistors Ra1, Ra2, Ra3 and the second diodes Db1, Db2,
Circuits (second series circuits) in which Db3 and Db3 are respectively connected in series are connected. In the unit circuit K0, a series circuit of a first resistor Ra0 and a second diode Db0 is connected between one end and the other end of the voltage source 1. The second diodes Db0, Db1, Db2, Db3 have a directionality in which they are forward biased by the voltage of the voltage source 1.

The first resistors Ra0, Ra1, Ra2, Ra3 and the second diodes Db0, Db in each unit circuit K0, K1, K2, K3.
A series circuit (third series circuit) of photodiodes S0, S1, S2, S3 and blocking diodes Dc0, Dc1, Dc2, Dc3 is connected to interconnection points P0, P1, P2, P3 of 1, Db2, Db3. ing. That is, the cathodes of the photodiodes S0 to S3 are connected to the points P0 to P3, and the anodes thereof are blocking diodes Dc0, Dc1, Dc2, Dc3 for preventing mutual interference of the photodiodes S0 to S3.
Is connected to the common current output line 3 via.

The current-voltage conversion circuit 2 includes an operational amplifier 4 and a feedback resistor R.
f and a feedback capacitor Cf. Operational amplifier 4
The inverting input terminal of is connected to the common current output line 3, the non-inverting input terminal is connected to the other end (ground) of the voltage source 1, and the feedback resistor Rf and the capacitor Cf are provided between the inverting input terminal and the output terminal. It is connected. Therefore, the photodiodes S0 to S3 are substantially connected in parallel to the second diodes Db0 to Db3. In addition, the photodiodes S0 to S3
Are connected so as to be reverse biased by the voltage of the voltage source 1. Therefore, the current flowing through the photodiodes S0 to S3 is extremely small.

An inverting amplifier 4a is connected between the current-voltage conversion circuit 2 and the output terminal 5.

Details of each part of the image sensor of FIG. 1 are as follows.

The voltage source 1 is composed of a circuit that generates a sawtooth wave, and periodically generates a sawtooth wave, that is, a sweep signal shown in FIG. The maximum amplitude width of the sawtooth wave in FIG. 2 is set to a value capable of turning on all the first and second diodes Da1 to Da3 and Db0 to Db3 in FIG.

The photodiodes S0 to S3 and the first diode Da1 to
Da3, the second diodes Db0 to Db3, and the blocking diodes Dc0 to Dc3 are PIN junction diodes, respectively, and include a hydrogenated amorphous silicon semiconductor layer, one electrode layer provided below the semiconductor layer, and a semiconductor. It is composed of the other electrode layer provided on the upper side of the layer, and is provided on a common insulating substrate (not shown). The surface areas of the first and second diodes Da1 to Da3 and Db0 to Db3 are 3 × 10 ⁻³ cm ² and the thickness is 0.3 μm.
It has an equivalent capacity of F.

Since the photodiodes S0 to S3 are reverse-biased, they are equivalently shown by the parallel circuit of the capacitance Cs and the current source Is proportional to the light intensity shown in FIG. In addition,
Equivalent capacitance Cs of the photodiodes S0 to S3
The value of the current flowing through is extremely small.

The first diodes Da1 to Da3 and the second diode Db0
The voltage across both ends, that is, the forward voltage Vf when Db3 is turned on is approximately 1V. The first resistors Ra0 to Ra3 each have a resistance of 30 kΩ, and the second resistors Rb1 to Rb3 each have a resistance of 3 kΩ.
It is 0 kΩ, and these are formed of a substance such as TiO ₂ , Ta-SiO ₂ or NiCr.

[Operation] In the image sensor of FIG. 1, when the sawtooth wave shown in FIG. 2 is generated from the voltage source 1, the first diode Da1 is generated.
.About.Da3 are sequentially turned on. As the ramp voltage of the sawtooth wave gradually increases, the voltage Vp0 at the point P0 changes as shown in FIG.
It gradually increases as shown in. As a result, when the potential Vp0 at the point P0 becomes the forward voltage Vf of the second diode Db0 of the unit circuit K0, the diode Db0 is turned on,
The voltage Vp0 at the point P0 has a substantially constant value (approximately Vf), that is, a saturated voltage value. The first diode Da1 of the unit circuit K1 is also turned on almost at the same time as the second diode Db0 of the unit circuit K0 is turned on. First of unit circuit K1
In the period in which the diode Da1 is non-conducting (OFF state), the cathode of the first diode Da1 is approximately 0 volt, but the first diode Da1 is in the ON state, and the voltage Vd of the voltage source 1 is higher. Then, the cathode voltage of the first diode Da1 becomes higher following the voltage Vd. That is, when the first diode Da1 is turned on, the voltage across the first diode Da1 is almost fixed to the forward voltage Vf, so that the power supply voltage Vd
A voltage obtained by subtracting the forward voltage Vf of the diode Da1 from is applied to both ends of the resistor Rb1. In addition, the second of the unit circuit K1
While the diode Db1 is non-conducting, the unit of the point P1 becomes substantially equal to the voltage across the second resistor Rb1. Therefore,
After the first diode Da1 is turned on, the point P1
The potential Vp1 of the voltage gradually rises as shown in FIG. When the potential Vp1 at the point P1 becomes the forward voltage Vf of the second diode Db1, this turns on, and the potential Vp1 at the point P1 becomes substantially constant (Vf). Almost at the same time when the second diode Db1 of the unit circuit K1 is turned on, the first diode Da2 of the unit circuit K2 is turned on, and the potential Vp2 is applied to the point P2 as shown in FIG. 4 (A). can get. When the ramp voltage of the sawtooth wave supplied from the voltage source 1 further increases, the first diode Da3 of the unit circuit K3 is turned on, and the potential Vp3 of FIG. 4A is obtained at the point P3. The potentials Vp0 to Vp3 at the points P0 to P3 are the fourth
When it changes sequentially as shown in the figure (A), each point P0-P
The photodiodes S0 to S3, which are connected via the current-voltage conversion circuit 2 between 3 and the ground, are sequentially driven. That is, the photodiodes S0 to S3 are electrically scanned.

In the circuit of FIG. 1, the photodiodes S0 to S3 are arranged one-dimensionally. This photodiode S0
When reading the optical information in S3 to S3, first, the voltage source 1 generates a voltage capable of turning on all the first diodes Da1 to Da3 and the second diodes Db0 to Db3. The voltage for turning on all of the first diodes Da1 to Da3 and the second diodes Db0 to Db3 can be given by a sawtooth wave shown in FIG.
That is, the maximum value of the sawtooth wave and the voltage value in the vicinity thereof can turn on all of the first and second diodes Da1 to Da3 and Db0 to Db3.

The first diodes Da1 to Da3 and the second diode Db0
During the period when all of Db3 to Db3 are on, the points P0 to P3 are
Forward voltage V of the second diodes Db0 to Db3 obtained at
Each photodiode S0 to S3 is reverse biased by f, and the capacitance Cs equivalently shown in FIG. 3 is charged. Since the equivalent capacitance Cs is extremely small, the charging of the equivalent capacitance Cs can be achieved by the minute current in the region before the point where the forward current of the blocking diodes Dc0 to Dc3 rises steeply.

When an optical signal obtained from an object (not shown) such as a facsimile document arranged opposite to the image sensor of FIG. 1 is input to the photodiodes S0 to S3, the presence or absence of the optical signal and the magnitude of the optical signal are dealt with. Then, the charge amount of the equivalent capacitance Cs of the photodiodes S0 to S3 changes. That is, in the photodiodes S0 to S3, the equivalent capacitance Cs is discharged in the one where the optical signal is input, and the equivalent capacitance Cs is not discharged in the one where the optical signal is not input. Equivalent capacitance C
The amount of discharge of s changes with the amount of light. There are two methods of giving an optical input to the photodiodes S0 to S3.
One of them is a method of always giving a light input to the photodiodes S0 to S3, and the other is a method of giving a light input only during a predetermined period (for example, a period when the voltage Vd of the voltage source 1 is 0 volt). is there.

When the voltage Vd of the voltage source 1 linearly increases with time as shown in FIG. 2, potentials Vp0, Vp1, Vp2, Vp3 are obtained at points P0 to P3 as shown in FIG. 4 (A). Thus, the photodiodes S0 to S3 are sequentially reverse biased. In other words, the equivalent capacitance C shown in FIG.
The voltage for charging s is the photodiodes S0 to S3.
Applied to. At this time, the charging current flows to the ones which are discharged by the light input within the equivalent capacitance Cs of the photodiodes S0 to S3, but the charging current flows to the ones which are not discharged because there is no light input. Absent. Since the charging current of the equivalent capacitance Cs of the photodiodes S0 to S3 flows through the blocking diodes Dc0 to Dc3 and the current-voltage conversion circuit 2, the voltage Vout of the output terminal 5 changes depending on the presence or absence of the charging current. In the state where each equivalent capacitance Cs is discharged because the light input is given to all of the four photodiodes S0 to S3, the potentials Vp0 to Vp3 shown in FIG. 4A are sequentially applied to the photodiodes S0 to S3. When applied, the voltage Vout at the output terminal 5 changes each time a charging current flows through the photodiodes S0 to S3, as shown in FIG. 4 (B). That is, as the potentials Vp0 to Vp3 of the points P0 to P3 increase, the charging current of the equivalent capacitance Cs increases, and when the potentials Vp0 to Vp3 of the points P0 to P3 become saturated, the charging current decreases and the change of the charging current. An output voltage Vout corresponding to is obtained.

In FIG. 4 (C), four photodiodes S0, S1,
No optical input is given to S2 of S2 and S3, and S0 and S3
When the change of the voltage Vout of the output terminal 5 when the light input is given only to 1 and S3 is shown, the charging current does not flow in the photodiode S2. That is, the output voltage Vout corresponding to the potential Vp2 shown in FIG. 4 (A) does not change.

By the way, the first diodes Da1 to Da3 and the second diodes Db0 to Db3 have equivalent capacitances. That is, taking the potential circuit K1 as an example, as shown in FIG. 5, the first diode Da1 has a capacitance Ca and the second diode Db1 has a capacitance Cb. Therefore, the potential change at point P1 is affected by the following three time constants.

τ1 = Ca · Rb τ2 = {CaCb / (Ca + Cb)} · Ra τ3 = Cb · Ra In the present embodiment, the first and second resistors Ra0 to Ra3, Rb1 to
Rb3 is set to 30 kΩ respectively, and the equivalent capacitances Ca and Cb of the first and second diodes Da1 to Da3 and Db0 to Db3 are set.
Are set to 97 pF, respectively. As a result, the mutual time interval (average read time interval) of the current changes corresponding to the plurality of photodiodes S0 to S3 in the current Iout obtained in the common current output line 3 is 2.5 μs, which is twice (5 μm).
The time constant is smaller than s).

FIGS. 6 (B), (C) and (D) show the output currents Iout and P0 to P3 in response to the sawtooth wave voltage Vd shown in FIG. 6 (A).
The relationship between the potential Vp at the point and the time constant τ3 is shown. That is, the sixth
The figure (B) shows Cb = 32.3 pF, Ra = 30 kΩ, τ3.
The change in each value when = 0.97 μs is shown in FIG. 6 (C).
Is Cb = 97 pF, Ra = 30 kΩ, τ3 = 2.9 μs
The change of each value at the time of is shown, and FIG. 6 (D) shows Cb = 323.
The change in each value when pF, Ra = 30 kΩ, and τ3 = 9.7 μs is shown.

Three types of capacitance of the second diodes Db0 to Db3 32.3p
F, 97 pF, and 323 pF have an area (substantially the same as the surface area) of this pin junction of 1 × 10 ⁻³ cm ² , 3 × 10 ⁻³ cm ² , 1
It can be obtained by changing to 3 steps of × 10 ^-2 cm ² .
Measurement of Vp and Iout in FIGS. 6 (B), (C), and (D) is performed by using all the photodiodes S0 to S3 as in FIG. 4 (B).
It is performed under the condition that the optical input is given to the.

When the sawtooth voltage Vd shown in FIG. 6 (A) is generated in synchronization with zero on the horizontal axis (time width) of FIGS. 6 (B) to (D), the potentials Vp0 to Vp3 at points P0 to P3 are generated. Start up in sequence. The potentials Vp0 to Vp3 cannot rise faster than the speed of the sawtooth wave in FIG. 6 (A). The vertical axis voltage Vd in FIG. 6 (A) is the vertical axis voltage Vp in FIGS. 6 (B) to (D).
Is shown more compressed than. If the second diodes Db0 to Db3 do not have the equivalent capacitance Cb, the potential Vp at the points P0 to P3 depends on the voltage-current rising characteristics of the diodes Db0 to Db3 and the rising speed of the sawtooth voltage Vd. Changes. However, in reality, since the second diodes Db0 to Db3 have the equivalent capacitance Cb, the equivalent capacitance Cb
The potential at points P0 to P3 rises following the charging of. P0
The rising rate of the potentials Vp0 to Vp3 at the P3 point is the time constant τ3 = Ra.
-Depends on Cb. As shown in FIGS. 6B and 6C, when the time constant τ3 is relatively small, the current change corresponding to the photodiodes S0 to S3 of the output current Iout becomes large, and the outputs of the photodiodes S0 to S3 are separated. (Dot separation) improves.

On the other hand, as shown in FIG. 6 (D), when the time constant τ3 becomes large, the rising speed of the potentials Vp0 to Vp3 at the points P0 to P3 slows down, and rises well before the saturation point of each potential Vp0 to Vp3. Starting, the overlapping of rising periods of the potentials Vp0 to Vp3 increases, and eventually the separation (dot components) corresponding to the photodiodes S0 to S3 in the output current Iout becomes poor. Since the average read time interval corresponding to the time interval in which the first diodes Da1 to Da3 are turned on sequentially is 2.5 μs in this embodiment, it is shown in FIGS. 6 (B) and 6 (C).
In the case of (D), the ratio of the time constant to the average read time interval is 0.39 times, 1.2 times, and 3.9 times. It has been confirmed that when the ratio is 2 times or less, the separation characteristics of the photodiodes S0 to S3 (dot components) of the output current Iout are good. It is even better if this is set to 1.5 times or less.

Now, considering the time constant τ3, the time constants τ1, τ2,
It is desirable that all of τ3 be less than or equal to twice the average read time interval. In this example, Ra = Rb = 30 kΩ and Ca = Cb = 67 pF, so τ1 = 2.9 μs τ2 = 1.5 μs τ3 = 2.9 μs.

As described above, according to this embodiment, the photodiodes S0 to S0
The sequential driving (scanning) of S3 can be performed by a diode without using a transistor. A diode is easier to manufacture than a field effect transistor because a gate electrode is unnecessary. For example, when the bit interval is 125 μm, the width of the wiring conductor can be set to 20 μm or more,
Manufacturing yield is greatly improved. It should be noted that when the switch element is composed of a field effect transistor, the width of the wiring conductor needs to be about 10 μm.

Further, by setting the time constant of each part to be equal to or less than twice the average read time interval, it becomes possible to reliably separate the dot components in the common output current Iout.

[Modification] The present invention is not limited to the above-described embodiments, and the following modifications are possible, for example.

(1) Connecting blocking diodes Dc0 to Dc3 for preventing mutual interference of the photodiodes S0 to S3 in series with the first resistors Ra0 to Ra3, or the second resistor Rb1.
~ Rb3 and the first resistors Ra1 to Ra3 can be connected.

(2) When the number of read pixels of the image sensor according to the embodiment is increased, the drive voltage Vd must be increased accordingly. Therefore, it is desirable to set the maximum number of read pixels to several tens. When the number of pixels is larger than this, the image sensor may be divided into a plurality of blocks and driven.

In FIG. 7, n unit circuits corresponding to the unit circuits K1 to K3 in FIG. 1 are divided into m circuit blocks B1 to B2 ... Bm. Each circuit block B1 to Bm has a first
The circuit includes several to several tens of units corresponding to the unit circuits K1 to K3 in the figure, and corresponds to a circuit in which the voltage source 1 is omitted from the image sensor circuit in FIG. Each circuit block B1 to B
m is connected to the voltage source 1a via the multiplexer 10. The output terminals of the circuit blocks B1 to Bm are commonly connected via the amplifiers A1 to Am. Voltage source 1a
Is generated by repeating the sawtooth wave (triangular wave) shown in FIG. The multiplexer 10 distributes the sawtooth wave of FIG. 8 (A) to the circuit blocks B1 to Bm as shown in FIGS. 8 (B) and (C). The light input to each photodiode of each circuit block B1 to Bm is always given as shown in FIG.

9 and 10 show another driving method of the image sensor. Also in FIG. 9, n unit circuits corresponding to the unit circuits K1 to K3 in FIG. 1 are divided into m circuit blocks B1 to Bm just as in FIG. Each of the circuit blocks B1 to Bm is connected to the voltage source 1, respectively. The voltage source 1 shown in FIG. 9 generates the sawtooth wave shown in FIG. 10 (A) similarly to that shown in FIG. Saw wave is first
0 circuit blocks B1 to Bm as shown in FIGS.
Can be supplied at the same time. As a result, each circuit block B1
At Bm, the scanning starts at the same time and the outputs occur at the same time. The output of each circuit block B1 to Bm is sent to the signal processing circuit 11 including a memory. The signal processing circuit 11 arranges the outputs of the circuit blocks B1 to Bm on the common time axis so as to correspond to the arrangement order of the circuit blocks B1 to Bm. In the image sensor of FIG. 9, as shown in FIG. 10D, the light output to the photodiode is given during the period when the drive voltage Vd is zero.

(3) The sawtooth wave can be a stepwise sawtooth wave as shown in FIG. 11 and a sawtooth wave increasing quadratically as shown in FIG.

(4) The polarities of the diodes and the voltage source 1 can be reversed.

(5) In the embodiment, the diodes Da1 to Da3 and Db0 to Db3 are used.
Hydrogenated amorphous silicon as (amorphous silicon)
However, amorphous silicon carbide or the like can also be used.

(6) Diodes Da1 to Da3, Db0 to Db3, Dc0 to D
c3 may be any of PIN, PI, IN, Schottky junction diode and the like.

(7) A voltage may be applied to the cathode terminals of the diodes Db0 to Db3. That is, a bias voltage can be applied between the diodes Db0 to Db3 and the ground to expand the dynamic range.

(8) In FIG. 1, change the value of the second resistor from Rb1 to Rb.
You may set so that it may become large gradually toward b3.
That is, Rb0 <Rb1 <Rb2 <Rb3 may be set.

(9) As shown in FIG. 13, the current-voltage conversion circuit 2
The load resistance RL between the common current output line 3 and ground.
May be connected, and this voltage may be taken out via the AC component extracting capacitor C.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide an image sensor having a simple configuration. Further, it becomes possible to reliably take out the output of the photodiode.

[Brief description of drawings]

1 is a circuit diagram showing an image sensor according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing a sawtooth wave, FIG. 3 is an equivalent circuit diagram of a photodiode, and FIG. 4 (A) is FIG. Showing potential changes at points P0 to P3 of the circuit of FIG. 4, and FIG. 4 (B) shows voltage changes of the output terminal of the circuit of FIG.
FIG. 4 (C) is a diagram showing a state where light is input to all the two photodiodes. FIG. 4 (C) shows a voltage change at the output terminal of the circuit of FIG.
FIG. 5 is a diagram showing a state where light is inputted to only three of the photodiodes, FIG. 5 is a diagram for explaining a time constant of the unit circuit, and FIG. 6 (A) is a waveform diagram showing a sawtooth voltage. , (B), (C) and (D) of FIG. 6 show changes in the time constant and P0 to P3.
FIG. 7 is a block diagram showing the relationship between the point potential and the change in output current. FIG. 7 is a block diagram showing the principle of the photodiode driving method when the number of unit circuits is large. FIG. 8 is the state of each part in FIG. 9 is a block diagram showing in principle the driving method of the photodiode when the number of unit circuits is large as in FIG. 7, and FIG. 10 is a diagram showing the state of each part of FIG. 11 and 12 are waveform diagrams showing a modification of the sawtooth wave, and FIG. 13 is a circuit diagram showing a modification of the current-voltage conversion circuit. 1 ... Voltage source, Da1 to Da3 ... Diode, Ra1 to Ra3
...... First resistance, Rb1 to Rb3 ...... Second resistance, S1 to S
3 ... Photodiode, 2 ... Current-voltage conversion circuit.

Claims (1)

[Claims] 1. A plurality of first electrodes each having a voltage source (1) for supplying a sawtooth wave which continuously or stepwise increases or decreases with time, and a first electrode and a second electrode. It is a circuit in which diodes (Da1 to Da3) are connected in series, one end of which is connected to the voltage source (1), and the forward current of each first diode (Da1 to Da3) changes to the sawtooth wave. Each of the first diodes (Da1 to Da3) has a directionality such that the first diodes (Da1 to Da3) of the respective first diodes (Da1 to Da3) flow.
Of the first series circuit in which the electrodes are arranged on the side of the voltage source (1), and the first resistors (Ra1 to Ra3) and the second diodes (Db1 to Db3) are connected in series. A second diode (Db1 to Db3) each connected between the second electrode of the first diode (Da1 to Da3) and the other end of the voltage source (1). ), A plurality of second series circuits each having a second diode (Db1 to Db3) having a directivity such that the forward current flows based on the sawtooth wave, and each first diode ( The second of Da1 to Da3)
A plurality of second resistors (Rb1 to Rb3) respectively connected between the electrodes of the first voltage source (1) and the other end of the voltage source (1), the first resistors (Ra1 to Ra3) and the second diode ( Db1 to Db3) and a plurality of photodiodes (S1 to P3) connected to the common current output line (3) and the connection points (P1 to P3) with reverse bias.
S3), and a current-voltage conversion circuit (2) connected between the common current output line (3) and the other end of the voltage source (1), wherein the first diode ( The equivalent capacity of Da1 to Da3) is C
When the equivalent capacitance of the second diode (Db1 to Db3) is Cb, the value of the first resistor (Ra1 to Ra3) is Ra, and the value of the second resistor (Rb1 to Rb3) is Rb, τ1 = CaRb τ2 = {CaCb / (Ca + Cb)} Ra τ3 = CbRa All the time constants represented by the first diode (Da1 to Da3) are sequentially turned on in response to the sawtooth wave. An image sensor characterized in that it is smaller than twice the mutual time interval for switching to a state.