JPH04134224A - Light detection device - Google Patents

Abstract

PURPOSE:To output signal light at high sensitivity by dividing photoelectric current, using one side for signal detection to input in a first current-voltage conversion amplification means, and using the other side for low frequency detection to flow into a first amplification means from a second current-voltage conversion amplification means. CONSTITUTION:When the frequency band of a current-voltage conversion amplification means 4 is set sufficiently low to the frequency band of an A.C. component by signal light with feedback capacity 10, the output voltage V02 of the amplification means 4 includes a non-A.C. component by the signal light to become output voltage having only D.C. and low frequency components by disturbance light, consequently current Ic, flowing into a current-voltage conversion amplification means 3 via a resistance 5, becomes current having only the D.C. and the low frequency components. This can actually exclude current having only the D.C. and the low frequency components out of the divided current I1 of photoelectric current Ip from a photoelectric transfer element 11 by the current Ic. Thus only photoelectric current, having an A.C. component by the light signal out of the photoelectric current I1, is added to the input end of the amplification means 3, consequently only the photoelectric current by the light signal can be outputted without need for lessening a gain in the first amplification means 3, permitting the extreme improvement of sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a photodetection device that amplifies and outputs only signal light of photocurrent photoelectrically converted by a photoelectric conversion element such as a photodiode. .

[Conventional technology]

In general, a photodetection device consisting of a photoelectric conversion element such as a photodiode and an amplifier that converts the photocurrent photoelectrically converted into current and voltage and amplifies it, simultaneously detects the signal light to be detected and detects sunlight, etc. Disturbing light enters the photoelectric conversion element. In this case, is it necessary to amplify and output only the photocurrent caused by the signal light? In particular, when the signal light is AC modulated light or pulse modulated light and the disturbance light is DC light or light with a frequency considerably lower than the frequency of the signal light, the DC component is extracted from the photoelectrically converted photocurrent in the amplifier circuit. By removing the low frequency components, only the photocurrent caused by the signal light can be extracted and output.

As such a conventional device, for example, Japanese Patent Application Laid-Open No. 64-20
There is one disclosed in Publication No. 418.

In this device, two photodiodes are used as photoelectric conversion elements, and the amplifier circuit provided on the output side of one photodiode is designed to extract only low frequency components, and this low frequency component is used to convert the other photodiode. The low frequency components in the output of the diode are canceled out. In this way, the low frequency component corresponds to noise such as disturbance light, and as a result, light detection that is not affected by disturbance light becomes possible.

In this case, the two photodiodes are configured as shown in FIG. 3, for example. The figure is a plan view thereof, and a first photodiode 31 for detecting signal light and a second photodiode 32 for detecting disturbance light are provided concentrically and close to each other with a dead area 33 in between.

[Problem to be solved by the invention]

However, if an image of an electric light or other disturbance light is formed as shown in the dotted line area 34 in FIG. 3, accurate detection of the disturbance light becomes impossible. In other words, when a bright image such as a light is focused on the first photodiode 31, the DC or low frequency noise component in the signal output increases, whereas the second photodiode 32 produces less noise. If the image is focused on the second photodiode 32, the signal output from the first photodiode 31 does not contain much noise, but the second photodiode 32 detects a large amount of noise. An output corresponding to the noise appears. For this reason, for example, if the above-mentioned photodetection device is used in a camera's autofocus circuit, the signal may or may not be detected accurately depending on the distribution of light and darkness of the subject to be photographed, the brightness of the background, etc. .

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a photodetection device that can stably and accurately detect only the component of signal light.

[Means to solve the problem]

A photodetection device according to the present invention includes a photoelectric conversion element, a resistance means that divides the photocurrent from the element into a first and a second photocurrent, and outputs the divided photocurrent, and a resistor that converts the first photocurrent into a voltage. a first current/voltage conversion amplification means for amplifying;
a second current/voltage conversion amplification means that has a frequency bandwidth narrower than the frequency bandwidth of the voltage conversion amplification means and responds to the low frequency component of the second photocurrent, converts it into a current, and amplifies it; voltage/current conversion/amplification means which converts the output voltage from the second current/voltage conversion/amplification means into a current with a predetermined gain and applies the converted voltage to the input terminal of the first current/voltage conversion/amplification means; The present invention is characterized in that the current from the conversion and amplification means flows into the input terminal of the first current/voltage conversion and amplification means in a direction that cancels out the first photocurrent.

Second, the dynamic range of the second current/voltage conversion/amplification means with respect to the amount of light incident on the photoelectric conversion element is
The dynamic range of the first current/voltage conversion/amplification means is larger than the dynamic range of the first current/voltage conversion/amplification means with respect to the amount of light incident on the photoelectric conversion element when no current flows from the current conversion means to the input terminal of the first current/voltage conversion/amplification means. You can.

[Effect]

In the present invention, a photocurrent output from a photoelectric conversion element due to incident light is divided into a first and a second photocurrent by a resistance means, and the first photocurrent is divided by a first current/voltage conversion amplification means. Convert to voltage, amplify and output. By the way, the present invention further includes a second current/voltage conversion amplification means that converts the second photocurrent into a voltage and amplifies it, and a second current/voltage conversion amplification means that converts this output voltage into a current and inputs it to the first current/voltage conversion amplification means. It is equipped with a voltage/current conversion means to be applied at the end. This second current/voltage conversion/amplification means has a frequency bandwidth narrower than that of the first current/voltage conversion/amplification means and responds only to the low frequency component of the second photocurrent, so it will be output from now on. The resulting voltage is due to the disturbance light among the light incident on the photoelectric conversion element, and this voltage flows in a direction that cancels out the first photocurrent. Thereby, of the first photocurrent flowing into the first current/voltage conversion/amplification means, the DC voltage component and low frequency component caused by the disturbance light can be reduced, and the AC component caused by the signal light can be outputted with high sensitivity. . At this time, the first and second photocurrents are obtained by distributing the output (photocurrent) of a single photoelectric conversion element using resistance means.
Stable signal detection becomes possible regardless of the shape of the image, brightness distribution, etc. caused by disturbance light on the light receiving surface of the photoelectric conversion element.

Further, the dynamic range of the second current/voltage conversion/amplification means for the distributed second photocurrent is set to be larger than the dynamic range of the first current/voltage conversion/amplification means for the distributed first photocurrent. By doing so, even in the case of strong disturbance light, the AC component of the signal light can be outputted with high sensitivity and accuracy without saturating the first current/voltage conversion/amplification means.

〔Example〕

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a block diagram of an embodiment of a photodetecting device according to the present invention. The illustrated photodetection device includes one photoelectric conversion element 11,
The photocurrent photoelectrically converted by the photoelectric conversion element 11 is
and a resistor 1.2 distributing to the second photocurrent 11I2,
First and second current/voltage conversion and amplification means 3 that convert these distributed photocurrents 1 and I into voltages, respectively.
.

The photoelectric conversion element 11 consists of a photodiode,
This is connected to parallel-connected resistors 1 and 2 for charge current distribution. Here, the resistance values of resistor 1.2 are R and R, respectively.
Then, B RA/RB 1 It is configured to satisfy the relationship (1).

The current/voltage conversion amplification means 3 includes a high-cane inverting amplifier 6 and a feedback resistor 7 for feeding back the output to its input side.
The current/voltage conversion amplification means 4 consists of a high gain inverting amplifier 8, a feedback resistor 9 and a feedback capacitor 10 for feeding back the output to its input side.

The DC current/voltage conversion gain in the current/voltage conversion amplification means 3.4 is determined by each feedback resistor 79.
It is determined by the resistance values R and H of. SunaPi
F2 That is, the gains of the current/voltage conversion amplification means 3 and 4 are R and R, respectively. In addition, current/voltage change FI F
The cutoff frequency f of the current/voltage conversion amplification means 4 is determined by the capacitance Cr of the feedback capacitance 10. is fo-1/2πCFR
F2 - (2), as shown in Figure 2,
The bandwidth f2 of the current/voltage conversion/amplification means 4 is considerably smaller than the bandwidth f1 of the current/voltage conversion/amplification means 3.

A resistor 5 connecting the output end of the current/voltage conversion/amplification means 4 and the input end of the current/voltage conversion/amplification means 3 converts the output voltage from the current/voltage conversion/amplification means 4 into a current to perform current/voltage conversion. It is used to add to the human power of the amplification means 3, and functions as a voltage/current conversion means as described above. In addition,
Assuming that the resistance value of this resistor 5 is R8, the voltage/current conversion gain due to this resistor 5 is 1/R. In a photodetecting device having such a configuration, when a signal light containing disturbance light is made incident on the photoelectric conversion element 11, the resistors 1 and 2 are divided into the first
and second photocurrents I 1 and I 2 flow respectively.

Since the resistance values R and R of resistors 1 and 2 are as shown in equation (1) above, the photocurrent 1. (3) Between (2) and (3), where k is a constant, (3) I, -, and -12. When the second photocurrent (2) is applied to the second current/voltage conversion/amplification means 4, the output voltage V of this current/voltage conversion/amplification means 4. 2 is V −-1-R--1-
RF2/k...(4)02 2F
2 1, and this output voltage V. 2, the current I flowing into the input terminal of the current/voltage conversion amplification means 3 via the resistor 5 is: I −vo2/R8 C −−I −R,/(k−Ro) − (5)
■ It becomes. Now, the resistance value RF2 of the feedback resistor 9 and the resistance value R of the resistor 5 are expressed as RF2/Ro-k,... (6)
If the selection is made so as to satisfy the relationship, from equation (5), the current I flowing into the input terminal of the current/voltage conversion and amplification means 3 through the resistor 5 becomes I--11...(7), and the photovoltaic The first photocurrent 11 outputted and distributed from the conversion element 11 can be canceled out, and the current flowing into the first current/voltage conversion/amplification means 3 can be made zero.

However, if the currents are canceled in all bands, not only the DC component and low frequency component due to the disturbance light but also the AC component due to the signal light will be lost. , only the current with low frequency components is canceled out. That is, as shown in FIG. 2, the frequency band (bandwidth f) of the current/voltage conversion/amplification means 4 should be set sufficiently lower than the frequency band (bandwidth f2) of the AC component due to the signal light using the feedback capacitor 10. For example, the output voltage vo2 of the current/voltage conversion/amplification means 4 includes an AC component due to the signal light, a DC component due to the disturbance light, and only low frequency components. The current flowing into the circuit ■ also has only DC and low frequency components. As a result, the DC component of the shunt (1) of the photocurrent I from the photoelectric conversion element 11,
Only the low frequency components can be substantially removed by the current I.

In this way, only the AC component of the photocurrent 11 due to the signal light is applied to the input terminal of the current/voltage conversion/amplification means 3, so that the first current/voltage conversion/amplification means 3 is Since only the photocurrent can be converted into a voltage with the gain R11 and output without the need to reduce the gain, the sensitivity can be significantly improved.

At this time, the two photocurrents 1 and I are obtained by distributing the output photocurrent I of the single photoelectric conversion element 11 using the resistor 1.2, so the state of the disturbance light becomes as shown in Figure 3. However, the low frequency component is distributed to the photocurrent at a predetermined ratio of 1.12. ■ In other words, it is theoretically possible that the low frequency component due to the disturbance light increases only on the photocurrent ■ side or conversely only on the photocurrent I2 side because a single photoelectric conversion element is used. impossible. Therefore, stable signal detection becomes possible.

In addition, in the photodetector shown in Fig. 1, resistors 1 and 2, current and
When the voltage conversion amplification means 4 and the resistor 5 are not provided and the current from the resistor 5 is not allowed to flow into the input terminal of the current/voltage conversion amplification means 3, the inverting amplifier 6 with the saturated output voltage vlH operates without being saturated. The limit photocurrent (■) for which the voltage is applied is HT 1-V/R (8) IT)I THPi. On the other hand, when the current I from the resistor 5 is caused to flow into the input terminal of the current/voltage conversion and amplification means 3 as in this embodiment, the dynamic range becomes significantly wider. That is, assuming that the inverting amplifier 8 has the same saturation output voltage VTH as the inverting amplifier 6, the limit photocurrent I that allows the inverting amplifier 8 to operate normally without saturating is TH 1-V/R (9 ) 2THTHF2. As can be seen by comparing equation (9) with equation (8), the dynamic range of current/voltage conversion/amplification means 4 is (1-R)/(1 -R) ITHPl 2THF2 = k" RFl/RF2...
・It will increase by (10) times.

In this way, the photodetector of this embodiment has a large dynamic range with respect to disturbance light and a large current/voltage conversion gain with respect to signal light as AC modulated light, so even if strong disturbance light is incident along with signal light, Only the optical signal generated by the signal light can be amplified with high sensitivity, which makes it possible to simplify the signal processing circuit connected to the latter stage of the current/voltage conversion amplification means 3, thereby reducing the overall circuit scale. be able to.

Further, since the device of this embodiment has an overall negative feedback, and does not give negative feedback to the DC component and low frequency component of the disturbance light, it is possible to stably operate the entire device. For this reason, Yong;
The capacitance value Cp of 10 may be set to a small value, so that the capacitance 10 is combined with the photoelectric conversion element 11, the distribution resistors 1 and 2, the inverting amplifiers 6 and 8, the resistor 7.9, and the signal processing circuit (not shown). It can be integrated into a single monolink IC.

Note that the current/voltage conversion amplification means 3 is based on equations (3) to (7).
) If the condition of the equation is satisfied, there is no response at all to the DC component and low frequency component of the disturbance light, or there is a difference in the resistance ratio R/RB of the distribution resistors 1 and 2, or the resistance ratio RF of the resistance value RR.
It is also conceivable that the current/voltage conversion and amplification means 3 slightly responds to the DC component and low frequency component of the disturbance light due to the deviation F2 c of 2/Ro. For this purpose, a coupling capacitor is connected in series between the current/voltage conversion/amplification means 3 and a signal processing circuit (not shown) at the subsequent stage, and disturbance light that may be output from the current/voltage conversion/amplification means 3 is connected in series. The slight direct current component and low frequency component may be removed by a coupling capacitance and manually applied to a subsequent signal processing circuit.

In the embodiment shown in FIG. 1, the values of the resistors 1 and 2 are fixed, but variable resistors may be used to adjust the distribution ratio of the photocurrent I. In this case, it may be possible to dynamically set the distribution ratio at the time of shipment as a product (photodetector) or during actual use, depending on the ratio of the levels of the disturbance light and the signal light. That is, since the input impedance increases as the frequency of the signal light increases, it is conceivable to adjust the resistors 1 and 2 at the time of shipment according to the frequency band of the signal to be used. It is also conceivable to dynamically adjust the resistors 1 and 2 according to the level of ambient light during actual use of the device.

The values of the resistors 2 and 3 may be changed or adjusted not only in their relative values but also in their absolute values. That is, the values of the resistors 1.degree.2 may be adjusted in height depending on the level of the signal light itself.

〔Effect of the invention〕

As described in detail above, according to the present invention, the output (photocurrent) of the same photoelectric conversion element is divided into first and second photocurrents, one of which is used for signal detection, and the first current - Input the current into the voltage conversion amplification means, use the other one for low frequency detection and input it into the second charge voltage conversion amplification means, and use the current in response to the low frequency component in a direction that cancels out the photocurrent for signal detection. Since it is made to flow into the input terminal of the first current/voltage conversion and amplification means, regardless of the circumstances in which the disturbance light is incident,
Only the alternating current component of the signal light can be output in a stable state with high sensitivity.

6.8--inverting amplifier, 7,9... feedback resistor, 10-
- Feedback capacitance, 31... First photodiode, 32.
...Second photodiode, fo...Cutoff frequency, f, f2...Frequency bandwidth.

■

Claims (1)

[Scope of Claims] 1. A photoelectric conversion element, a resistance means for distributing the photocurrent from the photoelectric conversion element into first and second photocurrents, and converting the first photocurrent into a voltage. a first current/voltage conversion/amplification means for amplifying the current/voltage;
a second current/voltage conversion/amplification means that responds to the low frequency component of the photocurrent, converts it into a current, and amplifies it;
converting the output voltage from the current/voltage conversion/amplification means into a current with a predetermined gain, and applying a voltage to the input end of the first current/voltage conversion/amplification means in a direction that cancels out the first photocurrent; - A photodetection device characterized by comprising current conversion and amplification means. 2. The second ratio with respect to the amount of light incident on the photoelectric conversion element
The dynamic range of the current/voltage conversion amplification means is defined as the dynamic range of the current/voltage conversion/amplification means with respect to the amount of light incident on the photoelectric conversion element when no current flows from the voltage/current conversion means to the input terminal of the first current/voltage conversion/amplification means. 2. The photodetecting device according to claim 1, wherein the dynamic range is larger than the dynamic range of said first current/voltage conversion/amplification means.