JPH03216523A - Photometry circuit - Google Patents

Abstract

PURPOSE:To enable realization of an expensive and compact photo detecting sensor by forming the sensor using a specified analog circuit comprising a diode, a constant current means, a variable voltage generation means, a photoelectric conversion means and a comparison means. CONSTITUTION:This apparatus is made up of diodes D1 and D2, constant current means R1 and R2, a photodiode PD1, a comparison means CP1 and the like. When a power source switch is turned ON, current 2 with I1=I2 is supplied to resistances R1 and R2 from a power source line Vc in proximity by a specified setting of resistances R1-R3. Therefore, current, a half of the D1, per unit area flows to a diode junction of the diodes D2 and D3 and a difference V1-V2 of an anode voltage is multiplied by R4/R3 through a differential amplifier OP1 to be developed in an output voltage Vo to give a voltage Vo=V1 -R4/R3(V2-V1). On the other hand, photocurrent Ip generated from the PD1 converted to a compression voltage through the D4 and developed at an output terminal of the OP2 as Vp. As an output inversion of a comparator CP is Vo=Vp, Ip at this point is given by Ip=I1/2<r>.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a photometric circuit that converts an incident luminous flux into an electrical signal according to intensity, and particularly to a photometric circuit used in cameras, optical measuring instruments, etc.

[Prior Art] In recent years, photometric circuits for optical devices such as cameras use photodiodes to convert the incident light beam into 1! This is converted into a logarithmically compressed voltage. In this case, a diode is used to logarithmically compress the photocurrent, but the reverse saturation current of the diode has temperature characteristics, so to remove this effect, another diode is used in addition to the diode for photocurrent compression. The reverse saturation current generated in the photocurrent compression diode is canceled out by the reverse saturation current generated therein. In a circuit configuration using a bipolar process IC, the position of the diode can be set freely regardless of the power line due to its structure, so conventionally,
Photometric circuits such as those described above have been manufactured using bipolar ICs.

On the other hand, processing circuits such as exposure control circuits and display circuits that receive and process the output of photometric circuits are becoming more digital due to reasons such as noise resistance and ease of processing, and CMOS-ICs, which are easy to manufacture digital ICs, are becoming more and more popular. It is used. Therefore, in optical devices such as cameras, at least two types of ICs, bipolar ICs and CMOS-ICs, have been used.

[Problems to be Solved by the Invention] The above-mentioned bibolar IC has a low integration density of digital circuits, and if a digital processing circuit after digitally converting a photoelectric conversion signal is configured on the same IC chip, the chip area of the IC becomes large. This is causing a cost problem. Although C-MOS process ICs are suitable for digital processing in terms of integration density and cost, they have drawbacks such as the fact that they can only be configured with a diode connected to a power supply line, making them unsuitable for analog processing. On the other hand, recently, BiCMOS process ICs have been used that take advantage of the advantages of both to compensate for these disadvantages. However, the problem of complicated process and high cost remains.

The present invention has been made in view of the above problems, and is based on pure C.
The present invention attempts to provide a photometric circuit that compensates for the shortcomings of CMOS analog circuits by configuring the photometric circuit using an analog circuit that can be realized with a MOS process IC.

[Means and effects for solving the problems] In order to solve the above-mentioned problems, the present invention provides a first diode and a current having a first value per unit area of the diode junction in the forward direction of the first diode. a first constant current means for causing a current to flow, a second diode, and a current having a second value that is a predetermined ratio of the current of the first value to flow per unit area of the diode junction in the forward direction of the second diode; a second constant current means;
variable voltage generating means for varying and amplifying the difference between the respective terminal voltages of the diode and the second diode, and outputting a first voltage added to the terminal voltage of the first or second diode; a photoelectric conversion means including a photodiode that generates a second voltage according to the logarithmic compression value;
and comparing means for comparing the voltage of the first voltage with the first voltage.

[Embodiment] First, a photometric circuit according to a first embodiment of the present invention will be explained based on FIG. This photometry circuit is a circuit that compares a photocurrent generated according to the amount of incident light with a reference current, and is a circuit that detects brightness and darkness. Power supply line Vc is connected to the anode of diode D1 via resistor R1,
The cathode of this diode D1 is connected to earth GND. Further, the power supply line Vc includes a resistor R2 having the same resistance value as the resistor R1, and diodes D2 and D.
It is connected to the earth GND through three parallel circuits. The junction areas of the diode D1 and the diodes D2 and D3 are formed to be equal to each other. The connection point of the above resistor R1 and diode D1 and the differential amplifier OPI
A resistor R3 is connected between the inverting input terminals of the resistor R3, and a non-inverting input terminal thereof is connected to a connection point between the resistor R2 and the anodes of the diodes D2 and D3. Above power line V
c is sufficiently higher than the saturation voltage of the diodes D1 to D3, and the resistance value of R3 is set to a large value so that the current flowing through resistor R3 is sufficiently small. A resistor R4 is connected between the inverting input terminal and the output terminal of the differential amplifier OPI.
is connected.

A photodiode PDI that performs photoelectric conversion into photocurrent according to the amount of incident light is connected between both input terminals of the differential amplifier OP2, the inverting input terminal and the output terminal are directly connected, and the non-inverting input terminal has a logarithmic compression terminal. It is connected to the ground GND via a diode D4.

The outputs of the differential amplifier OPI and the differential amplifier OP2 are respectively connected to the input terminal of the comparator CPI.

In addition, except for the photodiode PDI mentioned above, the CMO is integrated.
It is composed of S-IC.

The operation of the first embodiment of the present invention configured as described above will be explained.

When a power switch (not shown) is turned on, the diode D1 receives current through the resistor R1, and the diodes D2 and D3 connected in parallel receive current through the resistor R2. As mentioned above, the power supply line Vc is sufficiently high with respect to the saturation voltage of the diode, the resistance value of R3 is set large so that the current flowing through resistor R3 is sufficiently small, and (the resistance value of Rl )=(R
Since the resistance value is set to 2), approximately the resistance R1
A current 2×I1=I2 is supplied from the power supply line Vc to the resistor R2 and the resistor R2, respectively. Therefore, diode D
Half the current per unit area of the diode D1 flows through the diode junctions 2 and D3, so that the voltages Vl and v2 at their respective anode terminals are well known. Here, K: Boltzmann constant, T: temperature, q:
Electron charge amount, (b): Represents the saturation current in the opposite direction, respectively. In the above formula, it can be considered that 2XI1=I2, so
The difference between v1 and v2 is KT V2-Vl- 1n2 (3)q.

In the differential amplifier OPI, the value of V2-Vl is R4/R
Since it is multiplied by 3 and appears as the voltage vO of the output terminal, R4 Vo= Vl − (V2 − Vl)・−
(4) R3 On the other hand, the photocurrent Ep generated in the photodiode PDI flows through the diode D4 and is converted into a compressed voltage, which appears as a voltage Vp at the output terminal of the differential amplifier OP2, so KT Ip Vp= I n...(5)q
Ib becomes. The output of the comparator CP is inverted when Vo
= Vp, so from equations (4)-(5), the photocurrent Ip at this time is Ip if r=R4/R3.
= If/2' (6).

FIG. 2 is a graph showing the relationship between the photocurrent 1p and the radical II I2 output from the photodiode FD and the voltage. The horizontal axis is proportional to the logarithm of the current value, and one scale corresponds to a twice change. The vertical axis is proportional to the voltage value. Therefore, if the relationship shown in this figure exists, when the photocurrent Ip is 1/8 of the reference current ■1, the differential amplifier O
Since the output Vo of PI and the output vp of the differential amplifier OP2 are equal, if R4/R3 = 3, Ip = I1/8, and the output of the comparator CP will be inverted from L level to H level. .

In this way, according to the first embodiment, the diodes (D1 to D1) whose cathodes are connected to the negative power supply (earth GND')
Using only D3), the photocurrent II) and the reference current
This magnitude relationship can be detected by comparing 1/2'. Therefore, for this comparison, the reverse saturation current Ib
is not included, so it has the effect of being able to be detected without being affected by temperature changes.

Next, a second embodiment of the present invention will be described with reference to FIG.

In the first embodiment, since the photodiode PDI was grounded through the diode D4, the photodiode PDI could not be integrated with the CMOS-IC. To solve this problem, in the first embodiment, the photocurrent of the photodiode PDI was converted into a logarithmically compressed voltage using the diode D4, but in the second embodiment, the photocurrent of the photodiode PD2 is converted into a logarithmically compressed voltage. II)
Compression is performed using the diode junction of the photodiode itself, thereby omitting diode D4 and making it possible to connect the photodiode to ground.

In FIG. 3, the non-inverting input terminal of the comparator CPI is directly connected to the anode of the photodiode PD2, and the cathode of this photodiode FD2 is connected to the earth GND. Therefore, corresponding elements are given the same reference numerals and explanations of the configurations will be omitted.

In the case where the photometric circuit of the second embodiment is configured using a CMOS-IC, the photodiode and the CMOS-IC are
This will be explained with reference to FIG. 7, which is a schematic cross-sectional view when S-ICs are configured on the same chip.

In Figure 7, the P region is a so-called P-type silicon substrate, N1 and N2 are N-type silicon layers made by diffusing N-type impurities, and N3ΦN4 is an epitaxial layer made by growing N-type silicon on the substrate. layer, Pi
-P2@P3 is a P-type silicon layer made by diffusing P-type impurities into the epitaxial layer. Further, a-i is silicon oxide for insulation, A-J is a conductor formed as an electrode and is metal aluminum or polysilicon, m is an insulating film such as silicon oxide, and n is an aluminum film for light shielding.

In a semiconductor chip configured in this way, as is well known, by connecting electrodes K and D to a negative power source (ground) and electrode H to a positive power source, electrodes A and C can be connected as drain and source. Then, an N-channel transistor with electrode B as the gate, a P-channel transistor with electrodes E and G as the drain and source, and electrode F as the gate are formed. In addition, a diode is created with electrode ■ as the anode and electrode J as the cathode, as shown in Figure 7.
If electrode (2) is made of a transparent conductive film without forming a light-shielding film (n) on it, a photodiode can be obtained. In a CMOS-IC chip with a light-shielded diode or an unshielded photodiode, the cathode electrode is connected to a negative power supply. If the cathode electrode is not connected to the negative power supply but connected to an intermediate potential, each layer of P3-N4-P will form an emitter-base-collector parasitic PNP transistor, and will not operate as a diode accurately.

In Fig. 7, we have explained the case where the substrate is P-type silicon, but in the case of N-type silicon, the anode of the diode is connected to the positive power supply, P-type silicon becomes N-type, N-type silicon becomes P-type, and so on. N-channel transistors replace P-channel transistors, and P-channel transistors replace N-channel transistors.

FIG. 8 is a diagram of the chip surface of a second embodiment in which photodiodes are formed on the same chip.

The shaded portion PD does not have a light shielding film, but a diode is formed in this portion, and when exposed to light, a photocurrent is generated and acts as a photodiode. Further, in the figure, diodes are provided under the shaded film at diagonally shaded symbols D1 to D3, and since they are not exposed to light, they operate as normal diodes. Other elements other than the diodes D1 to D3 are also formed under the light shielding film.

To explain the operation of the second embodiment configured in this way, the equivalent circuit of the photodiode is shown as shown in FIG. It flows through Dp, and the voltage shown in equation (5) is generated at both ends. Therefore, since the diodes D1, D2, and D3 and the photodiode Dp are manufactured in the same process, the comparator CPI is inverted in FIG. 3 as well as in FIG. 1 when equation (6) holds.

Photodiode PD (Dp) of this second embodiment
Since the cathode of is also connected to the negative power supply line (earth GND), the photodiode can also be integrated on the CMOS-IC chip, and as shown in Figures 7 and 8, it is a sensor. A photodiode and a photocurrent processing circuit can be configured on the same chip.Also, a digital circuit can also be configured on this chip.In this way, a bandgap voltage generating diode can be installed relatively close to the same IC chip. By configuring the photodiode FD with (DI to DB), variations in process conditions can be reduced, and the temperature difference between the respective diodes can be reduced.

Next, a third embodiment of the present invention is shown in FIG. In contrast to the first and second embodiments, which were only capable of determining brightness and darkness, this third embodiment converts the brightness of incident light into an AD converter, and uses a current source using a MOS transistor as a reference current source. The difference is that a Mira constant current source is used.

In FIG. 4, the power supply line Vc is connected to MOS transistors TR2, TR3, TR whose gate electrodes are connected in common.
The gate and drain of the MOS transistor TRI are short-circuited and connected to a constant current source (not shown) that supplies a constant current. The drains of MoS transistors TR2 and TR3 are connected in common and are also connected to ground GND via a diode D1.

Also, the drain of the MOS} radiator TR4 is a diode D.
2 to earth GND. Differential amplifier OP
Resistors R4 to Rn are inserted between the inverting input terminal and output terminal of I, and a fixed terminal connected to the connection point of resistors R6 to R7 and a movable terminal connected to the connection point of resistors R4 to R7. A switch SW2 has a fixed terminal connected to the output end of the differential amplifier OPI, and a switch SW1 has a movable contact connected to each connection point of the resistors R7 to Rn. The resistance values of each of the above-mentioned resistors have the following relationship.

R4=R5=R6=R3/3 R7=R8=・Φ・=Rn=R3 The output terminal of the differential amplifier OPI is connected to the inverting input terminal of the comparator CP2, and the non-inverting input terminal of this comparator CP2 is The photodiodes PDI and PD2 are connected to the anodes of photodiodes PDI and PD2, which have different photometric sensitivity distributions, through a switch SW3, and the cathodes of the photodiodes PDI and PD2 are connected to the ground GND, respectively. In addition,
The photodiodes PDI and PD2 and the photometric circuit are constructed on the same CMOS-IC chip.

The operation of the third embodiment configured in this way will be explained. A constant current (1) is supplied to the MOS transistor TRI, and the transistor TR is connected by a current mirror.
Since the current 1 also flows through the transistors 2 and TR3, a current twice the constant current I1 flows through the diode D1. Similarly, the constant current ■1 flows through the MOS transistor TR4, so the constant current ■1 flows through the diode D2.

By switching the switches SWI and SW2 from a control circuit (not shown), the values of the feedback resistors R4 to Rn can be changed and various levels of photocurrent can be determined. switch SWI
By independently switching SW2 and SW2, the photocurrent can be determined every 21/3 times the reference current (1). Furthermore, by switching the switch SW3, it is possible to switch between the two photodiodes PDI and PD2 for measurement.

In this third embodiment, since current is supplied to the diodes using a current mirror circuit, the diodes DI, D
The current value flowing through FD2 becomes a constant value, making it possible to perform highly accurate photometry.In addition, since the photodiodes PDI, FD2 and the photometry circuit are configured on the same chip, even when connecting a large number of photodiodes, the IC There is no need to increase external wiring, which is very effective.

Next, a fourth embodiment of the present invention will be described based on FIG. 6. As is well known in C-MOS differential amplifiers, the input offset voltage is larger than that in bipolar differential amplifiers, sometimes exceeding ±20 mV. On the other hand, in photometric circuits such as those shown in Figures 1, 3, and 4, if the ratio of the reference currents r1 and ■2 is 1:2 and the diodes D1 to D3 are made of silicon, the difference The difference between the two input voltages v1 and V2 (7) of the dynamic amplifier is only about 18 mV at 25°C. Therefore, an amplification circuit using a differential amplifier has the drawback that the input signal and the offset voltage are amplified together, making it impossible to accurately amplify the signal. This fourth embodiment solves this drawback.

In the photometric circuit shown in FIG. 6, the anode of the diode D1 is connected to one movable contact of the switch SW4, and the other movable contact is connected to the connection point between the drain of the MOS transistor TR3 and the anode of the diode D2. The fixed contact SW4a of the switch SW4 is connected to the inverting input terminal of the differential amplifier OPI via the resistor R3, and the resistors R4 to Rn are connected between the inverting input terminal and the output terminal via the switch SW1. It is connected. Also, the differential amplifier O
One end of a resistor RIO is connected to the inverting input end of PI, the other end of which is connected to a movable contact SW5a of a switch SW5, and the other movable contact SW5b to an output end of a differential amplifier OPI. Here, the resistance value of resistor RIO is R10=R4+
The value is set to R5+●●-+Rn.

A fixed contact of the switch SW5 is connected to the ground GND via a capacitor C1. The output terminal of the differential amplifier OPI is connected to the inverting input terminal of the comparator CP3, and the non-inverting input terminal of this comparator CP3 is connected to the earth GND via the diode D3, and the switch SW2.
and is connected to the power supply line Vc via photodiodes PDI and PD2. The output terminal of this comparator CP3 is connected to the control circuit CS, and a part of this output is connected to the control terminals of the switch SWI and switch SW2. The parts other than the above explanation of the structure are the same as those of the third embodiment described in FIG. 4, so the same members are given the same reference numerals and the explanation will be omitted.

The operation of the photometric circuit of the fourth embodiment configured as described above will be explained.

First, before the A/D conversion operation, the control circuit CS sets the switches SW4 to SW5 to the side shown in the figure, sets the switch SW1 so that the feedback resistors R4 to Rn reach their maximum values, and sets the fixed contact of the switch SW1 to the movable side of the resistor Rn. Switch to connect to the contact point. By switching the switch in this way, the input terminal of the differential amplifier OPI is short-circuited via the resistor R3, so the input signal of the differential amplifier OPI becomes 0, and the resistor R3 only has the offset voltage of the operational amplifier. is added, and the resistor R3 has Io= Vofs
/R3 current flows, and only the offset voltage of differential amplifier circuit OP1 (R3+R4+...+R n ) /
R Since it will be amplified three times, the differential amplifier OPI
The output terminal voltage ■0 is Vo=V1+Vofs+Vofsx(R4+R5+...
・+Rn)/R3, and the terminal of the capacitor C1 on the switch SWS side is charged to the potential of this vO.

Next, the control circuit CS switches the switches SW4 and SW5 to the opposite side from that shown. Then, at both ends of resistor R10,
The voltage of Vo-Vl = Vo f s is applied and the resistance R
A current of Ido=Vofs/R3 flows through IO to the inverting input terminal of the differential amplifier OPI. This current is a current due to the input offset voltage of the differential amplifier OPI.
The absolute values are equal and the directions are opposite, so the differential amplifier OPI
can cancel the amplification of offset voltage caused by After canceling the offset voltage in this way, the control circuit CS moves the switch SW1 from the resistor Rn side to the resistor R4 side.
When the output of the comparator CP is inverted, the switching of the switch SW1 is stopped. By doing so, the switching position of the switch SW1 can be obtained as a digital value of the photocurrent Ip. Normally, the time required to switch the switch SWI is extremely short, so as shown in Figure 6,
An offset canceling circuit using R can also provide a practically sufficient effect.

[Effects of the Invention] According to the present invention, the current compression circuit can be realized only by diodes connected to the positive or negative power supply line, and C
- A circuit suitable for MOS process can be obtained. In addition, by adding a few steps to the CMOS-IC process, it is possible to create a light receiving element and a photocurrent processing circuit within the same chip, thereby realizing an inexpensive and compact light receiving sensor.

[Brief explanation of drawings]

FIG. 1 is an electric circuit diagram showing a first embodiment of the present invention, and FIG.
The figure is a diagram showing the relationship between current and compression voltage, FIG. 3 is an electric circuit diagram showing the second embodiment of the present invention, and FIG. 4 is a diagram showing the third embodiment of the present invention.
An electric circuit diagram showing an embodiment, Fig. 5 is an equivalent circuit diagram of a photodiode, Fig. 6 is an electrical circuit diagram showing a fourth embodiment of the present invention, and Fig. 7 is a photodiode installed in a CMOS process IC. FIG. 8 is a surface view of a case in which photodiodes are constructed on the same chip of a CMOS-IC. D1...first diode D2...second diode R1, R2, TRI~TR4...constant current means CPI~
CP3...Comparison means

Claims (3)

[Claims] (1) a first diode; a first constant current means for passing a current of a first value per unit area of the diode junction in the forward direction of the first diode; a second diode; a second constant current means for flowing a current of a second value at a predetermined ratio to the current of the first value per unit area of the diode junction in the forward direction of the diode, and each of the first diode and the second diode. variable voltage generating means for varying and amplifying the difference between the terminal voltages of the first and second diodes and outputting a first voltage added to the terminal voltage of the first or second diode; A photometric circuit comprising: photoelectric conversion means including a photodiode that generates a voltage; and comparison means that compares the second voltage with the first voltage. (2) means for changing the amplification factor of the variable voltage generation means; and control means for switching the amplification factor of the variable voltage generation means based on the comparison result of the comparison means to perform analog/digital conversion. A photometric circuit according to claim 1, characterized in that: (3) The photometric circuit according to claim 1 or 2, wherein the first and second diodes and the photodiode are arranged in the same package.