JPH03220973A - Image sensor - Google Patents

Abstract

PURPOSE:To accurately obtain a scanning output by making all time constants less than twice as long as mutual time intervals where 1st diodes turn on in order in response to a saw-tooth wave. CONSTITUTION:All the time constants tau1-tau3 shown by equations I are made less than twice as long as the mutual time intervals where the 1st diodes Da1-Da3 turn on in order in response to the saw tooth wave. Here, Ca is the equivalent capacity of the 1st diodes Da1-Da3, Cb the equivalent capacity of 2nd diodes Db1Db3, Ra the value of 1st resistances Ra1-Ra3, and Rb the value of 2nd resistances Rb1-Rb3. Thus, the time constants are set and then the potentials at the connection points P1-P3 of photodiodes S1-S3 increase in rising speed, so that the separation of current components of the photodiodes S1-S3 obtained on a common current output line 3 is improved. Consequently, the outputs of the photodiodes S1-S3 can accurately be extracted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image sensor that sequentially scans a plurality of photodiodes based on a sawtooth voltage.

[Prior art and problems to be solved by the invention] Image sensors include multiple photoelectric conversion elements for exchanging optical information into electrical signals, and select electrical signals by electrically scanning the multiple photoelectric conversion elements. It has an analog switch for obtaining For example, the analog switch is disclosed in Japanese Patent Application Laid-Open No. 1983-
As disclosed in Japanese Patent No. 2377, it is made of a field effect transistor (FET) and is arranged near a plurality of photoelectric conversion elements.

By the way, in an image sensor with an integrated circuit configuration,
The width of one photoelectric conversion element, that is, one pixel (for example, 125
One field effect transistor must be placed to fit within .mu.m).

However, it is not easy to form a field effect transistor to fit within such an extremely narrow width, and the three wiring conductor layers for the drain, source, and gate of the field effect transistor are placed in predetermined locations on the substrate. When the three wiring conductor layers are provided within the same width, the width of the three wiring conductor layers inevitably becomes narrower, and the yield of manufacturing the image sensor becomes lower.

In order to solve this kind of problem, a method of scanning photodiodes using a series circuit of diodes was proposed in Japanese Patent Application No. 190848/1984 and Japanese Patent Application No. 198279/1999, which is an application claiming domestic priority. Disclosed. However, a specific technique for obtaining accurate output is not disclosed.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an image sensor that can accurately obtain a scanning output.

[Means for solving the WA problem] The present invention for achieving the above object will be described with reference to the reference numerals in the drawings showing the embodiments. It is a circuit in which a voltage source 1 for supplying a voltage and a plurality of first diodes Da1 to Da3 each having a first electrode and a second electrode are connected in series, one end of which is connected to the voltage source 1. each of the first diodes Da1 to Da3 has a directionality such that the forward current of each of the first diodes Da1 to Da3 flows based on the sawtooth wave; a first series circuit in which the first t[i of Da1 to Da3 is arranged on the side of the voltage source 1; and a first series circuit, each of which has a first resistor Ral to Ra3;
and second diodes Db1 to Db3 are connected in series, each of the first diodes Da1 to D
a3 respectively connected between the second electrode and the other end of the voltage source 1, and each second diode Db
Each of the second diodes Db1 has a directionality such that a forward current of 1 to Db3 flows based on the sawtooth wave.
~Db3 has a plurality of second series circuits, and a plurality of second series circuits each connected between the second electrode of each of the first diodes Da1 to Da3 and the other end of the voltage source 1. 2 resistors Rb1 to Rb3 and the first resistor Ra1
A plurality of photodiodes 81 to S3 each connected with reverse bias directionality between connection points P1 to P3 between ~Ra3 and the second diodes Db1 to Db3 and the common current output line 3. and a current-voltage conversion circuit 2 connected between the common current output line 3 and the other end of the voltage source l, the equivalent capacitance of the first diodes Da1 to Da3 is Ca, Said second
The equivalent capacitance of the diodes Db1 to Db3 is cb, the value of the first resistor Ra1 to Ra3 is Ra, and the second resistor Rb1 to R
When the value of b3 is Rb, τ1 = Ca - Rb r2 = (Ca cb / (Ca + cb))
- all of the time constants represented by Raτ3=Cb-Ra are smaller than twice the mutual time interval during which the first diodes Da1 to Da3 sequentially turn on in response to the sawtooth wave; This is related to an image sensor.

[Function] When the time constant is set according to the present invention, the rising speed of the potential at the connection points P1 to P3 of the photodiodes 81 to S3 becomes faster, and the plurality of photodiodes 31 to S3 obtained on the common current output line 3 The separation of current components is improved.

[Embodiment] A one-dimensional image sensor according to an embodiment of the present invention shown in FIG. 1 includes a voltage source 1, four unit circuits KO, K1, K2, K3 corresponding to four pixels, that is, bits, and a current - voltage conversion circuit 2.

This -dimensional image sensor is configured to be able to detect more than four pixels. but,
Since it is difficult to show the entire configuration of this -dimensional image sensor in the drawing, only the -part thereof is shown in FIG.

1, K2, and K3 are the first
diodes Da1, Ca2, Ca3, second diodes Db1, Db2, Db3, and first resistor Ra1.
Ra2, Ra3, and the second resistor Rb1. Rb2, Rb
3, photodiodes S1, S2, S3 for light detection, and blocking diodes Del, Ca2, DC3. Another unit circuit KO placed between the pressure source 1 and the unit circuit 1 is isolated from the second diode DbO, the first resistor RaO, the photodiode SO, and the blocking diode DCO. . The unit circuit KO has a first diode Dal, Ca2, Ca3 in 1, K2, K3 and a second resistor Rb1, R in another unit circuit.
b2, does not have a counterpart to Rb3, but
Unit circuits KO can also be connected to other unit circuits Kl, K2, K3 corresponding to the first diode and second resistor. Furthermore, the first stage unit circuit KO can be omitted from the image sensor shown in FIG. 1 if necessary.

Three first diodes Da1. have an anode (first electrode) and a cathode (second electrode). Ca2, Ca
One end (left end) of the circuit in which the 3 are connected in series with each other (first series circuit) is connected to one end of the voltage source 1. 1st
The diodes Dal, Ca2, and Ca3 have directionality that is biased in the forward direction by the voltage of the voltage source 1.

That is, the anodes (first voltage [i+) of the first diodes Da1 to Da3 are arranged on the voltage source 1 side. Note that 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.

Cathode of the first diode Dal, Ca2, Ca3 (
between the second electrode) and the other end (ground) of the voltage source 1 are first resistors Ral, Ra2, Ra3 and second diodes Db1. A circuit (second series circuit) in which Db2 and Db3 are connected in series is connected to each other. In the unit circuit KO, a series circuit of a first resistor RaO and a second diode DbO is connected between one end of voltage #1 and the @ end. Second diode DbO, DbL
Db2 and Db3 have the directionality of being biased in the forward direction by the voltage of the voltage source 1.

First resistors Ra01Ral, Ra2, Ra3 and second diode D in each unit circuit KO1K1, K2, K3
bO1Db1. Interconnection point PO5P1 of Db2 and Db3
, P2, P3 have photodiodes so, si, S2, S
3 and blocking diode DcO1Dc1, Ca2,
A series circuit (third series circuit) with Ca3 is connected. That is, the cathodes of photodiodes SO to S3 are at point P.
Connected to O~P3, the anode is photodiode SO
Blocking diode DCO1Dc1.~S3 to prevent mutual interference. It is connected to a common current output line 3 via Ca2 and Ca3.

The current-voltage conversion circuit 2 includes an operational amplifier 4 and a feedback resistor R.
f and a feedback capacitor Cf.

The inverting input terminal of the operational amplifier 4 is connected to the common current output fIA3, the non-inverting input terminal is connected to the other end (ground) of the voltage source 1, and the feedback resistor Rf and capacitor Cf are connected to the inverting input terminal and the output terminal. connected between. Therefore, the photodiode s.

~S3 are substantially connected in parallel to the second diodes DbO~Db3. Also photodiode s.

~S3 are connected so as to be reverse biased by the voltage of voltage source 1. For this reason, photodiodes SO~S3
The current flowing through is extremely small.

Note that 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 shown in FIG. 1 are as follows.

The voltage source 1 is comprised of a sawtooth wave generating circuit and periodically generates a sawtooth wave or sweep signal as shown in FIG. The maximum amplitude width of the sawtooth wave in FIG. 2 is set to a value that can turn on all the first and second diodes Da1 to Da3 and DbO to Db3 in FIG. 1.

Photodiodes SO to S3, first diode Da1
~Da3, the second diodes DbO~Db3, and the blocking diodes DcO~Dc3 are each PIN junction diodes, and include a hydrogenated amorphous silicon semiconductor layer, one electrode layer provided below this semiconductor layer, The other electrode layer is provided on the upper side of the semiconductor layer, and is provided on a common insulating substrate (not shown).

Note that the first and second diodes Dal ~Da3, D
The surface area of bO~Db3 is 3×10′ cd, the thickness is 0.3 μm, and it has an equivalent capacitance of 97 pF.

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

When the first diodes Da1 to Da3 and the second diodes DbO to Db3 are turned on, the voltage across them, that is, the forward voltage Vf is approximately 1V. First resistance RaO~R
a3 are each 30Ω, and the second resistors Rb1 to R
b3 are respectively 30Ω and these are T i O2
, T a S i O 2 or NiCr.

[Operation] In the image sensor shown in FIG. 1, when the sawtooth wave shown in FIG. 2 is generated from the voltage source 1, the first diode Da
1 to Da3 become conductive in sequence.

As the slope voltage of the sawtooth wave gradually increases, the potential VpO at point PO gradually increases as shown in FIG. 4(A). As a result, the potential VpO at point PO becomes the second potential of the unit circuit KO.
When the forward voltage Vf of the diode DbO reaches the forward voltage Vf of the diode DbO, the diode DbO turns on, and the potential VpO at the point PO becomes a substantially constant value (approximately Vf), that is, a saturation voltage value. Almost at the same time as the second diode DbO of the unit circuit NIKO is turned on, the first diode Da1 of the unit circuit is also turned on. During the period in which the first diode Dal is non-conducting (off state) in unit cycle #IK1, the cathode of the first diode Dal is at approximately zero volts, but when the first diode Da1 is in the on state, the voltage is further increased. voltage source l
When the voltage Vd of the first diode Da1 increases, the cathode voltage of the first diode Da1 increases following the voltage Vd.

That is, when the first diode Da1 is turned on, the voltage across it is almost fixed to the forward voltage Vf, so a voltage obtained by subtracting the forward voltage Vf of the diode Da1 from the power supply voltage Vd is applied across the resistor Rb1. . Further, during a period in which the second diode Db1 in the unit circuit is non-conductive, the voltage becomes approximately equal to both f4 voltages of the unit and second resistor Rbl at the point P1. Therefore, after the first diode Da1 is turned on, the potential Vp1 at the point P1 or the potential Vpl at the six points P1, which gradually increases as shown in FIG.
When the forward voltage Vf of the diode Dt11 reaches the forward voltage Vf of the diode Dt11, it turns on, and the potential Vp1 at the point P1 becomes an almost constant value (
Vf). 1 second diode Db in the unit circuit
Unit times FI almost at the same time as switching to ON state of 1? IK2
The first diode Da2 turns on, and the point P2
As shown in FIG. 4(A), a potential Vp2 is obtained.

When the slope voltage of the sawtooth wave supplied from the voltage source 1 further increases, the first diode Da3 of 3 is connected to the unit circuit.
changes to the on state, and the potential Vp of FIG. 4 (A>
3 is obtained. When the potentials VpO to Vp3 of points PO to P3 change sequentially as shown in FIG. 4(A), each point PO to
Photodiodes SO to S3 connected between P3 and the ground via the current-voltage conversion circuit 2 are sequentially driven. That is, the photodiodes SO to S3 are electrically scanned.

In the circuit of FIG. 1, photodiodes SO to S3 are arranged one-dimensionally. This photodiode SO
~ When reading optical information in S3, first, the first diodes Da1 to Da3 and the second diodes DbO to Db
The voltage that can turn on all 3 is the voltage source l.
Generate from. Note that the first diodes Da1 to Da
The voltage for turning on all of the diodes DbO to Db3 and the second diodes DbO to Db3 can be given in the form of a sawtooth wave as shown in FIG. That is, the maximum value of the signal wave and the voltage value in the vicinity are the same as those of the first and second diodes Da1 to Da.
3 and all of DbO to Db3 can be turned on.

During the period when all of the first diodes Da1 to Da3 and the second diodes DbO to Db3 are in the on state, the point P
The forward voltage Vf of the second diode DbO-Db3 obtained at O-P3 causes each photodiode, the ode SO-S
3 is reverse biased and a capacitance CS, equivalently shown in FIG. 3, is charged. Note that the equivalent capacitance Cs
is extremely small, so the blocking diode DCO~
The equivalent capacitance Cs can be charged by a small current in the region before the point where the forward current of Dc3 rises steeply.

When an optical signal obtained from an object (not shown) such as a facsimile document placed opposite to the image sensor in Fig. 1 is input to the photodiodes SO to S3, the presence or absence and size of the optical signal are detected. As a result, the amount of charge in the equivalent capacitance CS of the photodiodes SO to S3 changes. That is, among the photodiodes SO to S3, the equivalent capacitance CS is discharged in the one to which the optical signal is input, and the equivalent capacitance CS is not discharged in the one to which the optical signal is not input. equivalent capacitance C
The amount of discharge of s changes depending on the amount of light.

There are two methods for providing optical input to the photodiodes SO to S3. One of them is photodiode SO~S3
One method is to always provide optical input to the input device, and the other method is to provide optical input only during a predetermined period (for example, a period when the voltage Vd of the voltage source 1 is 0 volts).

When the voltage Vd of the voltage source 1 increases linearly with time as shown in FIG. 2, the potentials VpO1Vp1. VO2 and VO2 are obtained, which sequentially reverse bias the photodiodes SO to S3. In other words, a voltage for charging the equivalent capacitance CS shown in FIG. 3 is applied to the photodiodes SO to S3. At this time, the photodiode S
Among the equivalent capacitances Cs of O to S3, a charging current flows to those that are discharged due to optical input, but no charging current flows to those that are not discharged due to no optical input. Equivalent capacitance C of photodiode SO~S3
The charging current of s is the blocking diode DCO~DC3
and the current-voltage conversion circuit 1i2, the voltage Vout at the output terminal 5 changes depending on whether charging is flowing or not. In a state where each equivalent capacitance CS is discharged because optical input is given to all four photodiodes SO to S3, the potential VpO to shown in FIG. 4(A)
When Vp3 is sequentially applied to the photodiodes SO to S3, the voltage vout at the output terminal 5 changes each time a charging current flows through the photodiodes SO to S3, as shown in FIG. 4(B). That is, the potentials VpO to V at each point PO to P3
As p3 increases, the charging current of the equivalent capacitance Cs increases, and when the potentials VpO to Vp3 at each point PO to P3 are saturated, the charging current decreases, and an output voltage Vout corresponding to this change in charging current is obtained.

FIG. 4(C) shows four photodiodes 5O1S1,
No optical input is given to 82 of S1, S3, SO, S
When it is shown that the voltage Vout at the output terminal 5 changes when optical input is applied only to S3, no charging current flows to the photodiode S2, that is, as shown in FIG.
A change in the output voltage Vout corresponding to the potential Vp2 shown in A) does not occur.

By the way, the first diodes Da1 to Da3 and the second diodes DbO to Db3 have equivalent capacitances.

That is, taking the unit cycle NK1 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, potential changes at 21 points are affected by the following three time constants.

τ1 = Ca - Rb? -2= (Ca Cb / (Ca + Cb))
・Raτ3 = Cb −Ra In this example, the first and second resistors RaO to Ra3, R
bl to Rb3 are each set to 30 Ω, and the equivalent capacitances Ca and Cb of the first and second diodes Da1 to Da3 and DbO to Db3 are each set to 97 pF. This makes the time constant smaller than twice (5 μs) the mutual time interval (average readout time interval) of 2.5 μs between the current changes corresponding to the plurality of photodiodes SO to S3 in the current Iout obtained at the common current output +i13. Become.

FIG. 6(B)(C)(D) shows the output current l0ut and 10 to 9 in response to the sawtooth voltage Vd shown in FIG. 6(A).
Figure 6 (B) shows the relationship between the potential Vp at three points and the time constant τ3, that is, Cb = 32.3 pF, Ra = 30, Ω,
Figure 6 (C) shows the change in multi-value when v3 = 0.97 μs, where Cb = 97 pF, Ra = 30, Ω, τ3 =
Figure 6 (D) shows the change in multi-value at 2.9 μs.
b=323pF, Ra=30Ω, r3=9.7μ
It shows the change in multi-values when s.

Three types of capacitance 32 of second diodes DbO to Db3.
3pF, 97pF, and 323pF are the area of this p1n junction (almost the same as the surface area) lXl0-3aJ, 3X10-
3cd, lXl0-2-. Vp, Iout in Fig. 6 (B) (C) (D)
The measurement was carried out under the condition that optical input was applied to all photodiodes SO to S3 as in FIG. 4(B).

When the sawtooth voltage Vd shown in FIG. 6(A) is generated in synchronization with zero on the horizontal axis (time width) in FIGS. 6(B) to (D), the potentials VpO to Vp3 at points 10 to 93 are generated. rise one after another.

Each of the potentials VpO to Vp3 cannot rise faster than the sawtooth wave speed shown in FIG. 6(A). In addition, Fig. 6 (A
) is shown more compressed than the voltage Vp on the vertical axis in FIGS. 6(B) to (D). If the second diodes DbO to Db3 do not have an equivalent capacitance cb, it depends on the voltage-current rise characteristics of the diodes DbO to Db3 and the rise speed of the sawtooth voltage Vd.
The potential Vp at ~93 points changes. However, in reality, since the second diodes DbO to Db3 have the equivalent capacitance Cb, the potentials at points 10 to 93 rise following the charging of the equivalent capacitance cb. The rising speed of the potentials VpO to Vp3 at points 10 to 93 depends on the time constant τ3=Ra−Cb. Figure 6 (B
) As shown in (C), when the time constant τ3 is relatively small, the amount of current change corresponding to the photodiode SO~S3 of the output current I out becomes large, and the photodiode SO~
M (separation of dots) is improved by the output of S3.

On the other hand, as shown in FIG. 6(D), when the time constant becomes 3, the rising speed of the potentials VpO to Vp3 at points 10 to 93 becomes slow, and the rising speed of the potentials Vpo to Vp3 at points 10 to 93 becomes long before the saturation point. starts, the rising periods of the potentials VpO to Vp3 overlap more, and as a result, the separation of the components (dot components) corresponding to the photodiodes SO to S3 in the output current Iout becomes worse.The first diode Da1
Since the average readout time interval corresponding to the time interval in which Da3 is turned on sequentially is 2.5 μs in this embodiment, the average readout time constant in the case of FIGS. 6(B), (C), and (D) The ratio to the time interval is 0.39 times, 1.2 times, 3
．． It is 9 times more. It has been confirmed that when this ratio is twice or less, the separation of the photodiode SO to S3 components (dot components) of the output current Iout is good. In addition,
It is even better if this is 1.5 times or less.

I just thought about the time constant τ3, but the time constants τ1, τ2,
All of τ3 are now relative to the average readout time interval, and the time constant τ
3, it is desirable that the time constants τ1, τ2, τ3 are all less than or equal to twice the average readout time interval. In this example, Ra = Rb = 30Ω, Ca = Cb = Since it is 97 pF, τ1 = 2.9 μs τ2 = 1.5 μs τ3 = 2.9 μs.

As described above, according to this embodiment, the photodiode SO~
Sequential driving (scanning) of S3 can be performed by a diode without using a transistor. Compared to field effect transistors, diodes are easier to manufacture because they do not require a gate electrode.
Manufacturing yield is significantly improved. Note that when the switch element is constituted by a field effect transistor, it is necessary to set the width of the wiring conductor to about 10 μm.

Further, by setting the time constant of each part to twice the average readout time interval or less, it becomes possible to reliably separate the dot component in the common output current l0ut.

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

(1) Producing diodes DcO to DC3 to prevent mutual interference of photodiodes SO to S3 are connected in series to first resistors RaO to Ra3, or second resistors Rb1 to Rb3 and first resistor are connected in series. It is possible to connect between Ra1 to Ra3.

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

In FIG. 7, n unit circuits corresponding to 1 to 3 are added to the unit circuit in FIG. 1, or m circuit blocks 81 to B2...
- Divided into Bn+. Each circuit block 81~Bn
+ includes several to several tens of units corresponding to 1 to 3 in the unit circuit of FIG. 1, and corresponds to a circuit in which the voltage source 1 is omitted from the image sensor circuit of FIG. Each circuit block B1 to Bl is connected to a voltage source 1a via a multiplexer 10. The output terminals of each circuit block 81-Bn are commonly connected via amplifiers A1-AI.

The voltage source 1a is a sawtooth wave (triangular wave) shown in FIG. 8(A).
occurs repeatedly. The multiplexer 10 is shown in FIG.
) As shown in (C), the sawtooth wave of FIG. 8(A) is distributed to circuit blocks 81 to Bn. Each circuit block 81
The optical input to each photodiode of ~B- is shown in Figure 8 (
Always give as shown in D>.

9 and 10 show another method of driving the image sensor. In FIG. 9, just as in FIG. 7, n units corresponding to 1 to 3 are added to the unit circuit in FIG. 1. The circuit is divided into m circuit blocks 81 to Bn. Each of the circuit blocks 81 to BIN is connected to the voltage source 1, respectively. The voltage source 1 in FIG. 9 generates a sawtooth wave shown in FIG. 10 (A>) in the same way as that in FIG.
As shown in FIGS. 0 (B) and (C), circuit blocks 81 to Bn
are supplied at the same time. As a result, each circuit block 81~
Scanning starts at the same time in Bl, and output occurs at the same time. The output of each circuit block 81-B^ is sent to a signal processing circuit 11 including a memory. The signal processing circuit 11 arranges the outputs of the circuit blocks 81 to BIl on a common time axis so as to correspond to the arrangement order of the circuit blocks 81 to BIl. In addition,
In the image sensor of FIG. 9, as shown in FIG. 10 (D>), optical output is given to the photodiode during a period when the drive voltage Vd is zero.

(3) The sawtooth wave can be a stepped sawtooth wave as shown in FIG. 11, or a sawtooth wave that increases in a quadratic curve as shown in FIG.

(4) The polarity of each diode and the polarity of the voltage source 1 can also be reversed.

(5) In the example, diodes Da1 to Da3, DbO
~ Although hydrogenated amorphous silicon (amorphous silicon) was used as Db3, amorphous silicon carbide or the like may also be used.

(6) Diodes Da1 to Da3, DbO to Db3,
DcO to Dc3 may be any of PIN, PI, IN, Schottky junction diodes, etc.

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

(8) In FIG. 1, the value of the second resistor may be set to gradually increase from Rb1 to Rb3. That is, it may be set to Rbo<Rbl<Rb2<Rb3.

(9) As shown in Figure 13, the current-voltage conversion circuit 2
A load resistor R1 is connected between the common current output line 3 and the ground.
It may also be configured such that this voltage is extracted via the AC component extraction capacitor C.

[Effects of the Invention] As described above, according to the present invention, an image sensor with a simple configuration can be provided. Furthermore, it becomes possible to reliably extract the output of the photodiode.

[Brief Description of the Drawings] Fig. 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, Fig. 4 (A)
is a diagram showing the potential change at each point PO to P3 of the circuit in Figure 1, and Figure 4 (B) is a diagram showing the voltage change at the output terminal of the circuit in Figure 1.
Figure 4 (C) shows the voltage change at the output terminal of the circuit in Figure 1 when all three photodiodes receive optical input.
Figure 5 is a diagram to explain the time constant of the unit circuit; Figure 6 (A) is a waveform diagram showing sawtooth voltage. , Figure 6(B) (
C) (D) is a diagram showing the relationship between changes in time constant and changes in PO to P3 point potential and output current. Figure 7 shows the principle of the photodiode drive method when the number of unit circuits is large. 8 is a block diagram showing the state of each part in FIG. 7, and FIG. 9 is a block diagram showing the principle of the photodiode drive method when there are many unit circuits, similar to FIG. 7. FIG. 10 is a diagram showing the state of each part in FIG. 9, FIGS. 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-Da3... Diode, R
al~Ra3...first resistance, Rb1~Rb3...
Second resistor, 81-S3... Photodiode, 2.
...Current-voltage conversion circuit.

Claims (1)

[Claims] [1] A plurality of voltage sources each having a voltage source (1) for supplying a sawtooth wave that increases or decreases continuously or stepwise with time, and a first electrode and a second electrode. A circuit in which first diodes (Da1 to Da3) are connected in series, one end of which is connected to the voltage source (1),
In addition, each of the first diodes (Da1 to Da3) has a directionality such that the forward current of each of the first diodes (Da1 to Da3) flows based on the sawtooth wave.
has, and each first diode (Da1 to D
a3) a first series circuit in which the first electrode is placed on the side of the voltage source (1); a first resistor (Ra1 to Ra3) and a second diode (Db1 to Db3), respectively; It consists of a circuit in which the first diodes (Da1 to Da3) are connected in series.
and the other end of the voltage source (1), and each second diode (D
The respective second diodes (
Db1 to Db3) have a plurality of second series circuits, and between the second electrode of each of the first diodes (Da1 to Da3) and the other end of the voltage source (1), respectively. A plurality of connected second resistors (Rb1 to Rb3), a connection point (P1 to P3) between the first resistor (Ra1 to Ra3) and the second diode (Db1 to Db3), and a common current output a plurality of photodiodes (S1 to S3) each connected with a reverse bias direction between the common current output line (3) and the voltage source (1); In an image sensor comprising a current-voltage conversion circuit (2) connected between the other end, the equivalent capacitance of the first diode (Da1 to Da3) is Ca, and the equivalent capacitance of the second diode (Db1 to Db3) is Ca. Cb is the equivalent capacitance of , and Ra is the value of the first resistance (Ra1 to Ra3).
, the value of the second resistors Rb1 to Rb3) is Rb, τ1=Ca・Rb τ2={CaCb/(Ca+Cb)}・Raτ3=Cb
characterized in that all of the time constants represented by Ra are smaller than twice the mutual time interval during which the first diodes (Da1 to Da3) sequentially turn on in response to the sawtooth wave; image sensor.