JPH04279879A - Photo detection circuit - Google Patents

Abstract

PURPOSE:To obtain a photo detection circuit in which the detection sensitivity of received light is not reduced even when the temperature around is changed. CONSTITUTION:The reduction in bias voltage due to the dark current of an APD 12 generated at high temperature is offset by the variation in the voltage of a posistor 15, by inserting the posistor 15 having positive temperature coefficient characteristic and a resistor 16 for temperature characteristic adjustment, between a resistor 9 for current limitation and a variable resistor 10.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photodetection circuit for a distance measuring device or the like that uses a laser beam.

0002

2. Description of the Related Art FIG. 8 is a block diagram showing a distance measuring device equipped with a conventional photodetection circuit. In the figure, 1 is a laser oscillator,
2 is a transmitting optical system, 3 is a receiving optical system, and 4 is, for example, a PD (P
5 is a photodetector using an APD (Avalanche Photo Diode) etc.
), and 6 is a counting circuit. In this distance measuring device, a pulsed laser beam emitted from a laser oscillator 1 is beam spread-adjusted by a transmitting optical system 2 and irradiated toward a target, and at the same time, a portion of the laser beam is directed to a photodetector 4. The input signal is converted into a pulsed electrical signal, ie, a start pulse signal, indicating the timing at which the laser beam is emitted. The laser beam reflected from the target is focused by the receiving optical system 3, and the light detecting circuit 5
It is converted into a pulsed electrical signal, ie, a stop pulse signal, indicating the time when the laser beam hits the target and returns. In this way, the laser oscillator 1, the transmission optical system 2,
A receiving optical system 3, a photodetector 4, and a photodetecting circuit 5 generate a laser beam for distance measurement, and a start pulse signal and a stop pulse signal indicating the time when the laser beam was emitted are generated. The distance R to the target can be determined by calculating the following equation.

0003

[Math 1]

Note that since the laser beam reflected from the target is weak, the photodetection circuit 5 that receives it and generates a stop pulse signal is generally an AP having a light multiplication function.
It is devised to make the most of the APD's functions by using D and applying a low bias voltage to the APD in the vicinity where the APD causes a breakdown phenomenon. FIG. 9 is a diagram showing the circuit configuration of this photodetection circuit 5, in which 7 is a high voltage circuit, 8 and 9 are current limiting resistors, 10 is a variable resistor, 11 is a changeover switch, and 12 is a light shielding circuit. The first APD is for temperature compensation, and 13 is the second APD for light detection.
, 14 is an amplifier. The output voltage of the high voltage circuit 7 is VH, and the breakdown voltage of the APD 12 is VB.
D, and a bias voltage VB is applied to the APD 13.

[0005] Here, the sensitivity of APD is generally determined by the breakdown voltage VBD of about 300 to 400V as shown in FIG.
The voltage rises steeply at , several tens of volts higher than this voltage.
The optimum operating state is achieved at a low voltage VOP. This voltage VO
P is called the operating voltage of the APD, but the operating voltage VOP
has a positive slope temperature fluctuation characteristic and changes with the ambient temperature, so the bias voltage that is directly proportional to the ambient temperature is set as AP.
It is necessary to supply it to D.

The photodetection circuit shown in FIG. 9 is an AP of the same type.
The above bias voltage is generated by utilizing the fact that VBD of D has almost the same temperature fluctuation characteristics as VOP, and APDs of the same type whose VBD and VOP are within a certain range are combined as a pair. , one of them is light-shielded and the APD 12 is used for temperature compensation, and the other is used as an APD 13 for light detection.
The voltage VH (
Approximately 500 to 600 V) is applied to this APD 12 to cause breakdown, and further, this breakdown voltage VBD is made approximately equal to the operating voltage VOP of the photodetecting APD 13 using a variable resistor 10, and this voltage is set as a bias voltage V.
B is supplied to the photodetection APD 13, and the photodetection APD 13 converts the received light into an electrical signal, thereby creating a photodetection circuit that adapts to the ambient temperature. In this case, the V of APD12 for temperature compensation is used as a combination of APDs.
When BD is higher than VOP of APD13 for photodetection (V
There are two cases: BD≧VOP) and a low case (VBD<VOP). In the former case, points a and c in FIG.
In the latter case, similarly connect points b and c in FIG. An appropriate bias voltage VB is applied, and if the temperature gradients of VB in the case of VBD≧VOP and VBD<VOP are 71 and 72, respectively, 71 and 72 are expressed as the following equations, and the temperature of these VB APDs are combined so that the gradient is as close as possible to the temperature gradient of VBD.

[0007]

[Math 2]

Specifically, the combination of APDs is 71, 7
2 is preferably selected to match the temperature gradient of VBD. In other words, if the coefficient is K, then 71 = K7, 72 =
It is preferable to use a combination of APDs such that K7 and K=1, but it is possible to obtain performance with no practical problems even if K=0.8 or more, so if this is the allowable limit, the above formula (2) (3 ), it is sufficient to select an APD that satisfies equation (4), and the above two APD pairs are combined so as to satisfy this condition.

[0009]

[Math 3]

[0010] The amplifier 14 is the above-mentioned APD 13 for photodetection.
After converting the received light detected in the form of current into voltage by
It serves to amplify and output to the counting circuit 6 as a stop signal.

[0011]

[Problems to be Solved by the Invention] In the conventional photodetection circuit as described above, from the viewpoint of the durability of the temperature compensating APD 12, the current limiting resistors 8 and 9 are set so that the current flowing through the APD 12 is several tens of microamperes. Although the resistance value is set to a high resistance value of 10 to 20 MΩ, the dark current of the APD generally increases exponentially as the ambient temperature rises, and at a temperature of 40 to 70°C, the dark current flowing through the photodetecting APD 13 increases by several times. This current, which is on the order of μA, flows through the current limiting resistor 8 and variable resistor 10, resulting in a voltage drop of several tens of volts, and as a result, the bias voltage VB of the photodetection APD 13 is also reduced by this voltage drop. Since the value becomes low, there is a problem in that the sensitivity decreases at high temperatures of 40 to 70°C. Also, if the output voltage of the high voltage circuit fluctuates due to fluctuations in the power supply voltage, etc.
There was a problem in that the sensitivity of the PD varied.

[0012] This invention was made to solve the above-mentioned problems, and even if the ambient temperature becomes high, the A
The object of the present invention is to provide a photodetection circuit that prevents the sensitivity of the PD from decreasing and protects the APD in the event of disconnection of a temperature-sensitive element such as a thermistor. Another object of the present invention is to obtain a photodetection circuit that does not cause fluctuations in sensitivity of the APD due to fluctuations in power supply voltage or the like.

[0013]

[Means for Solving the Problems] A photodetection circuit according to the present invention inserts a heat-sensitive element having a positive temperature coefficient characteristic between a current-limiting resistor and a variable resistor, so that the first A
The decrease in bias voltage due to the dark current of the PD is offset by the voltage change of the heat-sensitive element.

Further, the photodetection circuit according to the present invention inserts a heat-sensitive element having a negative temperature coefficient characteristic between the current limiting resistor and the variable resistor, thereby reducing the bias voltage due to the dark current of the first APD that occurs at high temperatures. The temperature drop is offset by the heat-sensitive element, and if the heat-sensitive element breaks, the connection to the second APD for photodetection is automatically cut off.
This protects the PD.

Furthermore, in the photodetection circuit according to the present invention, a constant voltage circuit is provided after the high voltage circuit, so that even if the power supply voltage fluctuates, a constant voltage is applied to the circuit that controls the bias voltage, thereby suppressing fluctuations in the bias voltage of the APD. It is intended to prevent

Furthermore, in the photodetection circuit according to the present invention, a constant voltage circuit is provided after the high voltage circuit, and a temperature sensing element is inserted between the current limiting resistor and the variable resistor to prevent fluctuations in the power supply voltage. In addition, a constant voltage is applied to the circuit that controls the bias voltage even when the temperature is high, and the bias voltage decrease due to the dark current of the first APD that occurs at high temperatures is offset, and the sensitivity of the APD is affected by fluctuations in the power supply voltage and temperature. This is to prevent this from happening.

[0017]

[Function] In the photodetection circuit of the present invention, the voltage change caused by the temperature sensing element inserted between the current limiting resistor and the variable resistor cancels out the decrease in bias voltage due to the dark current of the APD, and the APD at high temperature. Prevents sensitivity from decreasing.

Further, in the case of a thermistor disconnection, the voltage supply to the photodetecting APD is cut off to protect the APD. Further, a constant voltage circuit provided after the high voltage circuit always supplies a constant voltage to the APD to eliminate sensitivity changes due to power supply voltage fluctuations.

[0019]

[Example] Example 1. FIG. 1 is a diagram showing an embodiment of the present invention, in which 7 to 14 are the same as conventional ones, 15 is a POSISTOR having positive temperature coefficient characteristics, and 16 is a resistor for adjusting temperature characteristics. In such a circuit configuration, the high voltage circuit 7
, current limiting resistors 8, 9, variable resistor 10, changeover switch 11, and temperature compensation APD 12, the bias voltage control circuit controls the ambient temperature based on the circuit connection according to the combination of APDs, similar to the conventional photodetection circuit. A bias voltage suitable for the photodetection APD 13 is supplied to the photodetection APD 13, and the received light is converted into an electric signal by the photodetection APD 13 and the amplifier 14, amplified, and finally outputted to the counting circuit 6 as a stop signal. Generally, the dark current of an APD has the characteristic of increasing exponentially as the ambient temperature rises.
This dark current flows through the current limiting resistor 8 and the variable resistor 1.
0, a voltage drop occurs due to the above-mentioned photodetection A.
The bias voltage VB applied to the PD 13 becomes lower than the operating voltage VOP of the APD at high temperatures as shown in FIG. 2, and the sensitivity of the APD correspondingly decreases as shown in FIG. 3.

Therefore, as shown in FIG. 4, a voltage that has the opposite curve to the voltage drop characteristic, that is, the same temperature change as the difference voltage between VOP and VB, is generated as an offset voltage, and this is added to the bias voltage VB. By doing so, it is possible to prevent the bias voltage from decreasing due to fluctuations in the dark current of the APD. The above POSISTOR 15 and the resistor 1 for temperature characteristic adjustment
The resistor component 6 is used to generate this offset voltage, and the POSISTOR 15 generates a temperature fluctuation voltage with a positive slope by using the positive temperature coefficient characteristic, and the resistor component 6 is connected in parallel with the temperature characteristic adjustment resistor component 15. The resistor 16 is used to offset the temperature fluctuation characteristics of the fluctuating voltage according to the characteristics shown in FIG. 4, and the resistor component is inserted into the circuit so that the offset voltage is added to the bias voltage VB. .

Specifically, the resistance value of the POSISTOR 15 is R
1 (T) (where T is temperature), let R2 be the resistance value of the resistor 16 for adjusting the temperature characteristics, and let VA be the voltage generated at these resistance components connected in parallel. VA is expressed by the following equation (5). Therefore, the above resistance value R2 is adjusted so that VA has the same temperature fluctuation characteristics as the offset voltage in FIG.
In addition, the resistor component is inserted between the variable resistor 10 and the current limiting resistor 9 so as to increase the voltage division ratio of the bias voltage control circuit so that the voltage VA is added to the bias voltage VB. With this circuit configuration, the voltage VA generated in the resistor components is
offsets voltage drops due to dark current fluctuations of the APD, and always supplies the ideal operating voltage VOP to the photodetecting APD 13.

[0022]

[Math 4]

Example 2. In the first embodiment, a temperature fluctuation voltage with a positive slope is generated using a POSISTOR 15 having a positive temperature coefficient characteristic and a temperature characteristic adjustment resistor 16, and this is added to the bias voltage VB to darken the APD. Although the reduction in bias voltage due to current is offset, a thermistor with a negative temperature coefficient and a resistor for adjusting temperature characteristics are used to generate a temperature fluctuation voltage with a negative slope, and this is used to reduce the bias voltage VB. However, the reduction in bias voltage due to the dark current of the APD can be offset. FIG. 5 shows an embodiment in this case. In the figure, 17 is a thermistor having a negative temperature coefficient characteristic, and this and a temperature characteristic adjustment resistor 16 are connected to a current limiting resistor 8 and a variable resistance regulator 10. Everything else between them is the same as in the first embodiment.

With this circuit configuration, the thermistor 17 and the temperature characteristic adjustment resistor 16 generate a temperature fluctuation voltage with a negative slope, that is, a temperature fluctuation voltage with a curve exactly opposite to the offset voltage shown in FIG. Furthermore, since these resistor components are inserted in circuit positions that reduce the voltage division ratio of the bias voltage control circuit, the temperature fluctuation voltage acts to reduce the bias voltage, and as a result, the result of the embodiment 1 is generated to cancel the voltage drop due to the dark current fluctuation of the APD, so that the ideal operating voltage VOP is always supplied to the photodetecting APD 13. Furthermore, in general, failures in resistors are often due to disconnection, but if such a disconnection failure occurs in the resistor component, the connection to the photodetection APD 13 is simultaneously disconnected to eliminate the abnormal voltage at the time of the failure. The light detection APD 13 is protected from being applied to the light detection APD 13.

FIG. 6 shows still another embodiment of the present invention, which is the same as the conventional photodetector circuit 5 except that a constant voltage circuit 18 is provided after the high voltage circuit 7. With this circuit configuration, the constant voltage circuit 18 functions to keep the output voltage of the high voltage circuit 7 stable, so that even if the output voltage of the high voltage circuit 7 fluctuates due to fluctuations in the power supply, the voltage applied to the bias circuit system will remain constant. is always constant. Here, in a conventional photodetection circuit without the constant voltage circuit 18, the temperature compensation circuit A
AP for photo detection from breakdown voltage VBD of PD12
When it is lower than the operating voltage VOP of D13, the temperature gradient of the bias voltage applied to the photodetection APD 13 is expressed by the above formula (
3), the output voltage VH of the high voltage circuit 7
It changes depending on. As a result, in the conventional photodetection circuit, the sensitivity of the photodetection APD 13 was sometimes affected by fluctuations in the power supply, but by adding a constant voltage circuit as in this embodiment, this voltage can be stabilized. It is possible to prevent sensitivity fluctuations of the APD due to power fluctuations and the like.

FIG. 7 shows still another embodiment of the present invention, in which a conventional photodetector circuit includes a constant voltage circuit 18 after the high voltage circuit 7, and a POSISTOR 15 between the current limiting resistor and the variable resistor. A current limiting resistor 16 or a thermistor 17 and a current limiting resistor 16 are inserted. With such a circuit configuration, the constant voltage circuit 18 keeps the voltage applied to the bias circuit system constant and maintains the APD.
At the same time, the temperature fluctuation voltage generated by the resistor component offsets the decrease in bias voltage caused by the APD's dark current, thereby preventing the sensitivity of the APD from decreasing at high temperatures. Note that here, a case where a thermistor is used as the temperature sensing element is shown as a typical example.

[0027]

As described above, according to the present invention, a temperature fluctuation voltage with a positive slope is generated using a temperature sensing element having a positive temperature coefficient and a temperature characteristic adjustment resistor, and this fluctuation voltage is
By adding it to the D bias voltage, the decrease in bias voltage due to fluctuations in the dark current of the APD is offset, so there is an effect of preventing a decrease in the sensitivity of the APD at high temperatures.

Furthermore, a temperature fluctuation voltage with a negative slope is generated using a temperature sensing element having a negative temperature coefficient and a temperature characteristic adjustment resistor, and based on this fluctuation voltage, a bias voltage due to dark current fluctuation of the APD is finally determined. In addition, in the event of a breakage failure in the above-mentioned resistors, the connection to the photodetection APD is automatically disconnected, which prevents a decrease in the sensitivity of the APD at high temperatures. It has the effect of protecting APD.

Furthermore, the voltage applied to the bias voltage control circuit is set to a constant voltage using a constant voltage circuit, and the AP for photodetection
Since the bias voltage applied to D is not affected by fluctuations in the power supply, etc., there is an effect of preventing sensitivity fluctuations of the APD due to fluctuations in the power supply, etc.

Furthermore, by using resistance components such as a temperature sensing element and a temperature characteristic adjustment resistor at the same time as a constant voltage circuit,
First, the constant voltage circuit keeps the voltage applied to the bias voltage control circuit at a constant voltage so that the bias voltage applied to the photodetection AP is not affected by power fluctuations, etc., and the resistor component prevents the bias voltage from decreasing due to dark current of the APD. Since this counterbalances and prevents a decrease in APD sensitivity at high temperatures, it has the effect of preventing APD sensitivity fluctuations due to power supply fluctuations and ambient temperature.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of a photodetection circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between temperature, APD operating voltage, and APD bias voltage.

FIG. 3 is a diagram showing the relationship between temperature and APD sensitivity.

FIG. 4 is a diagram showing the relationship between temperature and offset voltage.

FIG. 5 is a diagram showing the configuration of a photodetection circuit according to a second embodiment of the present invention.

FIG. 6 is a diagram showing the configuration of a photodetection circuit according to a third embodiment of the present invention.

FIG. 7 is a diagram showing the configuration of a detection circuit according to a fourth embodiment of the present invention.

FIG. 8 is a configuration diagram showing a distance measuring device equipped with a conventional photodetection circuit.

FIG. 9 is a diagram showing the configuration of a conventional photodetection circuit.

FIG. 10 is a diagram showing the relationship between bias voltage and sensitivity of an APD.

[Explanation of symbols]

1 Laser oscillator 2 Transmission optical system 3 Receiving optical system 4 Photodetector 5 Photo detection circuit 6 Counting circuit 7 High voltage circuit 8 Current limiting resistor 9 Current limiting resistor 10 Variable resistor 11 Selector switch 12 Temperature compensation APD 13 APD for light detection 14 Amplification width 15 Posista 16 Temperature characteristic adjustment resistor 17 Thermistor 18 Constant voltage circuit

Claims (4)

[Claims] [Claim 1] A light-shielded first APD for temperature compensation (
Avalanche Photo Diode) and
a high-voltage circuit for applying a high voltage to the first APD to cause breakdown; a current-limiting resistor for limiting the current flowing out from the high-voltage circuit; and a variable voltage circuit for dividing the breakdown voltage. A resistor, a heat-sensitive element having a positive temperature coefficient characteristic inserted between the current limiting resistor and the variable resistor, a resistor for adjusting the temperature characteristic, and a voltage appropriately divided through the variable resistor. A second A converts the received light into current using the breakdown voltage as a bias source.
A photodetection circuit comprising a PD and an amplifier that converts the current from the second APD into voltage and amplifies it. 2. A light-shielded first APD for temperature compensation, a high voltage circuit for applying a high voltage to the first APD to cause breakdown, and a current for limiting the current flowing from the high voltage circuit. a limiting resistor, a variable resistor for dividing the breakdown voltage, a thermal element having a negative temperature coefficient characteristic inserted between the current limiting resistor and the variable resistor, and a resistor for adjusting the temperature characteristic. and a second A that converts the received light into a current using the breakdown voltage appropriately divided through the variable resistor as a bias source.
A photodetection circuit comprising a PD and an amplifier that converts the current from the second APD into voltage and amplifies it. 3. A first APD for temperature compensation shielded from light, a high voltage circuit for applying a high voltage to the first APD to cause breakdown, and a constant voltage circuit provided after the high voltage circuit. , a current limiting resistor for limiting the current flowing out from this constant voltage circuit, a variable resistor for dividing the breakdown voltage, and a breakdown voltage properly divided through the variable resistor. a second APD that converts received light into a current as a bias source;
What is claimed is: 1. A photodetection circuit comprising: an amplification device that converts and amplifies a current from an APD. 4. The photodetection circuit according to claim 3, further comprising a temperature sensing element having a positive or negative temperature coefficient characteristic between the current limiting resistor and the variable resistor.