JPH0832364A - Optical pulse current detection circuit - Google Patents

Abstract

PURPOSE:To improve the linearity, S/N, an error and a dynamic range of an optical pulse current by using an operational amplifier for negative feedback and using a transistor(TR) so as to extract a DC optical current flowing to a photoelectric conversion element via a switch. CONSTITUTION:A photo current flows to a photoelectric conversion element 1 to cause an output voltage VOUT through an NPN transistor(TR) 4 and a logarithmic transformation diode 2. The output voltage VOUT is applied to an inverting input terminal of an operational amplifier 7 whose noninverting input terminal connects to a cathode of a logarithmic transformation diode 3 and a constant current soure 8, a switch 9 is controlled by the operational amplifier 7 to apply negative feedback control to an NPN TR 5. Thus, only the pulsive constant current of the element 1 is extracted via the TR 5, by which the optical pulse current is detected with high accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulsed photocurrent detection circuit in a so-called active distance measuring device,
In particular, a DC photocurrent generated by external light from the output current of a photoelectric conversion element that receives reflected light of a pulsed light beam projected from a light beam projection unit such as an LED toward a distance measurement target and generates a pulsed photocurrent. The present invention relates to a pulsed photocurrent detection circuit that extracts a component and transmits only the pulsed photocurrent to a signal processing circuit in the next and subsequent stages.

[0002]

2. Description of the Related Art FIG. 5 is a circuit diagram of a conventional pulsed photocurrent detection circuit. In FIG. 5, reference numeral 21 denotes a photoelectric conversion element, 22
Is a logarithmic conversion diode, 23, 24 and 25 are NPN transistors, 26 is a capacitor, 27 is a switch, 28
Is an operational amplifier, and 29 is a constant current source.

FIG. 6 is a circuit diagram of a light beam projection means for irradiating a pulsed photocurrent detection circuit with pulsed light. In FIG. 6, numeral 30 is normally off and is synchronized with the turning off of the switch 27 (in the case of an embodiment described later, the switch 9 or 12 is used).
A switch that is turned on (in synchronization with), 31 is a switch 30
Light-emitting elements such as light-emitting diodes that emit light when the power is turned on
Reference numeral 2 is a transistor for controlling light emission, 33 and 34 are resistors, 35 is a DC power source, and 36 is an object to be measured.

The operation will be described with reference to FIG.
When the switch 30 is turned on, the transistor 32 is turned on and power is supplied from the DC power source 35, and the light emitting element 31 emits light. Then, the light of the light emitting element 31 is projected to the outside through the lens 37, hits the distance measuring object 36, is reflected, and enters the photoelectric conversion element 21 (SPD or PSD) through the lens 38.

On the other hand, in FIG. 5, the switch 27 is normally turned on, and is turned off at the timing when the pulsed light is projected and the pulsed photocurrent is generated in the photoelectric conversion element 21. In the normal state, the current flowing in the NPN transistor 23 due to the negative feedback via the operational amplifier 28 is I _0, and the current excluding the base current of the transistor 23 in the DC photocurrent component of the photoelectric conversion element 21 is the collector of the NPN transistor 24. Flow to. Also, when the pulsed photocurrent is generated, the switch 27
Is turned off, the base potential of the transistor 24 is held by the charge holding effect of the capacitor 26, and the collector current of the transistor 24 maintains the state immediately before generation of the pulse photocurrent. Therefore, only the pulsed photocurrent component flows into the base of the transistor 23, the transistor 23 receives the amplification of h _FE times, and the logarithmic conversion diode 22 flows a current of h _FE times the pulsed photocurrent. As a result, a logarithmically compressed output voltage corresponding to the pulsed light is output to V _OUT and is transmitted to the next stage circuit.

In the conventional circuit shown in FIG. 5, in a normal state in which the switch 27 is on, a pulsed photocurrent is generated in the current and voltage of each part and the photoelectric conversion element, and the switch 27 is turned off at the same timing. The current and voltage of each part of is as follows. However, the contents of each symbol are as follows. I _PD : Current of photoelectric conversion element I _DC : DC photocurrent component due to external light of photoelectric conversion element I _AC : Pulse photocurrent component due to pulsed light of photoelectric conversion element I _BN : Base current of transistor N I _CN : Transistor N Collector current I _EN : Emitter current of transistor N I _DN : Current of diode N K: Boltzmann constant T: Absolute temperature q: Electron charge I _S : Reverse saturation current of transistor h _FEN : h _{FE of} transistor N [normal state] Voltage and current of each part of

[0007]

[Equation 1] I _PD = I _DC

[0008]

## EQU00002 ## I _D22 = I _C23 = I ₀

[0009]

## _EQU3 ## I _C24 = I _PD -I _B23 = I _DC -I ₀ / h _FE23

[0010]

(4) V _OUT = V _T × ln (I _D22 / I _S ) = V _T × ln (I ₀ / I _S ).

[0011]

[Formula 5] V _T = KT / q [Voltage / current of each part in pulsed light receiving state]

[0012]

[Equation 6] Since the current of I _PD = I _DC + I _AC I _C24 maintains the normal state immediately before by the capacitor 26,

[0013]

[ _Equation 7] I _C24 = I _PD −I _B23 = I _DC −I ₀ / h _{FE23 From} ( _Equation 6) and ( _Equation 7)

[0014]

[ _Equation 8] I _B23 = I _AC + I ₀ / h _FE23

[0015]

[Equation 9] _{∴I D22 = I C23 = I B23} × h FE23 = I AC × h FE23 + I 0

[0016]

∴V _OUT = V _T × ln (I _D22 / I _S ) = V _T × ln {(I _AC × h _FE23 + I ₀ ) / I _S } FIG. 7 shows the time of each part. A chart is shown.
In FIG. 7, (a) shows the on / off state of the switch 27, (b) shows the on / off state of the switch 30,
(C) shows the lighting state of the light emitting element, (d) shows the photocurrent I _PD of the photoelectric conversion element 21, (e) shows the transistor 2
4 shows the collector current I _C24 , and (f) shows the collector current I _C23 of the transistor 23. Note that this time chart also has the same waveform in the embodiment.

[0017]

However, when the pulsed photocurrent becomes small in the above-mentioned configuration, (Equation 1)
0), the I ₀ component affects the output voltage as an error current. In order to suppress this effect, if I ₀ is reduced, the current flowing through the transistor 23 and the logarithmic conversion diode 22 in the normal state is reduced, and the transistor 23 is reduced.
There is a problem that the impedance of the logarithmic conversion diode 22 becomes high, the noise level of the output signal becomes high, and the S / N ratio is deteriorated.

The reason is as follows. The input impedance r _b23 of the base of the transistor 23, which is the input terminal, is r _b23 = h _FEN · (KT / qI ₀ ). Therefore, when the current I ₀ becomes small, the input impedance r _b23 becomes large, and the noise easily rides on the external noise jumping into the input terminal. Also, the input noise current / i seen at the base of the transistor 23
The fluctuation of the output voltage due to _n ² (≈2qI _DC Δf), that is, the output noise voltage / v ₀ is / v ₀ = / i _n · h _FEN · (kT / qI ₀ ). Therefore, the output noise voltage / v ₀ increases as I ₀ decreases. Due to these two factors, when I ₀ is reduced, the noise level of the output signal increases.

Further, there is a problem that the sensitivity fluctuates when the pulsed photocurrent fluctuates due to the non-linearity of h _FE caused by the current dependency of h _FE of the transistor 23. Furthermore, I
_{Since the 0} component is included as the error current, the dynamic range of the input current was narrow. The object of the present invention is to solve the problems of such a conventional circuit, to reduce the noise level including the extrinsic noise, and to improve the S / N ratio of the output with respect to the pulsed photocurrent which is the input current. It is to provide a pulsed photocurrent detection circuit.

Another object of the present invention is to provide a pulse photocurrent detection circuit which has a good output linearity with respect to a pulse photocurrent which is an input current. Still another object of the present invention is to provide a pulsed photocurrent detection circuit having a small error and an excellent dynamic range of input current.

[0021]

According to another aspect of the pulse photocurrent detection circuit of the present invention, the anode of the photoelectric conversion element is a first NP.
The base of the N-transistor is connected to the collector of the second NPN transistor, the collector of the first NPN transistor, the cathode of the first logarithmic conversion diode, and the inverting input of the operational amplifier are connected to the output terminal, and the non-inverting input of the operational amplifier is connected. Is connected to one end of the first constant current source and the cathode of the second logarithmic conversion diode, and the output of the operational amplifier is connected to the base of the second NPN transistor via the first switch which is turned off when the photoelectric conversion element receives light. And a capacitor, and the cathode of the photoelectric conversion element and the anodes of the first and second logarithmic conversion diodes are connected to the power source, and the second NP is connected.
The other end is grounded in the N emitter and the other end of the first constant current source of the capacitor of the transistor, a pulsed photocurrent of photoelectric conversion elements in the first NPN transistor multiplied by h _FE, h _FE and multiplied by the current first Logarithmic conversion is performed by the logarithmic conversion diode of, the negative feedback is applied from the output voltage V _OUT by the operational amplifier, the DC photocurrent flowing in the photoelectric conversion element is extracted by the second NPN transistor, only the pulse photocurrent is detected, and the output terminal is output. Output to.

According to another aspect of the pulse photocurrent detection circuit,
In addition to claim 1, the second constant current source is a power source and a first NP.
It is connected to the collector of the N-transistor so that a constant current flows through the first NPN transistor when the first switch is turned on and negative feedback is applied. Claim 3
In addition to claim 1 or 2, the pulsed photocurrent detection circuit described in the first constant current is added to the collector of the first NPN transistor via a second switch that is turned on and off in synchronization with the first switch. When one end of a third constant current source having the same current value as the source is connected, the other end of the third constant current source is grounded, and the first switch and the second switch are turned off when the photoelectric conversion element receives light. In addition, the current flowing through the first logarithmic conversion diode is subtracted to make the error current zero in principle.

[0023]

According to another aspect of the pulse photocurrent detection circuit of the present invention, the photoelectric conversion element is connected to the base of the first NPN transistor, and the collector of the first NPN transistor is connected to the logarithmic conversion diode. Without output terminal,
Negative feedback for extracting the DC photocurrent from the output terminal is applied to detect the pulse photocurrent with high accuracy. In the circuit of the conventional example, since the negative feedback is applied by the base potential of the first NPN transistor which is the preceding stage of the output terminal, in addition to the offset of the differential amplifier, the first logarithmic conversion diode and the first logarithmic conversion diode are used.
In the pulsed photocurrent detection circuit of the present invention, the first logarithmic conversion diode that determines the output voltage is directly negatively fed back to the constant current source. Since a current I ₀ corresponding to the current of (1) is passed, the variation in the base-emitter voltage of the first NPN transistor is excluded from the error factor,
High accuracy.

By applying the negative feedback as described above, the following improvements in claims 2 and 3 can be easily realized, and as a result, the error current due to the increase of the current I ₀ shown in (Equation 10) is obtained. It is possible to solve the contradictory problem between an increase in the noise and an increase in the noise due to the decrease in the current I ₀ . The pulsed photocurrent detection circuit according to claim 2 further causes a constant current to flow from the second constant current source to the output terminal, that is, the collector of the first NPN transistor, and the constant current flows to the first NPN transistor. The linearity with respect to the fluctuation of the signal component of the pulsed photocurrent is improved.

The pulsed photocurrent detection circuit according to claim 3 is
Furthermore, in order to reduce the detection error as much as possible and improve the width of the dynamic range, especially the accuracy at low current, the third constant current source is connected to the output terminal, that is, the cathode of the first logarithmic conversion diode, and the second constant current source is connected. Switch to constant current first
Subtract from the current flowing in the logarithmic conversion diode of.

[0026]

FIG. 1 shows a first embodiment (corresponding to claim 1) of a pulsed photocurrent detection circuit according to the present invention. In FIG. 1, 1 is a photoelectric conversion element, 2 and 3 are logarithmic conversion diodes, 4 and 5 are NPN transistors, 6 is a capacitor, and 7
Is an operational amplifier, 8 is a constant current source, and 9 is a switch that is turned off when receiving pulsed light. In this circuit, the inverting input of an operational amplifier 7 for applying negative feedback for extracting a DC photocurrent is connected to the output terminal V _OUT, that is, the collector of the transistor 4 in the first stage and the cathode of the logarithmic conversion diode 2.

In the pulse photocurrent detection circuit of this embodiment, the photoelectric conversion element 1 is connected to the base of the first NPN transistor 4, and the collector of the first NPN transistor 4 is connected to the logarithmic conversion diode 2. The connection point is used as an output terminal, and negative feedback for extracting the DC photocurrent from the output terminal is applied to detect the pulse photocurrent with high accuracy. In the circuit of the conventional example, since the negative feedback is applied by the base potential of the first NPN transistor 23 which is the previous stage of the output terminal, in addition to the offset of the differential amplifier 28, the first logarithmic conversion diode 22 and the first NPN transistor 22 are provided. Although the variation in the voltage between the base and the emitter of the transistor 23 affects the error, in the pulse photocurrent detection circuit of the present embodiment, the first logarithmic conversion diode 2 that determines the output voltage is directly negatively fed back to obtain the current I. _{Since 0} flows, the first NPN transistor 4
The variation in the base-emitter voltage V _BE of the above is excluded from the error factors, resulting in high accuracy.

By applying the negative feedback as described above, the improvement as shown in the following embodiment can be easily realized, and as a result, the error current increases due to the increase of the current I ₀ shown in (Equation 10). Thus, it is possible to solve the problem that conflicts with the increase of noise due to the decrease of the current I ₀ . A second embodiment (corresponding to claim 2) of the pulsed photocurrent detection circuit of the present invention is shown in FIG. In this circuit, a constant current source 10 is further connected between the power supply and the collector of the transistor 4 in the circuit of FIG. 1, so that a current I ₁ independent of I ₀ is supplied to the transistor 4 in a normal state where negative feedback is applied. By making I ₁ sufficiently larger than the current of h _FE times the pulse photocurrent, the fluctuation of the sensitivity at the time of pulse photocurrent fluctuation due to the current dependency of h _FE of the transistor 4 is reduced, and First-stage current amplification with excellent linearity can be realized regardless of the photocurrent level, and as a result, the pulsed photocurrent detection performance can be improved.

A third embodiment (corresponding to claim 3) of the pulsed photocurrent detection circuit of the present invention is shown in FIG. This circuit is obtained by adding a switch 12 that turns off in synchronization with the switch 9 of FIG. 1 and a constant current source 11 to the circuit of FIG. By setting the current value I ₀ ′ of the constant current source 11 as I ₀ ′ = I ₀ , the current I ₀ flowing in the logarithmic conversion diode 2 in the normal state can be set to ₀ when the pulse photocurrent is generated. It is possible to flow a current that is h _FE times the true input signal pulse photocurrent through the logarithmic conversion diode 2 and ideally make the error current zero.

FIG. 4 shows a fourth embodiment (corresponding to claim 3) of the pulsed photocurrent detection circuit of the present invention. This circuit realizes an optimum pulsed photocurrent detection circuit by combining the configurations of FIG. 2 and FIG. In the circuit of FIG. 4, when the switch 9 and the switch 12 are in the normal state, the current and voltage of each part and the pulse photocurrent are generated in the photoelectric conversion element, and the switch 9 and the switch 1 are synchronized at the same timing.
The current and voltage of each part in the pulsed light receiving state in which 2 is turned off are as follows. However, the contents of each symbol are as follows.

I _PD : Current of photoelectric conversion element I _DC : DC photocurrent component due to external light of photoelectric conversion element I _AC : Pulsed photocurrent component due to pulsed light of photoelectric conversion element I _BN : Base current of transistor N I _CN : Collector current of transistor N I _EN : Emitter current of transistor N I _DN : Current of diode N K: Boltzmann's constant T: Absolute temperature q: Electron charge I _S : Reverse saturation current of transistor h _FEN : Transistor N h _FE [Voltage and current of each part in normal state]

[0032]

[Equation 11] I _PD = I _DC

[0033]

[Equation 12] I _D2 = I ₀

[0034]

[Equation 13] I _D2 + I ₁ = I _C4 + I ₀ '

[0035]

[Equation 14] ∴I _C4 = I ₁

[0036]

[Equation 15] I _C5 = I _PD −I _B4 = I _DC −I ₁ / h _FE4

[0037]

## EQU16 ## V _OUT = V _T × ln (I _D2 / I _S ) = V _T × ln (I ₀ / I _S ) [Voltage / current of each part in pulsed light receiving state]

[0038]

Since the current of I _PD = I _DC + I _AC I _C5 is maintained in the normal state immediately before by the capacitor 6,

[0039]

[Equation 18] I _C5 = I _PD −I _B4 = I _DC −I ₁ / h _{FE4 From} ( _Equation 17) and ( _Equation 18)

[0040]

[Formula 19] I _B4 = I _AC + I ₁ / h _FE4

[0041]

[Number 20] _{∴I C4 = I B4 × h FE4} = I AC × h FE4 + I 1

[0042]

[Expression 21] I _D2 = I _C4 −I ₁ = I _AC × h _FE4

[0043]

Equation 22] _{∴V OUT = V T × ln (} I D2 / I S) = V T × ln (I AC × h FE23 / I S) Here, the light beam projection means, for example, those similar to the prior art Is used.

[0044]

According to the pulse photocurrent detection circuit of the first aspect of the present invention, by using the inverting input of the negative feedback operational amplifier as the collector of the first NPN transistor, the negative feedback is performed by the current amplified by h _FE times. The pulsed photocurrent can be detected with high accuracy.

According to the pulse photocurrent detection circuit of the second aspect, the constant current I ₁ independent of the negative feedback current I ₀ is caused to flow from the second constant current source to the first NPN transistor, whereby the first constant current I ₁ is supplied. The sensitivity variation due to the current dependence of h _FE of the NPN transistor during pulsed photocurrent fluctuations is reduced, and first-stage current amplification with excellent linearity is realized regardless of the pulsed photocurrent level, resulting in pulsed photocurrent detection performance. Can be improved.

According to the pulse photocurrent detection circuit of the third aspect, the error current is idealized by subtracting the current I ₀ 'equal to the negative feedback current I ₀ from the first logarithmic conversion diode when the pulse photocurrent is generated. Can be made zero, and as a result, the dynamic range of the input current can be expanded to the low current side.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment of a pulsed photocurrent detection circuit of the present invention.

FIG. 2 is a circuit diagram of a second embodiment of a pulsed photocurrent detection circuit of the present invention.

FIG. 3 is a circuit diagram of a pulse photocurrent detection circuit according to a third embodiment of the present invention.

FIG. 4 is a circuit diagram of a pulse photocurrent detection circuit according to a fourth embodiment of the present invention.

FIG. 5 is a circuit diagram of a conventional pulsed photocurrent detection circuit.

FIG. 6 is a circuit diagram showing an example of a light beam projection means.

FIG. 7 is a time chart showing the operations of the light beam projection means and the pulsed photocurrent detection circuit.

[Explanation of symbols]

 1 photoelectric conversion element 2 logarithmic conversion diode 3 logarithmic conversion diode 4 NPN transistor 5 NPN transistor 6 capacitor 7 operational amplifier 8 constant current source 9 switch 10 constant current source 11 constant current source 12 switch

Claims (3)

[Claims] 1. The anode of the photoelectric conversion element is a first NPN.
The base of the transistor and the collector of the second NPN transistor are connected, the collector of the first NPN transistor, the cathode of the first logarithmic conversion diode, and the inverting input of the operational amplifier are connected to the output terminal, and the non-inverting of the operational amplifier is connected. The input is connected to one end of the first constant current source and the cathode of the second logarithmic conversion diode, and the output of the operational amplifier is connected to the second switch via the first switch which is turned off when the photoelectric conversion element receives light. The base of the NPN transistor and one end of the capacitor are connected, the cathode of the photoelectric conversion element and the anodes of the first and second logarithmic conversion diodes are connected to a power source, and the emitter of the second NPN transistor and the capacitor are connected. A pulsed photocurrent detection circuit in which the other end and the other end of the first constant current source are grounded. 2. A second constant current source is connected between a power source and a collector of the first NPN transistor, and the first switch is turned on to provide negative feedback to the first NPN transistor. The pulsed photocurrent detection circuit according to claim 1, wherein a constant current is allowed to flow. 3. A collector of the first NPN transistor is provided with a third constant current source of the same current value as that of the first constant current source via a second switch which is turned on / off in synchronization with the first switch. One end is connected, the other end of the third current source is grounded, and when the first switch and the second switch are turned off when the photoelectric conversion element receives light, the current flows to the first logarithmic conversion diode. The pulsed photocurrent detection circuit according to claim 1 or 2, wherein the current is subtracted.