JPH0435016B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a low current current-to-voltage conversion circuit, and particularly to a photometric circuit using a photoelectric element such as a photodiode.

BACKGROUND ART In recent years, in the field of optical measurement, there has been a demand for measurement of weak light, and it has become necessary to accurately measure the low photocurrent region of photoelectric elements.
In addition, photoelectric elements with a wide dynamic range,
For example, a photodiode has a power of several pA to several tens of μA.
In order to make effective use of this dynamic range, it has become necessary to measure the low photocurrent range of photoelectric devices.

A conventional photometric circuit using a photo diode has been realized with a circuit as shown in FIG.
In this circuit, the feedback resistor 22 is switched by the switching means 23, and the amplification degree of the circuit is changed to correspond to the wide dynamic range of the photoelectric element 1. However, unless the switching means 23 is a relay or switch whose leakage current is smaller than the photocurrent, an error will occur in the measurement in the low photocurrent range of the photoelectric element 1.
For this reason, a relay or a switch is used as the switching means 23, but this has problems such as poor responsiveness and high power consumption, making it difficult to apply to battery-powered photometers and the like.

Additionally, as shown in Figure 5, a circuit has been devised that can be applied to battery-powered photometers, etc.
58-37489). However, in this circuit, the leakage current of the field effect transistor (hereinafter abbreviated as FET) 25 is several hundred pA, which is about the same as the photocurrent of photodiode 1, making it impossible to use photodiode 1 in the low photocurrent range. .

In addition, as an amplification circuit using an operational amplifier,
For example, as in Japanese Utility Model Application Laid-open No. 53-163186, there is a device that obtains a large amplification degree by dividing the output of an operational amplifier by resistors and feeding back the output voltage from the dividing point. In order to change the degree of amplification using such a method, it is necessary to change the amount of feedback by changing the resistance division ratio. Therefore, it is necessary to use the aforementioned relays and switches, and it cannot support low power consumption operation like battery drive.

Problems to be Solved by the Invention In such conventional circuits, it is necessary to use relays (current consumption of several tens of mA) and switches, so low power consumption cannot be achieved, and automatic measurement by switching measurement ranges is not possible. This makes it difficult to apply to photometers that require battery power. Also,
In the above-mentioned circuit using an FET, the leakage current of the FET has a large effect, making it difficult to use the photoelectric device in the low photocurrent range, and making it impossible to utilize the wide dynamic range inherent to the photoelectric device.

The present invention has been made in view of the above points, and realizes optical measurement with low power consumption by switching the amplification degree over a wide dynamic range from the low photocurrent range to the high photocurrent range of photoelectric elements, and is battery-powered. The present invention provides a photometry circuit that can be applied to photometers and the like.

Means for Solving the Problems In order to solve the above problems, the present invention includes: an amplifier that amplifies the photocurrent from a photoelectric element; a first resistance section having a variable element that changes the resistance value; It includes a second resistance section connected in series to the resistance section, and a feedback resistance that provides negative feedback from a connection point between the first resistance section and the second resistance section to the input of the amplifier. A series circuit of the second resistor section is connected between the output terminal of the amplifier and the ground, and the resistance value of the feedback resistor is made larger than the resistance value of the second resistor section, and the resistance value of the variable element is changed according to the value of the output of the amplifier. This is a photometric circuit having a control means for controlling the

Effects With the above-described configuration, the present invention controls the resistance value of the first resistor section by changing the resistance value of the variable element using the control means, depending on the feedback amount of the negative feedback system of the amplifier according to the output of the amplifier. ing. By automatically changing the amplification degree of the circuit system in this way, it is possible to utilize a wide dynamic range from the low photocurrent range of the photoelectric element. Also, the first resistance part
It is composed of a variable element such as a FET that can change the resistance value, and the resistance value of the feedback resistor connected to the connection point between the first resistor part and the second resistor part is the resistance of the second resistor part. Because it is set larger than the value, the influence of leakage current when the variable element shows the maximum resistance value (when shutting off in the case of FET) flows more easily to the second resistor, and is less likely to flow to the feedback resistor. Changes in feedback amount due to leakage current can be reduced, and a measurement circuit with good accuracy and wide dynamic range can be obtained.

Embodiment FIG. 1 shows an embodiment of the photometric circuit of the present invention,
This uses an operational amplifier as an amplifier. In FIG. 1, 1 is a photoelectric element such as a photodiode, 2 is an operational amplifier, 3 is a feedback resistor, 4 is a fixed resistor A as a second resistance section, 5 is a fixed resistor B,
6 is an FET as a variable element, 7 is a control circuit as a control means for controlling the FET 6, 8 is an output, and the fixed resistor B5 and the FET 6 constitute a variable resistance section 10 as a first resistance section.

The operation of the photometric circuit of this embodiment configured as described above will be described below.

When light is incident on a photoelectric element 1 such as a photodiode, a photocurrent is generated depending on the amount of incident light. This photocurrent is input to the operational amplifier 2, where it is amplified and converted into a voltage value. Here, the degree of amplification is determined by the amount of feedback fed back by the feedback resistor 3. The output 8 of the operational amplifier 2 is voltage-divided by a fixed resistor A4 and a variable resistor section 10. moreover,
Fixed resistor B5 and FET that constitute variable resistor section 10
6 are connected in series, and the fixed resistor B5 is grounded via FET6. Fixed resistance A4 and variable resistance section 10
The output of the operational amplifier 2 divided by the feedback resistor 3
The amount of feedback is changed by changing the resistance value of the variable resistance section 10 and changing the voltage division ratio between the fixed resistance A4 and the variable resistance section 10. In other words, if FET6 is operated as an analog switch, feedback resistor 3
The amount of feedback to changes.

Here, the operation of the circuit centering on the FET 6 will be explained in more detail.

The output of the operational amplifier 2 is set within the range from the lower limit value V _L of the output voltage to the upper limit value V _H of the output voltage (the dynamic range of the operational amplifier 2 is smaller than that of the photoelectric element 1, so the amplification degree is changed to (in order to set the upper and lower limits of the output voltage), the control circuit 7 detects V _L and V _H , and based on this, the FET
Controls the conduction state of 6. For example, the control circuit 7 can be realized by the circuit shown in FIG. Now, when the output of the operational amplifier 2 exceeds the upper limit value _VH , the control circuit 7
Make FET6 non-conductive. If the ratio of the feedback resistor 3 to the fixed resistor A4 and the fixed resistor B5 is large (feedback resistor 3>fixed resistor A4+fixed resistor B5), the output of the operational amplifier 2 is added to the feedback resistor 3. FET
When FET 6 is conductive, the output of operational amplifier 2 is divided by fixed resistor A4 and fixed resistor B5 and applied to feedback resistor 3. Therefore, when FET 6 is non-conductive, the feedback resistor 3 is lower than when it is conductive. The amount of feedback that goes through is large, and the amplification degree of the circuit is small (because it is fed back to the negative input). Therefore, the output of the operational amplifier 2 becomes smaller than the upper limit value _VH .

Furthermore, when the output of the operational amplifier 2 becomes smaller than the lower limit value V _L , the above operation occurs in reverse.

In addition, in the conventional circuit, the leakage current of the FET was a problem, but in this embodiment, since the feedback resistor is the fixed resistor A4, most of the leakage current of the FET6 flows through the fixed resistor A4, so the photocurrent amplification is The impact on Furthermore, if the drain D of FET6 is grounded, the leakage current between the gate G and drain D of FET6 will not affect the photocurrent amplification, and the leakage current between the gate G and drain D will be smaller than the leakage current between the source S and drain D. The only effect is the leakage current between the two.

As described above, according to this embodiment, the photocurrent from the photoelectric element is supplied to the operational amplifier, and the fixed resistor A is connected to the output terminal of the operational amplifier, and the fixed resistor A is connected to the other end of the fixed resistor A. A variable resistor section whose other end is grounded and the resistance value is changed by turning the FET ON/OFF using the operational amplifier output, and a negative feedback is applied by connecting the connection point between the fixed resistor and the variable resistor section to the negative input of the operational amplifier. By providing a feedback resistor, making the resistance value of the fixed resistor A smaller than the resistance value of the feedback resistor, and allowing the leakage current of the FET to flow through the fixed resistor A, the FET can be turned on/off.
It eliminates changes in the feedback amount of the operational amplifier due to FET leakage current during OFF operation, and allows stable optical measurement by switching the amplification degree and exceeding the dynamic range of the operational amplifier.

A second embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram of a photometric circuit showing a second embodiment of the present invention. In FIG. 3, 1 is a photoelectric element such as a photodiode, 2 is an amplifier (operational amplifier), 3 is a feedback resistor, and 4 is a fixed resistor A, which is the same as the configuration shown in FIG. 1. What differs from the configuration in FIG. 1 is the configuration of the variable resistance section 11.
Fixed resistance B12, fixed resistance C13, fixed resistance D1
4 in parallel and one of these fixed resistors
One fixed resistor D14 is directly grounded, and the other fixed resistors B12 and C13 are connected to the FET1
5, 16, and these
A control circuit 17 for controlling the FETs 15 and 16 is provided.

The operation of the photometric circuit of the second embodiment configured as described above will be described below. However, the operations of the photoelectric element 1, the operational amplifier 2, and the feedback resistor 3 are the same as in the first embodiment, and their explanations will be omitted here.

In order to set the output of the operational amplifier 2 within the range of the output voltage from the lower limit value V _L to the upper limit value V _H , the control circuit 17
Detect V _L and V _H , and based on this, FET15,
Controls the conduction state of FET16. Fixed resistance B1
2, C13, and D14 have a relationship of fixed resistance B12<fixed resistance C13<<fixed resistance D14. Assume now that FET 15 is conductive and FET 16 is non-conductive. At this time, the amount of feedback fed back via the feedback resistor 3 is determined by the ratio of the fixed resistor A4 to the fixed resistor B12. Fixed resistance B12《Fixed resistance D14
Therefore, the fixed resistance D14 can be ignored. Here, if the output of operational amplifier 2 exceeds the upper limit value _VH ,
The control circuit 17 makes the FET 15 non-conductive and the FET 16 conductive. Then, since the feedback amount is determined by the ratio of the fixed resistance A4 and the fixed resistance C13, the feedback amount becomes large (because the fixed resistance B12<fixed resistance C13).
Therefore, since the degree of amplification becomes smaller, the output of the operational amplifier 2 becomes smaller than the upper limit value _VH . In this way, when the FET 15 is non-conducting and the FET 16 is conducting, the output of the operational amplifier 2 becomes even larger and exceeds the upper limit value _VH , and the control circuit 17
5 and 16 are made non-conductive. Then, the amount of feedback is
Since it is determined by the ratio between the fixed resistor A4 and the fixed resistor D14, the amount of feedback becomes large (because the fixed resistor C13<<the fixed resistor D14). Therefore, since the degree of amplification becomes smaller, the output of the operational amplifier 2 becomes smaller than the upper limit value _VH .

Furthermore, when the output of the operational amplifier 2 becomes smaller than the lower limit value _VL , the above operation occurs in reverse.

In addition, in this photometry circuit, FET15 and 16 are fixed resistor D14 rather than leakage current flowing when non-conducting.
If the current flowing through the FET is set to be large enough, the leakage current of the FET can be ignored.

As described above, according to this embodiment, the fixed resistance B
12, 13 and FET 15 are connected in series and grounded, and a fixed resistor D14 is installed in parallel to this, and by constantly flowing a current larger than the leakage current of FET 15 through the fixed resistor D14, the FET
Eliminating the influence of leakage current of FET15,
Optical measurements can be made by switching the amplification degree while turning it on and off.

In the first embodiment, the case where one FET is used has been described, but it goes without saying that by providing a plurality of FETs as in the second embodiment, the amplification degree can be switched in three or more stages. Further, in the first embodiment and the second embodiment, it goes without saying that a feedback capacitor is provided in parallel to the feedback resistor 3 to stabilize the operation of the operational amplifier 2.

Effects of the Invention The photometric circuit of the present invention includes an amplifier that amplifies a photocurrent from a photoelectric element, a feedback resistor of this amplifier, a series circuit of first and second resistors resisted by the output of the amplifier, and an amplifier. The feedback resistor is connected to the connection point of the first and second resistor parts, and the resistance value of the feedback resistor is changed to the second resistor. By making the resistance value larger than that of the resistor part, the leakage current of variable elements such as FETs that control the resistance value of the first resistor part is not affected, and a wide dynamic range from the low photocurrent range of the photoelectric element is used. It can be driven by batteries, and its practical effects are great.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the photometric circuit in the first embodiment of the present invention, FIG. 2 is a diagram showing the configuration of the control circuit, and FIG. 3 is a diagram showing the photometric circuit in the second embodiment. FIGS. 4 and 5 are diagrams showing examples of conventional photometric circuits. 1...Photoelectric element, 2...Operation amplifier, 3...Feedback resistor, 4...Fixed resistor A, 5...Fixed resistor B,
6...FET, 7...Control circuit, 10...Variable resistance section, 11...Variable resistance section, 12...Fixed resistance B, 13...Fixed resistance C, 14...Fixed resistance D,
15, 16...FET, 17...control circuit.

Claims (1)

[Scope of Claims] 1. A photoelectric element, an amplifier for amplifying the photocurrent of the photoelectric element, a first resistor section having one end grounded and a variable element for changing the resistance value, and a first resistor section connected in series with the first resistor section. a feedback resistor that provides negative feedback from a connection point between the first resistor section and the second resistor section to the input of the amplifier; A series circuit of a resistor section is connected between the output end of the amplifier and the ground, and the resistance value of the feedback resistor is made larger than the resistance value of the second resistor section. A photometric circuit having a control means for changing the resistance value of an element. 2. When the first resistance section includes a series circuit of a variable element and a first fixed resistor, and a second fixed resistor connected in parallel to the series circuit, and the resistance value of the variable element is maximum. 2. The photometric circuit according to claim 1, wherein the resistance value of said second fixed resistor is smaller than the resistance value of said series circuit. 3. The variable element is a field effect transistor,
2. The measuring circuit according to claim 1, wherein the source terminal of the field effect transistor is connected to a first fixed resistor, and the drain terminal of the field effect transistor is grounded.