JPS6020654A - Optical detecting circuit - Google Patents

Abstract

PURPOSE:To improve the maximum measuring level of incident optical power and to obtain a wide dynamic range by impressing a reverse bias voltage which is zero or minute at a region having a small incident light power and large at a region having large power to a photo diode. CONSTITUTION:When the incident light power of the photo diode 1 is increased gradually from zero, the current I0 of the photo diode 1 is also increased and a positive light detecting voltage V0 is obtained. On the other hand, a negative output voltage V1 in response to the light detecting current is obtained from an operational amplifier 6 for reverse bias of a reverse bias voltage generating circuit 5, the voltage is not fed back to a non-inverting input terminal until a rectifier diode 10 is forward-biased and the photo diode 1 is brought into the state where the diode 1 is impressed with a minute reverse bias voltage. In increasing the light power further, the negative output voltage V1 is also increased, the rectifier diode 10 is conducted and a negative voltage is applied to the non-inverting input terminal and the negative voltage at an inverting input terminal is impressed to the photo diode 1 as the reverse bias voltage.

Description

Detailed Description of the Invention Technical Field The present invention relates to a photodetection circuit used in optical power meters and the like, and more specifically, to a photodetection circuit that can improve the maximum measurement level of light received by a photodiode and obtain a wide dynamic range2. Regarding circuits.

Conventional technology Through the practical application of optical application technology centered on optical communication technology,
The M requirements for devices (optical power meters) that measure optical power (level or amount of light) have improved.

Photodetectors originally used in power meters used in the field of optical communications can be roughly divided into ℃ heat exchange type photodetectors and photoelectric conversion type photodetectors. The former type of thermal conversion type photodetector includes a thermomobile detector, which is used in standard optical power meters. However, it has the drawback of low photodetection sensitivity and inability to provide a wide dynamic range. On the other hand, there is a photodetector using a photodiode as the latter photoelectric conversion type photodetector. This photodiode photodetector can easily measure weak optical power and can perform photodetection with excellent linearity over a relatively wide dynamic range.

FIG. 1 shows a conventional photodetection circuit jjfl' capable of measuring weak original power. In this photodetection circuit, one end of the photodiode ill is connected to one input terminal (inverting input terminal) of a high input impedance operational amplifier (2) of FET input type, and the other end of the photodiode ill is connected to the other end of the photodiode ill The other input terminal (non-inverting input terminal) of each is grounded. Rf is a feedback resistor, which is connected between one input terminal and the output terminal of the operational amplifier (2). +31 is the zero point. It is a variable resistor for adjustment, and the output voltage V when the incident light of the photodiode [11 is made zero.

Y! - It is a resistor that is adjusted to zero V.

In this circuit, a photodetection voltage fiI corresponding to the incident light.

When is generated from the photodiode il+, since the arithmetic intensifier I (21 is the input impedance), the photodetection current does not flow there, but flows through the feedback resistor Rf.

On the other hand, since operational amplifier +21 operates so that the potential of the inverting input terminal is the same as the potential (OV) of the non-inverting input terminal, a virtual short circuit occurs between the inverting input terminal and the non-inverting input terminal. Therefore, V at the output terminal. = Engineering. A photodetection voltage of R7 is obtained.

Incidentally, the factor that determines the maximum light reception measurement repertoire of a photodiode photodetector is the series resistance, which is determined by the material of the photodiode and the light-receiving area.

Furthermore, the element that determines the minimum measurement level of light reception is the parallel resistance of the photodiode. Therefore, the use of photodiodes with low series impedance and high parallel impedance is a prerequisite for obtaining a wide dynamic range. In order to explain this point in detail, FIG. 2 shows an equivalent circuit N when a load RL is connected to the photodiode. Here, air is the current generated by the incident light (photodetection current J
, Iゎ is the diode current, ゎ is the parallel resistance current, Cj is the junction capacitance, R8H is the parallel resistance, R8 is the series resistance, ■. is the output current, and Vo is the output voltage. From this equivalent circuit. = 0, the output voltage m is the open circuit voltage V. Subtracting P gives the following formula.

Here, S: saturation current of the photodiode, q: electron charge, K: Boltzmann's constant, and T: absolute temperature of the element.

Also, the current when RL=0 and Vo=O, that is, the short circuit current s
When H is found, the following formula is obtained.

Open circuit voltage V. P is proportional to IPVC when the air is large, but generally it is V as shown in formula 111. Since P is not linearly proportional to air and is also affected by temperature, open circuit voltage V. It is not appropriate to measure the level of incident light based on P. For this reason short circuit electrician. It is desirable to use H for optical detection. The second term and the third term are related to the linearity of the short circuit current ISH in the +21 formula.

Generally, R8 is several Ω to several Ω, and RsH is 10'
Ω to 10Ω, and the second and third terms can be ignored over a fairly wide range. For example, in a silicon (Si) photodiode that is sensitive from visible light to near-infrared light, a linear proportional relationship is established between the M1 current sH and the photodetection/ll current IP. However, in germanium (GeJ) photodiodes, etc., which are sensitive to so-called long wavelength light (wavelength λ = 1.0 μm to 1.7 μm) used in optical communications, as the photodetection current Ip increases, the +21 type Second
The term can no longer be ignored, resulting in a short circuit. H becomes saturated.

The nonlinearity due to saturation of the short-circuit current ISH increases as the series resistance R8 of the photodiode increases.

In order to suppress the linearity of the short circuit current ISH to, for example, a deviation of 0.3% or less, it is necessary to set the series resistance R8 to about 50Ω or less. However, the series resistance R8 of a typical Ge photodiode currently on the market is in the range of 150Ω to several Ω, and sufficient linearity cannot be obtained in a region where the incident light power is large. If the series resistance Rs is lowered by lowering the resistance of the photodiode substrate, the non-M-line property due to the saturation of the short circuit current 18H can be improved. However, it is difficult to significantly reduce only the series resistance Rs by keeping the parallel resistance R8H thick.General photodiodes rather reduce the junction capacitance Cj by inserting a resistor in the substrate or by using a PIN structure. It is designed to reduce the amount of force and increase response speed.

The above-mentioned saturation of the short m current ISH in the region where the incident light power is large can be improved by applying a reverse bias voltage to the photodiode. FIG. 3 shows a conventional bias type photodetector circuit. This circuit consists of a photodiode tll
A photodetection voltage V corresponding to the voltage obtained across the load resistor 9 by connecting a rectification resistor RL to the K@ column and applying a reverse bias of V to the cathode of the photodiode 11+. is configured to obtain at the output terminal of the operational amplifier (41).However, this method has the following disadvantages.First of all, by reverse biasing the photodiode (11), the photodiode (11) Because the dark current Id increases, it is difficult to achieve high sensitivity.Especially with ordinary photodiodes, the dark current increases significantly in high-temperature environments, leading to large measurement errors. The dark current of a Ge photodiode is approximately 3 times higher than that of a Si photodiode used in the short wavelength region.
Since it is larger by ~4 orders of magnitude, practical measurement in a region where the wavelength of the light to be measured is 1 μm or more is extremely difficult.

Second, when a constant reverse bias voltage V is applied, the output voltage V across the load resistor 几corresponds to an increase in the photodetection current 1a. As t increases, the reverse bias voltage applied to the photodiode tl+ actually decreases. Therefore, there is a drawback that the photodetection sensitivity fluctuates due to changes in the incident light power. As a result, it is difficult to accurately measure the weak input optical power by using the circuit shown in FIG.

OBJECTS OF THE INVENTION Therefore, an object of the present invention is to provide a photodiode photodetection circuit that can increase the maximum measurement level of incident light power and obtain a wide dynamic range.

Structure of the Invention To achieve the above object, the present invention has a photodiode that generates a current corresponding to the power of incident light, a pair of input terminals and an output terminal, and one of the pair of input terminals is connected to the photodiode. an operational amplifier that is connected to one end of the photodiode, and the other of the pair of input terminals is connected to a ground level or a means for applying a constant voltage level; and feedback between the one input terminal and the output terminal of the operational amplifier. A feedback resistor connected in series is connected to the other end of the photodiode for applying a reverse bias voltage to the photodiode, and the reverse bias voltage becomes zero or very small in a region where the power of the incident light is small. , and a reverse bias voltage generating circuit formed so that the reverse bias voltage is large in a region where the power of the incident light is large.

Effects of the Invention According to the above invention, since the reverse bias voltage is maintained at zero or very small in the region where the incident light power is low, the incident light power can be accurately measured without being affected by dark current. I can do it. On the other hand, in a region where the incident light power is high, the reverse bias voltage increases, so the saturation of the short circuit is improved, and the maximum measurement level of the incident light becomes wide (thus, a wide dynamic range can be obtained). Note that the dark current based on the temporary application of the reverse bias voltage 1c at high incident light power is small compared to the photodetection current, so it has very little influence on the foot measurement degree.

First Embodiment Next, a photodetection circuit according to a first embodiment of the present invention will be described with reference to FIGS. 4 and 5. The basic parts of this photodetection circuit are the same as in Figure 1, and include a photodiode fi+ into which the light to be measured is incident, and an inverting input terminal (one input terminal) connected to the cathode [one end] of this photodiode (1). , an operational amplifier +21 whose non-inverting input terminal (the other input terminal) is connected to the ground (common ground line), and a feedback resistor Rf connected between the inverting input terminal and the output terminal of this operational amplifier (2). , and a zero point adjustment resistor (3).

(51 is a reverse bias voltage generating circuit, which is a newly added circuit according to the present invention. This reverse bias voltage generating circuit (51 is a reverse bias voltage generating circuit) The VC is configured to apply a reverse bias voltage corresponding to the optical power.More specifically, it includes an FET input type operational amplifier (6) for reverse bias voltage.

One of these input terminals (the inverted input terminal is a photodiode (
1), and the other input terminal (non-inverting input terminal) is connected to the voltage division point (7). t81
is a negative feedback resistor for setting the gain of current/voltage conversion, and is connected between the inverting input terminal and the output terminal of the operational amplifier (6). Additionally, a feedback resistor (91) and a rectifier diode (1) are connected between the output terminal of the operational amplifier (61) and the non-inverting input terminal.
0) is connected, and a variable resistor Uυ is connected between the non-inverting input terminal and the ground. In order to apply a negative voltage, the non-inverting input terminal is connected to the negative power supply terminal V'''' via a resistor 1101. To give an example of Naoyabe's constants, the resistance Rf is 1 Ω, the resistance +81 is 1.2 Ω, the resistance +91 is 2.2 Ω, and the variable resistor (ID is 2 Ω).
, resistor α0) is IMIQ, photodiode +11 is Ge
It is a photodiode.

In addition, it is generally preferable to select the resistor Rf and the resistor (8) from several + ohms to several zeros. t81.tlυ must be set.

Next, the operation of the circuit shown in FIG. 4 will be explained. Photodiode (
When the incident light power on the moon is zero, the photodetection current ■. Since is zero, the photodetection voltage ■. is zero volts. The output voltage y of the g1 amplifier +61 of the reverse bias voltage generating circuit (5) in the housing is also approximately zero volts. At this time, since a minute negative voltage of 1 to several mV is applied to the non-inverting input terminal of the reverse bias operational amplifier (61), the negative feedback effect via the resistor +81 also applies 1 to several mV to the inverting input terminal. A minute negative voltage is generated, which is applied to the anode (11) of the photodiode +11.
0 is applied as a reverse bias. This minute reverse bias voltage is provided to prevent a forward current from flowing through the photodiode +II for some reason, resulting in a forward bias state. Therefore, in the case of a circuit in which there is no possibility of becoming in a forward bias state, there is no need to apply it.

When the incident light power is gradually increased, the photodiode +11 current is generated. gradually increases, and Vo=I. A positive photodetection voltage can be obtained in relation to Rf. On the other hand, a negative output voltage V according to the photodetection current is obtained from the operational amplifier for reverse bias voltage (6).
and (lIJ) and is applied to the non-inverting input terminal. However, the output voltage Φ is not fed back to the non-inverting input terminal until the rectifier diode uOj is forward biased. In this case, an extremely low negative voltage (1 to 12 mV) is kept applied to the non-inverting input terminal through the resistor (10), and the voltage at the inverting input terminal of this operational amplifier (6) is also the same as that of the non-inverting input terminal. The voltage is maintained at the same value, and a minute reverse bias voltage is applied to the photodiode (1).

When the optical power is further increased, the photodetection voltage V. The output voltage of reverse bias operational amplifier +61 9 also increases accordingly, and the rectifier diode (I
O) conducts. As a result, the output voltage V is changed to the resistance (91
The voltage of the force divided between
Photodiode (11 cathodes, i.e. operational amplifier + 21
Since the inverting input terminal of is in the state of virtual ground (zero V),
The negative voltage at the inverting input terminal of the operational amplifier for reverse bias (61) is applied to the photodiode (11) at a reverse bias voltage of 7 degrees Celsius.Then, this reverse bias voltage is applied to the incoming light at 5 as shown in Fig. It increases as the power increases.If the photodiode tl+ is not a constant current source, the output voltage v1 of the operational amplifier +61 will tend to saturate due to positive feedback, but in this example, the photodiode tl+ is not a constant current source. Therefore, saturation of the operational amplifier (61) does not occur.

If the circuit of Fig. 4 is used, the reverse bias voltage generation circuit (51
If the circuit is removed, the circuit becomes the same as that shown in FIG. 1, so that the photodetection voltage is saturated as shown by v: in FIG. However, in this embodiment, a reverse bias voltage vRZ is applied that almost corresponds to the region where the conventional photodetection voltage v0 is non-linear and makes the rectifier diode QOI conductive and prevents saturation of the short-circuit current of the photodiode fi+. The photodetection voltage X maintains linearity even in the high incident light power region. can be obtained as shown in FIG. Therefore, the limit of the maximum measurement level of received light in the high incident light power region is small (both are the same as the limit in the circuit shown in FIG. 3).

The rectifier diode (101 uses the non-linear voltage-current characteristics of the forward voltage rise region of the photodiode tl+). In addition to having the function of rapidly increasing the reverse bias voltage R when the incident light power is saturated, for example, at a point where the incident light power is 00 μW or more, it also has the function of providing directionality of feedback of the operational amplifier (6). In other words, it has the function of raising the reverse Iasumi pressure R as shown in Fig. 5, and also prevents the non-inverting and inverting inputs of the operational amplifier from becoming positive if the output voltage Vl becomes positive for some reason. It has the function of preventing

In this embodiment, a small reverse bias voltage is applied to the photodiode (1) in a region where the incident light power is low, and a rapidly high reverse bias voltage is applied in a region where the incident light power is low. When a reverse bias voltage is applied in this way, a dark current naturally flows. However, in the region where the incident light power is low (FJ400
In the region below μW), the micro reverse bias voltage (1 to 3 mV
), so dark 1! The current is extremely small and can be ignored.

Even if the temperature of the Qe-PIN photodiode 111 is kept constant and a reverse bias voltage of 1.0 to 10.0 mV is applied.

It has been experimentally confirmed that there is virtually no change in sensitivity at °C.
There is. It has also been confirmed that the dark current becomes almost constant when the reverse bias voltage exceeds 5V.

On the other hand, as shown in Figure 5, when the incident light power is approximately 540 μW, the reverse bias voltage is 11 mV, and the IC analogy is 630 μW.
#20mV for W, 71mV for 1mW, 155mV for 2mW (7) 5 (< If the bias Y is made deeper, the dark current will also increase accordingly.

Light detection electrician. Since the ratio of dark current to K is small, the influence of dark current can be substantially ignored. In addition, photodetection current ■. Since the 5iC#8 is constructed so that the reverse bias voltage increases as the voltage increases.

The application of extra reverse bias voltage is restricted, and the influence of dark current is extremely reduced.

As is clear from the above (this embodiment has the following effects).

By applying an Iasumi pressure (inversely corresponding to the al photodetection current), it is possible to increase the maximum measurement level of the optical coupler by at least 6 dB or more.

On the other hand, in the region where the original power is low, the IA Sumi pressure is minuscule, so highly accurate measurement is possible without being substantially affected by dark current. Therefore, highly accurate measurement is possible over a wide dynamic range from a micro/J% optical power to several mW or more.

(Reverse based on bl photodetection current ■.) (Since the Iasumi pressure is obtained, it is a must-have converse) (Iasumi pressure can be easily obtained.

Since a very small voltage is applied through the cl resistor (101), it is possible to prevent a forward bias state from occurring for some reason when the incident light power is zero.

(d) Since rectifier diode u01 is provided, no force 1
Even if a positive output voltage ■1 occurs due to the cause of Let's talk about the photodetection circuit. However, in this FIG. 6 and FIGS. 7 to 9 explaining the modified example, the parts vclt that are common to FIG. The explanation of the sound μ is omitted here. It does not respond directly to the photodetection current of the inverse voltage generation circuit (51 is a photodiode) shown in FIG. It is configured as follows. Also,
Depending on the measurement range [Secondly, to change the value of the feedback resistor Rf,
Resistance R1 (100Ω), R, (1kΩ), Ra (10
Ω), R4 (100 kΩ), and R, (IMΩ), and a first switch S having a contact a1ga is provided to select these. In addition, in order to switch the reverse bias voltage value, resistors R, (1kΩ), Rv
(10kΩ) is provided.

Reverse bias calculation j for these resistors R, , R7 and ground
(2) A second switch S2 is provided for selectively connecting the inverting input terminal of the speaker +61. The second switch S has contacts S, ~b, and is interlocked with the first switch SI.

The non-inverting input terminal of the reverse bias operational amplifier 161 is grounded, and between the inverting input terminal and the output terminal there is a resistor (88 rectifier diode) and a resistor (91 is A resistor (9) and a diode (10) are connected between the output terminal of the operational amplifier +61 and the ground.
1 and a variable resistor aD are connected, and the voltage dividing point (7) is connected to the anode of the photodiode +11. The dividing point (7) Ic is also connected to a negative power supply terminal 2- for applying a minute reverse Iasumi pressure via a resistor tta.

The operation of this circuit will be explained next. Since the circuit shown in FIG. 6 includes changeover switches S+ and S2, a contact point matching the magnitude of the incident light power is selected.

That is, the output voltage ■ of the operational amplifier (2). For example, an analog-to-digital converter (not shown) measuring the
/I/VC, switch from contact Yaa toward 31. When the incident light power is small, the contacts a, b
Select 6. When light enters the photodiode 113 in this state, the output voltage o corresponding to the original voltage becomes the fourth
Obtained in the same way as in the figure. On the other hand, since the inverting input terminal of the inverse bias operational amplifier +61 is grounded through the contact point S2 of the switch S2, the output voltage VI becomes zero V. Therefore, a minute reverse bias voltage is applied to the photodiode +I+ through the resistor σl. Therefore, measurements are performed in a state where there is substantially no influence of dark current based on the reverse bias voltage.

When the incident optical power increases to full scale, contact va4. Switch to b, or a3, b3. Even if the contacts are switched in this way, they operate in the same way as contacts a6 and b and P1.

Since the incident light power is large, if the contacts a8 and bs are at full scale, the % contact albbt is selected.

At this time, the inverting input terminal of the operational amplifier (6) is connected to the output terminal of the preceding operational amplifier (21) via the contacts of the switch S and the resistor R7, and the photodetection voltage V is output. An inverted amplified output voltage v1 is obtained. At this time, resistor R7 is set to 10
Ω, resistance (81 to 2 Ω, diode (10
)'s anode voltage is approximately 1■. It becomes 15. The inverted output voltage V is divided by the resistor (9) and (ID) and applied as a reverse bias voltage to the anode of the photodiode fl+.Therefore, the same effect as the circuit shown in FIG. 4 can be obtained. Since the diode (10) operates in the same manner as in the case of FIG. 4, linearity in the range from low incident optical power to high incident optical power is ensured.

When the incident light power increases further, the contacts al and bl
Turn on. In this case, resistor R8 is set to 100. Q, if the resistor R6 is 1Ω, the photodetection voltage v0 becomes “/10” by switching from contact a2 to al. On the other hand, the operational amplifier (6
The gain of 1 becomes 2 when the contact b2 is switched from to closed.

Therefore, the same light-detecting electrician. In the case of , contact a1b
1 and albl, the same reverse bias voltage can be obtained.

Modifications The present invention is not limited to the above-described embodiments (for example, it includes various modifications such as the following).

(Al In Fig. 4 and Fig. 6, the photodiode 11
．． 1 and the rectifier diode polarity are reversed, and the resistor u
It may be connected to 21 positive W sources.

(Bl Let R+ -Rs in Fig. 6 be a single resistor Rf,
In addition, a single resistor R6 may be used for R6 to R1, and the circuit configuration shown in FIG. 7 may be adopted. In addition, in Fig. 7, the photodiode il
+ and the polarity of the rectifier diode curve is reversed and the resistor 11
2+ is connected to positive is. Even with this configuration, since the diode (101) is provided, a reverse bias voltage is not substantially applied in the low incident light power region, and an increase in dark current is prevented.

(0 reverse bias voltage generation circuit (51) may be configured as shown in FIG.
and photodetection current■. and 'c% better. In the circuit of Fig. 8, the resistors Ra,
1 photodetection voltage v by (8a) and operational amplifier (6a)
Obtain an inverted voltage V of 0, add this voltage V2 to a voltage divider circuit consisting of a resistor (9), a diode Q (II, and a resistor σD), and apply the voltage at the 1 division point +71 to the non-inverting input terminal of the operational amplifier (6b). The input formula 31i having two feedback resistors (8b) and the amplifier (6b) operate in the same manner as in FIG. 4, and the same effects can be obtained.

0 Photodetection current of photodiode 111■. If the saturation of the operational amplifier +21 becomes large and the saturation of the operational amplifier +21 has an adverse effect, a pair of transistors Q, , Q, and a pair of diodes D1 . A complementary emitter follower output circuit consisting of D2 and a pair of resistors Ro and Rn may be added. Also, the ninth output terminal 1c of the operational amplifier (6)
The output circuit of the complementary emitter follower shown in the figure may also be connected.

(E+ In place of the diode (tol), another non-linear element may be connected. Also, by combining the diode [0) with a resistor or another non-linear element, the photodiode (1
It is also possible to generate a reverse bias voltage that conforms to the characteristics of 1. In addition, the reverse bias current EE follows the increase in photodetection current from the low source power region to the high optical power region.
When increasing δ, the diode aO+ may be omitted. r) Operational amplifier f21 (The non-inverting input terminal of 61 may be connected to a constant voltage without being grounded.

(Not limited to GI Ge photodiodes,
Si photodiode, 1. It is also applicable when using a P photodiode or the like.

■ Photodetection photodiode (a control photodiode with substantially the same characteristics as the moon is provided, the same source as the photodetection photodiode il+ is provided to this photodiode, and a reverse bias voltage is generated based on the current VC of this control photodiode) It may be thinned.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional photodetection circuit, Figure 2 is an equivalent circuit diagram when a load is connected to a photodiode, and Figure 3 is a circuit diagram showing a conventional photodetection circuit using a reverse bias application method. Figure 4 shows a photodetection circuit 2 according to the first embodiment of the present invention.
The circuit diagram shown in FIG. 5 is a characteristic diagram showing the relationship between the incident light power, the photodetection voltage, and the reverse bias voltage to explain the operation of the photodetection circuit of FIG. 4, and FIG. A circuit diagram showing the photodetection circuit of the second embodiment, and FIGS. 7, 8, and 9 are circuit diagrams showing modification examples of the photodetection circuit, respectively. tl+... photodiode, +21... operational amplifier, (5)... reverse bias voltage generation circuit, (6j...
Reverse bias operational amplifier, (t(II...rectifier diode, Rf...#M resistor. Proxy Takano Noriji 4Figure 5Enter Liu-Ku-9W) 0Figure 1' L J

Claims (1)

[Scope of Claims] (11) comprising a photodiode that generates a current corresponding to the power of incident light, a pair of input terminals and an output terminal, and one of the pair of input terminals is connected to one end of the photodiode, an operational amplifier, the other of the pair of input terminals being connected to a ground level or means for providing a constant voltage level; and a feedback connected in series to a feedback circuit between the one input terminal and the output terminal of the operational amplifier. a resistor connected to the other end of the photodiode in order to apply a reverse bias voltage to the photodiode, and the reverse bias voltage is zero or very small in a region where the power of the incident light is small, and a region where the power of the incident light is large; [2+] A photodetector circuit comprising a reverse bias voltage generating circuit formed in 5 such that the reverse bias voltage increases at 5. [2+] The reverse bias voltage generating circuit increases the reverse bias voltage in response to the current of the photodiode. The photodetection circuit according to claim 1, which is a circuit including an operational amplifier for generating a reverse bias. +31 The reverse bias voltage generation circuit is responsive to the photodetection voltage obtained from the output terminal of the operational amplifier. The photodetection circuit according to claim 1, which is a circuit that generates the reverse bias voltage. (4) The reverse bias voltage generation circuit generates both the current of the photodiode and the photodetection voltage. 2. The photodetection circuit according to claim 1, which is a circuit that generates a reverse bias voltage in response to.