JP2004343441A - Light receiving circuit and distance measuring unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-noise and wide-band light receiving circuit amplifying a current signal generated by a photoelectric conversion section for output, and a distance measuring unit using the same. <P>SOLUTION: A constant bias current Ib supplied by a bias circuit 3 is superimposed on a current signal Ipd supplied from a photodiode 2. The resulting current signal Ipd+Ib is amplified m times by means of a current amplifier 4 composed of a current amplification type current mirror circuit 40. A voltage conversion circuit 5 converts an amplified current signal Ia into a voltage signal Vo. The bias current Ib positively operates the current mirror circuit 40 and is set to the minimum magnitude to secure a required response frequency (band). Thus, the current mirror circuit 40 is not brought into an inoperative state even when the current signal Ipd is extremely weak. In addition, the development of shot noise is controlled to the minimum. Thus, the detection of weak light and high-speed operation are compatible. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light receiving circuit that amplifies and outputs a current signal generated by a photoelectric conversion unit, and a distance measuring device using the same.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a light receiving circuit shown in FIGS. 10A and 10B using a photoelectric conversion element such as a photodiode and generating a signal corresponding to the intensity of irradiation light is known.
[0003]
The light receiving circuit 101 shown in FIG. 10A includes an amplifier circuit and the like after converting a current signal Ipd generated by the photodiode 102 into a voltage signal Vpd by a load resistor 103 connected in series to the photodiode 102. The signal processing circuit 104 is configured to amplify the voltage signal Vpd (for example, see Patent Document 1).
[0004]
The light receiving circuit 111 shown in FIG. 10B amplifies the current signal Ipd generated by the photodiode 112 by the current mirror circuit 113, and then amplifies the current signal Ipd by the transimpedance circuit 114 including an operational amplifier 114a and a feedback resistor 114b. It is configured to convert the amplified current signal Ia into a voltage signal Vo (for example, see Patent Document 2).
[0005]
[Patent Document 1]
JP-A-09-31181 (FIG. 1)
[Patent Document 2]
JP-A-2002-344252 (FIG. 1)
[0006]
[Problems to be solved by the invention]
Among these, in the light receiving circuit 101 that performs amplification after voltage conversion, it is necessary to increase the resistance value of the load resistor 103 in order to secure light receiving sensitivity. However, when the resistance value of the load resistor 103 is increased, thermal noise generated in the load resistor 103 increases, so that it becomes difficult to detect weak light (that is, the minimum light receiving current decreases) and the parasitic capacitance of the photodiode 102 increases. As a result, the response frequency (bandwidth of the gain) is reduced due to the influence of the light receiving circuit 101, so that the light receiving circuit 101 cannot be operated at high speed.
[0007]
On the other hand, in the light receiving circuit 111 that performs voltage conversion after current amplification, the resistance value of the feedback resistor 114b used for voltage conversion can be reduced, and thermal noise generated in the feedback resistor 114b can be reduced.
However, in the light receiving circuit 111, the current signal Ipd from the photodiode 112 is the base current of the transistors 113a and 113b constituting the current mirror circuit 113 as it is. Therefore, in a region where the current signal Ipd is small, the effect of the parasitic capacitance of the photodiode 112 and the transistors 113a and 113b increases, the response frequency of the transistors 113a and 113b decreases, and the on-voltage cannot be maintained, so that the transistor 113a , 113b cannot be driven. Therefore, also in this case, there is a problem that it is difficult to detect weak light (that is, the minimum light receiving current is reduced).
[0008]
When this type of light receiving circuit is applied to, for example, an in-vehicle laser radar device for obstacle detection, the minimum light receiving current determines the maximum detection distance of an obstacle, and the response frequency determines the response of detection. Since it is a factor and greatly affects the performance of the device, it is desired to improve these characteristics.
[0009]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a low-noise, wide-band light receiving circuit and a distance measuring device using the same, in order to solve the above problems.
[0010]
[Means for Solving the Problems]
In the light receiving circuit of the first invention made in order to achieve the above object, when the photoelectric conversion unit generates a current signal according to the intensity of the irradiation light, the bias circuit superimposes a bias current on the current signal, A current amplifier comprising a current amplification type current mirror circuit amplifies and outputs a current signal on which the bias current is superimposed.
[0011]
That is, in the light receiving circuit of the present invention, since the current signal amplified by the current amplifying unit may be converted into a voltage signal, it is possible to reduce thermal resistance superimposed on the load resistance that performs this conversion and, consequently, the load resistance. it can.
If the bias current is set to a value at which the desired response frequency (band) is ensured in the current amplifier, no matter how small the current signal, the current amplifier does not become inoperable.
[0012]
That is, according to the light receiving circuit of the present invention, it is possible to achieve both the detection of weak light and the high-speed operation.
However, as the bias current superimposed on the current signal increases, the shot noise generated in the transistors constituting the current mirror circuit of the current amplifier increases, and the minimum light receiving current deteriorates (increases). It is desirable to set a minimum size that can secure a necessary response frequency (band).
[0013]
By the way, a large light receiving surface is required as a photoelectric conversion unit, but when only a part of the light is to be irradiated with light to be detected (hereinafter, referred to as “detection light”), the background light is received in a part not irradiated with the detection light. Therefore, the noise current based on the background light increases, and the minimum light-receiving current deteriorates.
[0014]
Therefore, the photoelectric conversion unit is configured by a plurality of photoelectric conversion elements arranged so as to cover a predetermined light receiving surface, and a bias circuit and a current amplification unit are provided for each of the photoelectric conversion elements, and the plurality of photoelectric conversion elements are provided. A selection circuit for selecting a part of the current amplification unit and extracting an output from the selected current amplification unit may be provided.
[0015]
In this case, an output with a small noise current based on the background light can be obtained by appropriately selecting a current amplifying unit provided corresponding to the photoelectric conversion element positioned at the irradiation site of the detection light by the selection circuit.
Further, in this case, the light receiving surface of each photoelectric conversion element is reduced, and the parasitic capacitance of the photoelectric conversion element is reduced, so that the response frequency of the photoelectric conversion element and, consequently, the light receiving circuit can be improved.
[0016]
Note that a bipolar transistor or a MOS field effect transistor may be used as a transistor constituting the current mirror circuit of the current amplifier.
In particular, when a MOS field-effect transistor is used, the element size can be reduced as compared with a bipolar transistor, and the parasitic capacitance can be suppressed accordingly, so that the response frequency can be further improved.
[0017]
Further, it is desirable that the photoelectric conversion unit, the bias circuit, and the current amplification unit be formed on a single semiconductor substrate, that is, be configured as a monolithic integrated circuit.
Next, in the distance measuring device of the second invention, the laser irradiating means sweeps and irradiates the space with the laser light, and the light receiving means receives the reflected light from the object existing in the irradiation area of the laser light. Then, the round trip time from when the laser light is irradiated to when the reflected light is received is measured, and based on the measured time, the distance to the object reflecting the laser light is obtained.
[0018]
The light receiving means includes a light receiving circuit according to claim 3 and a condenser lens for condensing the reflected light so that the reflected light is applied to a part of the light receiving surface. According to the irradiation direction of the laser beam by the means, the current amplification unit corresponding to a part of the photoelectric conversion elements to be irradiated with the reflected light condensed by the condenser lens is configured to be selected by the selection circuit. I have.
[0019]
That is, in the distance measuring device of the present invention, since the output using the light receiving circuit according to the third aspect of the present invention can be obtained with less background noise from the light receiving circuit, the minimum light receiving current of the light receiving circuit is reduced. In addition, since each photoelectric conversion element is small, its parasitic capacitance can be reduced, and as a result, the object detection capability (maximum detection distance) and the response of detection can be improved.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
FIG. 1 is a circuit diagram illustrating a configuration of the light receiving circuit 1 according to the first embodiment.
[0021]
As shown in FIG. 1, the light receiving circuit 1 of the present embodiment includes a photodiode 2 serving as a photoelectric conversion element for generating a current signal Ipd corresponding to irradiation light, and a constant bias applied to the current signal Ipd from the photodiode 2. The bias circuit 3 includes a bias circuit 3 for superimposing the current Ib and a current amplification type current mirror circuit 40 including a pair of NMOS field effect transistors (hereinafter simply referred to as “transistors”) 41 and 42. And a diode-connected PMOS field-effect transistor 51, which amplifies the current signal Ipd + Ib multiplied by m (>1; m = 10 in this embodiment). A voltage conversion circuit 5 for converting the voltage into a voltage signal, all of which are formed on a single semiconductor substrate. In the drawing, a capacitance Cpd indicated by a dotted line is equivalent to the parasitic capacitance of the photodiode 2.
[0022]
The connection terminal (drain terminal of the transistor 42) between the current amplification unit 4 and the voltage conversion circuit 5 is used as an output terminal to output a voltage signal Vo whose signal level changes according to the light intensity of the irradiation light. ing.
Further, the bias circuit 3 includes an NMOS field effect transistor 33 constituting a common source circuit, a voltage dividing resistor 34, 35 for dividing the power supply voltage Vcc to generate a gate voltage applied to the transistor 33, and a pair of PMOS field effect transistors. The current mirror circuit 30 includes transistors 31 and 32, and uses the drain current of the transistor 33 as an input current and supplies an output current of the same magnitude as the bias current Ib.
[0023]
However, each of the current mirror circuits 40 and 30 is configured as a so-called Widlar type current mirror circuit in which the drain and gate of the input-side transistors 41 and 31 are short-circuited.
In the light receiving circuit 1 of the present embodiment thus configured, when light is irradiated to the photodiode 2, the photodiode 2 generates a current signal Ipd corresponding to the intensity of the irradiated light, and the current signal Ipd , The bias circuit 3 superimposes the bias current Ib. Then, the current mirror circuit 40 of the current amplifier 4 amplifies the current signal Ipd + Ib to which the bias current has been added by m times, and the amplified current signal Ia (= m × (Ipd + Ib)) is converted by the voltage conversion circuit 5. It is converted to a voltage signal Vo and output from an output terminal.
[0024]
Here, FIG. 2A is a graph showing the relationship between the bias current Ib and the bandwidth based on the obtained frequency characteristics of the gain G (= Vo / Ipd) of the light receiving circuit 1 by simulation. It is.
FIG. 2B shows, by simulation, a noise voltage Vn superimposed on the voltage signal Vo based on shot noise and thermal noise generated in the transistors 31 to 33, 41, 42, and 51 constituting the light receiving circuit 1. 9 is a graph showing a relationship between a bias current Ib and an input-converted noise N (= Vn / G) based on the obtained result.
[0025]
However, it is assumed that the parasitic capacitance of the photodiode 2 is Cpd = 0.2 pF, and the current signal generated by the photodiode 2 is Ipd = 10 nA. In addition, since the input-converted noise N has frequency characteristics, the case of 1 kHz and the case of 10 MHz are shown here.
[0026]
That is, in the light receiving circuit 1, the bias current Ib is set based on the graph shown in FIG. 2A so that the required band (response frequency) B can be secured. However, the point where the graph changes discontinuously is that the transistors 41 and 42 become inoperable, and the bias current Ib needs to be set to at least a value larger than this point. Further, as the bias current Ib is increased, the shot noise of the transistor 41 is also increased. Therefore, the bias current Ib is set to a necessary minimum value.
[0027]
For example, if the band B needs to be 10 MHz or more, the bias current Ib may be set to 0.6 μA or more.
When the bias current Ib is determined, the band B can be determined from the graph shown in FIG. 2A, and the input-converted noise N can be determined from the graph shown in FIG. 2B. ) Can be estimated.
[0028]
In the light receiving circuit 1, since the bias current Ib is determined by the resistance values R1 and R2 of the voltage dividing resistors 34 and 35, at the time of design, the graphs of FIGS. Instead, it is convenient to use a graph converted so that the resistance value R1 or R2 or the resistance ratio R1 / R2 is on the horizontal axis.
[0029]
For example, when the resistance value R1 is set to 10 kΩ, there is a relationship shown in FIG. 2C between the resistance value R2 and the bias current. If the graph is converted to a graph having the resistance value R2 as the horizontal axis instead of the bias current Ib, the graphs shown in FIGS. 3A and 3B are obtained. FIG. 3C is a graph showing the relationship between the resistance value R2 and the gain G, and FIG. 3D is a graph showing the relationship between the resistance value R2 and the minimum light receiving current.
[0030]
That is, if the graphs shown in FIGS. 3A to 3C are used, the light receiving circuit 1 having desired characteristics can be easily designed only by adjusting the resistance value R2.
As described above, in the light receiving circuit 1 of the present embodiment, the current signal Ipd from the photodiode 2 is amplified by the current mirror circuit 40 of the current amplifying unit 4, and then converted to the voltage signal Vo by the voltage conversion circuit 5. ing. Therefore, a low-resistance voltage conversion circuit 5 can be used, and thermal noise superimposed on the voltage signal Vo in the voltage conversion circuit 5 can be reduced.
[0031]
Further, a bias current Ib is superimposed on the current signal Ipd supplied to the current mirror circuit 40 of the current amplifying unit 4, and the bias current Ib has a minimum value capable of securing a required response frequency (band B). Is set to Therefore, even when the current signal Ipd is very small, the current mirror circuit 40 can be reliably operated, and the occurrence of shot noise can be suppressed to the minimum necessary, and both the detection of weak light and the high-speed operation can be achieved. be able to.
[0032]
In the present embodiment, each transistor constituting the light receiving circuit 1 is configured using a MOS field effect transistor. However, for example, the bias circuit 3 and the current amplifying unit 4 are configured as in the light receiving circuit 1a shown in FIG. Instead of the MOS field-effect transistors 41, 42, 31-33, bipolar transistors 41a, 41b may be used, and a resistor 52 may be used instead of the transistor 51 forming the voltage conversion circuit 5. However, in FIG. 4, the details of the bias circuit 3 are omitted and described as a constant current source (the same applies to FIGS. 5 to 7 hereinafter).
[0033]
Further, in the present embodiment, as the current mirror circuit 40 of the current amplifying unit 4, a Widlar type in which the gate and the drain of the input-side transistor 41 are short-circuited is used. However, when a bipolar transistor is used, it is shown in FIG. As in the light receiving circuit 1b, a base current compensation type current mirror circuit in which a transistor 43a for supplying a base current to the transistors 41a and 42a is added may be used. The resistor 44 inserted between the base and the emitter of the transistors 41a and 42a stabilizes the base potential of the transistors 41a and 42a. Further, like the light receiving circuit 1c shown in FIG. 6, a transistor having a configuration in which the transistors 41a to 43a forming the light receiving circuit 1b shown in FIG. 6 are replaced with MOS field effect transistors 41 to 43 from bipolar transistors may be used. Good. In addition, a Wilson current mirror circuit or the like may be used.
[0034]
Further, as the voltage conversion circuit 5, a transimpedance circuit including an operational amplifier and a feedback resistor may be used instead of the diode-connected MOS field-effect transistor or resistor.
[Second embodiment]
Next, a second embodiment will be described.
[0035]
FIG. 7 is a circuit diagram showing a configuration of the light receiving circuit 60 of the present embodiment.
As shown in FIG. 7, the light receiving circuit 60 of the present embodiment includes p × q (p, q ≧ 1) light receiving blocks BKij (i = 1 to p, j = 1 to q) arranged in a matrix. A selection circuit for selectively extracting the output from one or a plurality of light receiving blocks BKij, and a diode-connected PMOS field effect transistor. The output (current signal) extracted by the selection circuit is converted into a voltage signal. And a voltage conversion circuit 65 for converting the voltage into a voltage.
[0036]
Among them, the light receiving block BKij is different from the light receiving circuit 1 of the first embodiment in that a switching NMOS field effect transistor 45 is added between the gates and sources of the transistors 41 and 42 constituting the current mirror circuit 40, and a voltage conversion circuit is provided. The configuration is exactly the same as that of the light receiving circuit 1 except that 5 is omitted.
[0037]
The selection circuit includes a row selection line LRi to which the gates of the transistors of the light receiving blocks arranged in the row direction are commonly connected, and a current mirror circuit 40 (transistors 41 and 42) of the light receiving blocks arranged in the column direction. ) Is provided with a column selection line LCj to which the output terminals are commonly connected, and an output line LO to which the column selection line LCj is commonly connected via the transistor TRj and to which the voltage conversion circuit 65 is connected. .
[0038]
However, the photodiodes 2 constituting each light receiving block BKij are arranged so as to cover a predetermined light receiving surface. That is, one light receiving block BKij is assigned to each of the divided light receiving surfaces obtained by dividing the light receiving surface in a matrix.
[0039]
In the light receiving circuit 60 configured as described above, the light receiving block BKxj connected to the row selection line LRx turns on the transistor 45 when the row selection line LRx is set to the high level. When the row selection line LRx is set to the low level, the operation of the current mirror circuit 40 is permitted by turning off the transistor 45.
[0040]
When the transistor TRy connected to the column selection line LCy is turned on, the output current of the light receiving block BKxy connected to the row selection line LRx set to low level flows through the column selection line LCy. That is, the sum of the output currents from all the light receiving blocks BKxy specified by the row selection line LRx set to the low level and the column selection line LCy whose transistor TRy is set to ON is output from the voltage conversion circuit via the output line LO. By flowing through 65, a voltage signal Vo corresponding to the sum of the output currents is obtained from the output line LO.
[0041]
Here, FIG. 8 is a schematic configuration diagram of an in-vehicle laser radar device configured using the light receiving circuit 60 of the present embodiment.
As shown in FIG. 8A, the on-vehicle laser radar device includes a light emitting circuit 71 that emits a semiconductor laser and outputs pulsed laser light (in this embodiment, a pulse width of 50 ns), a lens and a mirror. A scanning mechanism 72 for sweeping and irradiating a space with the laser light output from the light emitting circuit 71 using the configured scanner is provided. The on-vehicle laser radar device includes a condenser lens 73 for condensing reflected light from an object present in an irradiation area of the laser beam, and a light receiving device according to the present embodiment for receiving the reflected light condensed by the condenser lens 73. A circuit 60, a pixel selection circuit 74 for controlling a selection circuit of the light receiving circuit 60 to select a pixel (light receiving block BKij) used for light reception, an amplifier 75 for amplifying an output Vo of the light receiving circuit 60, and an output of the amplifier 75 , A light emitting circuit 71, a pixel selecting circuit 74, and a measuring unit 77, which includes a time measuring circuit 76 that measures the round trip time of the laser light that has reciprocated with the object reflecting the laser light. An arithmetic processing unit 78 is provided to execute a process of calculating the distance to the object reflecting the laser light from the round trip time of the laser light measured by the unit 77.
[0042]
The light emitting circuit 71 and the scan mechanism 72 correspond to a laser beam irradiating unit, and the condenser lens 73, the light receiving circuit 60, and the pixel selecting circuit 74 correspond to a light receiving unit. In particular, the pixel selecting circuit 74 corresponds to a selecting control unit.
The photodiode 2 of each light receiving block BKij constituting the light receiving circuit 60, that is, each pixel forming the light receiving surface is a spot of the reflected light collected by the light collecting lens 73 as shown in FIG. 8B. The diameter is set to be about the same as the diameter, and in the present embodiment, it is formed to have a size of about 200 μm × 200 μm (parasitic capacitance 0.2 pF).
[0043]
The pixel selection circuit 74 is configured to select only one or a plurality of light receiving blocks BKij to which the reflected light condensed by the condensing lens 73 is irradiated according to the direction in which the scanning mechanism 72 emits the laser light. Have been. That is, the light receiving block BKij located in the portion that receives only the background light is not operated.
[0044]
As described above, when the light receiving circuit 60 according to the present embodiment is used in a vehicle-mounted laser radar device, the light receiving surface is based on the background light as compared with the case where the light receiving surface is configured by a single photodiode. Noise can be significantly reduced, and as a result, the minimum light receiving current decreases, so that the ability to detect an obstacle (maximum detection distance) can be greatly improved.
[0045]
FIG. 9 is a graph showing the relationship between the parasitic capacitance of the photodiode 2 and the band (response frequency) of the light receiving block BKij. The photodiode 2 is configured so that the parasitic capacitance is about 0.2 pF. The light receiving circuit 60 of the present embodiment can obtain a band of 10 MHz or more.
[0046]
Specifically, as compared with the case where the conventional light receiving circuit 101 is used (photodiode size: about 10 mm × 3 mm, parasitic capacitance: about 80 pF), the input-converted noise N is reduced even when there is no influence of background light. It can be reduced to about 1/7. That is, when the influence of the background light is taken into consideration, the difference in performance from the conventional device is further increased.
[Brief description of the drawings]
FIG. 1 is a circuit diagram illustrating a configuration of a light receiving circuit according to a first embodiment.
FIG. 2 is a graph showing characteristics of a light receiving circuit.
FIG. 3 is a graph showing characteristics of a light receiving circuit using a resistance value R2 instead of a bias current.
FIG. 4 is a circuit diagram showing another configuration example of the light receiving circuit.
FIG. 5 is a circuit diagram showing another configuration example of the light receiving circuit.
FIG. 6 is a circuit diagram showing another configuration example of the light receiving circuit.
FIG. 7 is a circuit diagram illustrating a configuration of a light receiving circuit according to a second embodiment.
FIG. 8 is a block diagram illustrating a configuration of an on-vehicle laser radar device configured using the light receiving circuit of the second embodiment.
FIG. 9 is a graph showing a relationship between a parasitic capacitance of a photodiode and a band (response frequency).
FIG. 10 is a circuit diagram showing a configuration of a conventional device.
[Explanation of symbols]
Reference numerals 1, 1a to 1c, 60: light receiving circuit, 2: photodiode, 3: bias circuit, 4: current amplifier, 5, 65: voltage conversion circuit, 30, 40: current mirror circuit, 31 to 33, 41 to 43 , 45, 51, TRj: transistor, 34, 35: voltage dividing resistor, 44, 52: resistor, 71: light emitting circuit, 72: scanning mechanism, 73: condenser lens, 74: pixel selecting circuit, 75: amplifier, 76: time measurement circuit, 77: measurement unit, 78: arithmetic processing unit, BKij: light receiving block, LRi: row selection line, LCj: column selection line, LO: output line.

Claims (6)

A photoelectric conversion unit that generates a current signal according to the intensity of the irradiation light,
A bias circuit that superimposes a bias current on a current signal generated by the photoelectric conversion unit;
A current amplification unit including a current amplification type current mirror circuit that amplifies the current signal on which a bias current is superimposed by the bias circuit;
A light receiving circuit comprising: 2. The light receiving circuit according to claim 1, wherein the bias current is set to a minimum necessary value for operating the current amplifier at a desired response frequency. The photoelectric conversion unit includes a plurality of photoelectric conversion elements arranged to cover a predetermined light receiving surface,
A bias circuit and a current amplifier are provided for each of the photoelectric conversion elements,
The light receiving circuit according to claim 1, further comprising a selection circuit that selects a part of the current amplification unit and extracts an output from the selected current amplification unit. 4. The light receiving circuit according to claim 1, wherein a transistor constituting a current mirror circuit of the current amplifying unit is a bipolar transistor or a MOS field effect transistor. The light receiving circuit according to any one of claims 1 to 4, wherein the photoelectric converter, the bias circuit, and the current amplifier are formed on a single semiconductor substrate. Laser irradiation means for sweeping and irradiating a space with laser light,
Light receiving means for receiving reflected light from an object present in an irradiation area of the laser light irradiated by the laser light irradiation means,
A distance measuring device that measures a reciprocating time from when the laser light is irradiated to when the reflected light is received and, based on the measured time, obtains a distance to the object that reflects the laser light.
The light receiving means,
A light receiving circuit according to claim 3,
A condenser lens for condensing the reflected light so that the reflected light is applied to a part of the light receiving surface;
The selection circuit selects a current amplification unit corresponding to a part of the photoelectric conversion elements to be irradiated with the reflected light condensed by the condenser lens according to the irradiation direction of the laser light by the laser irradiation unit. Selection control means;
A distance measuring device comprising:

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