JPH0681008B2 - Light receiving circuit and optical coupling circuit - Google Patents

Description

The present invention relates to a photodetection system and a photodetection circuit and a photocoupling circuit used in any optical system using an optical coupling element, an optical fiber, or the like. is there.

For example, when an optical coupling element or an optical fiber is used for insulation of a video signal input terminal of a television set, there is a problem of variation in output due to variations and variations in DC transfer characteristics of these elements. The present invention provides a light receiving circuit and an optical coupling circuit that improve such output fluctuation.

<Prior Art> FIG. 5 shows a conventional circuit configuration example. FIG. 5 shows a light transmitting portion C1 including a light emitting diode LED and a light receiving element PD,
It includes a light-emitting element drive circuit B1 and a photocurrent amplifier circuit A4, and drives the light-emitting diode LED by a current in which an AC signal current is superimposed on a DC bias current, and the photocurrent output from the light-receiving element PD is used as it is. -Voltage conversion and linear amplification are performed, and an AC component is taken out from the output terminal V ₀ by capacitor (C ₂₂ ) coupling.

By the way, although the optical transmission part C1 can use an optical coupling element, an optical fiber, etc., the light emitting diode LED and the light receiving element P1
The current transfer rate α (CTR) between D has a considerable variation in each of them, and also changes due to aging and temperature fluctuations.

In the configuration example of FIG. 5, a variable resistor VR ₁ is provided in the photocurrent amplifier circuit A4 and the amplification factor of the amplifier is made variable, so that the variation of the current transmissibility α can be adjusted for the time being. However, in this example, it is difficult to adjust even changes due to changes with time and changes in temperature of the light transmission section C1.

Therefore, the applicant previously filed Japanese Patent Application No. Sho 62-241788 (September 1987).
We proposed a circuit configuration in which the output does not depend on the current transmissibility α in the application “Photo coupler circuit”, filed on March 25.

FIG. 6 shows an example of the configuration. B1 and C1 are a light emitting element drive circuit and a light transmission section similar to those in FIG. A5 is a logarithmic amplifier connected to the light receiving element PD side of the light transmitting section C1, B2 is a bias circuit of the logarithmic amplifier, and the photocurrent output I _P from the light receiving element PD
Is logarithmically converted by the PN junction (diode) between the base and emitter of the transistor Q ₄ . I1 is an integrating circuit, A6 is an antilogarithmic amplifier, these act as an AC differential amplifier,
Only the AC component of the photocurrent output that has been logarithmically converted is inversely logarithmically converted.

In the above, the light emitting diode LED has a DC bias current I
Driven by a current that superimposes the _AC signal current _{IF (AC)} on _{F (DC)} , and let the current transfer rate of the light transfer section C1 be α, the light receiving element PD
When output from, the photocurrent I _P is as follows.

I _P = α {I _{F (AC)} + I _{F (DC)} } This photocurrent I _P is a transistor that constitutes the logarithmic amplifier A5.
Current diode between base and emitter of Q ₄ - When voltage conversion, differential resistance .gamma.e the diode depending on the DC component of the photocurrent _{I P, γe = (kT /} q) × (1 / α · I F ( _DC) ), where k is the Boltzmann constant, q is the electron charge, and T is the absolute temperature. Since the AC signal current component of the photocurrent I _P is α · IF _(AC) , the AC signal voltage component generated by the diode is γe × α · IF _(DC) , and V _{O1 (AC)} = γe × α ・ IF _(AC) = {(kT / q) × (1 / α ・_{IF (DC)} } × α ・ IF _(AC) = (kT / q) × (IF _(AC) / I _{F (DC)} ).

That is, the AC signal voltage formation of the logarithmic conversion output V _O1 is proportional to the ratio of the DC bias current IF _(DC) and the AC signal current IF _(AC) , and depends on the current transfer rate α of the optical transfer section C _1. You can turn it off. When this is antilogarithmically converted by an AC differential amplifier including an integrating circuit I1 and an antilogarithmic amplifier A6, an output that is linearly proportional to the _AC signal current _{IF (AC)} is obtained as the AC output voltage V ₀ .

<Problems to be Solved by the Invention> By the way, in the circuit configuration of FIG.
_When the frequency f of the AC signal voltage component of _O1 satisfies the relation of time constant R ₉ × C _{2 of} the integrating circuit I ₁ and f >> 1 / (2π × R ₉ × C ₂ ), the two of the AC differential amplifier Input end T ₁
・ The voltage generated between T ₂ is the AC signal voltage component (kT / q) × (I
_{F (AC)} / I _{F (DC)} ). Then, when the amplification factor A _V is the differential input voltage on one side of the differential amplifier is small, a _{A V = (1/4) × R} 10 × I SS / (kT / q), the AC output voltage V ₀ is , V ₀ = (1/4) × R ₁₀ × I _SS × (I _{F (AC)} / I _{F (DC)} ), but if the differential input voltage increases, the AC output voltage V ₀
Is deviated from the above equation, and the linearity between the input and the output is deteriorated.

In view of the above points, the present invention is received or optically coupled including a direct current component and an alternating current signal component, logarithmically converting the photocurrent output from the light receiving element, and the logarithmically converted photocurrent logarithmically converted voltage ac. It is an object of the present invention to provide an optical coupling circuit including a light receiving circuit and a light emitting element driving circuit, in which linear deterioration is improved in a component that is subjected to inverse logarithmic conversion.

<Means for Solving the Problems> A light receiving circuit of the present invention includes a light receiving element for receiving light, a diode connected to the light receiving element for logarithmically converting a photocurrent output from the light receiving element, and a logarithm of the diode. It has an antilogarithmic amplifier that performs antilogarithmic conversion of the converted signal, and a negative feedback circuit unit that detects the DC component output of the antilogarithmic amplifier and holds the anode potential of the diode constant in terms of DC.

The negative feedback circuit unit includes, for example, an integrating circuit that integrates the output of the antilogarithmic amplifier and a differential amplifier that amplifies the difference between this integrated value and a reference voltage.The differential output makes the cathode potential of the diode negative. Feedback control is performed.

As the optical coupling circuit, the above is a light receiving side circuit, and a light emitting side circuit comprising the light receiving side circuit and a light emitting element drive circuit section for driving the light emitting element by superimposing an alternating signal current on a direct current bias current. Equipped with.

<Operation> With the above configuration, the photocurrent output from the light receiving element is logarithmically converted and the logarithmically converted AC voltage component is inversely logarithmically converted, so that it does not depend on the current transfer ratio α or the like of the light transmitting section reaching the light receiving element. Also, a stable output AC voltage can be obtained which is not affected by changes over time or changes in temperature. Furthermore, a negative feedback circuit that keeps the anode potential of the diode constant in terms of direct current allows the gain of the antilogarithmic amplifier to be constant even if the direct current component of a large current fluctuates, and as a result, it has excellent linearity and is more stable. An output AC voltage is obtained.

<Examples> Examples of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram and FIG. 2 is a circuit diagram showing a more specific example thereof.

The light emitting element drive circuit B ₁ applies a DC bias voltage obtained by dividing the power supply voltage V _CC1 and the ground GND by the resistors R ₁ and R ₂ to the base of the driving transistor Q ₁ to generate the AC input signal terminal voltage V
Even when there is no signal at _IN , a constant DC bias current _{IF (DC)} is made to flow.

Therefore, the current I _F due to the AC signal and the DC bias current I _{P (DC)} flow through the light emitting diode LED of the light transmitting unit C1, and the optical signal L proportional to the current obtained by superposing these two currents is received by the light receiving element PD. Be transmitted to. From the above, the LED driving current I _F is as follows. In the following, V _BE is the base-emitter voltage of the transistor Q ₁ .

I _F = I _{F (DC)} + I _{F (AC)} I _{F (DC)} (V _CC1 −V _BE ) × R ₂ / {R ₁ + R ₂ ) × R ₃ } I _{F (AC)} V _IN / R ₃ optical When the current transmissibility of the transmission part C1 is α, the photocurrent I _P of the light receiving element PD is as follows.

I _P = α (I _{F (DC)} + I _{F (AC)} ) Diode RD converts photocurrent I _P into current-voltage (logarithmic compression)
The differential resistance re of the diode RD is determined by the DC component of the photocurrent I _P , and is represented by re = (kT / q) × (1 / α · _{IF (DC)} ). Therefore, the AC voltage component V _AC appearing between the anode and cathode of the diode RD is V _AC = α · I _{F (AC)} x re = (kT / q) x (I _{F (AC)} / I _{F (DC)} ) And does not depend on the current transfer rate α of the light transfer section C1.
If this AC voltage component V _AC is subjected to inverse logarithmic conversion, a linear AC output voltage V ₀ that does not depend on α with respect to the AC input voltage V _IN can be obtained.

1 and 2, A ₁ is an emitter follower circuit, A ₂
Is an antilogarithmic amplifier, and A ₃ is a differential amplifier that amplifies the difference between the integrated value of the output voltage of the antilogarithmic amplifier A ₂ and the reference voltage value Vref to control the cathode voltage terminal of the diode D ₁ . The second
PS in the figure is a constant voltage circuit including a generation unit for the reference voltage value Vref,
O is an output circuit of the emitter follower.

By configuring a negative feedback circuit that feeds back from the emitter follower circuit A1, the antilogarithmic amplifier A2, the differential amplifier A3 to the diode RD, the anode potential of the diode RD is made constant,
The gain of the antilogarithmic amplifier A2 is fixed.

The antilogarithmic amplifier A2 is composed of a transistor Q _B2 and a resistor R _B6 , and its gain A _V is A _V ≈ R _B6 × I _C (kT / q) where I _C : Q _B2 collector current, where I _C = was _{h FE × I E / (h} FE +1) ≒ I E.

I _E : Q _B2 emitter current h _FE : Q _B2 h _FE

I _C is determined by the potential of the point, and if the output voltage of the differential amplifier A3, that is, the point potential is constant, it increases as α ・ IF _(DC) , which is the DC component of the photocurrent I _P , increases, and α ・If I _{F (DC)} becomes small, it decreases.

However, the differential amplifier A3 performs a negative feedback operation with respect to changes in α · _{IF (PC)} , and when α · _{IF (DC)} increases and the anode-cathode voltage of the diode RD increases by ΔV, the point potential is increased. Is ΔV
It only lowers and works to make the point potential constant.

Therefore, the collector current I _C of the transistor Q _B2 is constant, and the gain of the antilogarithmic amplifier A2 is A _V = R _B6 × I _C / (kT / q) is constant with respect to the direct current change of the photocurrent I _P.

Here, the operation of the differential amplifier A3 will be described in more detail.

One terminal of the differential amplifier A3 is connected to the reference voltage source E. The reference voltage source E operates from the power supply voltage V _CC2 to the reference voltage source output terminal ₍ point) so as to have a constant voltage Vref.

The integrated value (point potential) of the output of the antilogarithmic amplifier A2 is input to the + terminal of the differential amplifier A3. Let τ be the time constant of the integration circuit I1
If (= R ₉ × C ₂ ), only an AC signal of 1 / 2πτ or less and a DC-like output fluctuation are input to the differential amplifier A3, and the differential amplifier A3 works to suppress this fluctuation.

Since the differential amplifier A3 tries to make the point potential the same as the point potential, neglecting the DC voltage drop in the integrating circuit I1 and the potential difference between the points due to the input bias current of the differential amplifier A3, VCC2 becomes The collector potential difference of the transistor Q _B2 , that is, the DC potential difference V _DC of the resistor R _B6 becomes the same as the reference voltage Vref. Therefore, the collector current I _C flowing through the transistor Q _B2 becomes I _C = Vref / R _B6 (constant).

Since the AC input voltage V _AC of the antilogarithmic amplifier A2 is V _AC = (kT / q) × (I _{F (AC)} / I _{F (DC)} ), the AC output voltage V ₀ of the antilogarithmic amplifier A2 is Is V ₀ = A _V × V _AC ＝ Vref × (I _{F (AC)} / I _{F (DC)} ), which is the ratio I _{F (AC)} of the forward current I _F of the light emitting diode LED to the AC component and the DC component. In proportion to / _{IF (DC)} , the current transfer rate α of the light transfer section C1 and other influences can be prevented.

Another configuration example is shown in FIGS. The same functional parts as those in FIGS. 1 and 2 are designated by the same reference numerals.

Figure 3 is detected, and not directly from the inverse logarithmic amplifier A2, a current mirror composed of the transistor Q _B2 antilog amplifier A2 DC component of the output voltage in Figure 2 (1 / 2.pi..tau less low frequency components) The collector potential of the transistor Q _B21 is used.

That is, the base potential of Q _{B2 and} the base potential of Q _B21 are simultaneously controlled by the configuration of the current mirror circuit CM, and V _CC2
Hold the voltage between Q and the collector voltage of Q _B21 constant. Q _B21 and
The same DC current flows through Q _B2, resulting in a DC voltage between Q _B2 and V _CC2 is also kept constant.

With this, the DC voltage generated between the output terminals V _{0 and} V _CC can be arbitrarily changed. For example, if the value of R _B61 of the resistor R _B6 is doubled, the voltage generated across the resistor R _B6 is doubled by 2 × Vre.
It can be f.

Circuit configuration example of FIG. 3 in other words, one in which there is an advantage that arbitrarily select the gain A _V antilog amplifier A2.

In FIG. 4, a current output type differential amplifier is constructed so that negative feedback is applied to the diode RD by the current. A
3'is a differential amplifier with a gain of 1, AL1 is a voltage-current conversion circuit composed of transistors Q ₃₂ and Q ₃₃ which form a current mirror circuit, and a transistor Q ₃₁ which performs voltage-current conversion, AL2
Is a circuit for converting the feedback current into a current-voltage in the diode RD section.

For example, at the input of the differential amplifier A3 ′, when the point voltage becomes higher than the point voltage by ΔV ₁ , this potential difference is voltage-current converted by the transistor Q ₃₁ . However, I _DC0 outputs the current when the point voltage is the same as the point voltage. Then, in the current-voltage conversion circuit AL2, the current change of I _DC0 → I _DC1 is converted into a voltage change, and operates so as to raise the point potential by ΔV ₁ .

Thus, in this circuit configuration example, the potential difference between points is directly transmitted to the point voltage, and the gain of the negative feedback circuit from point to point is 1. That is, in this example, the noise generated in the negative feedback circuit is not amplified, and the noise of the light receiving side circuit can be reduced.

Incidentally, A2 'is an inverted logarithmic amplifier, provided the current mirror circuit CM, the resistance ratio of the resistors R _B6 and R _B61, inverse logarithmic amplifier A2 between the integration circuit I1' arbitrarily selected gain A _V of I am able to do it.

In each of the above-described embodiments, a part or all of the light-receiving-side circuit portion, excluding the light-receiving element PD or including the light-receiving element PD, can be a one-step monolithic integrated circuit. Therefore, when an optical coupling element is used for the light transmitting section C1, the light emitting diode LED, the light receiving element PD and other light receiving side circuits, or all the light receiving side circuits including the light receiving element PD are packaged in one package. It is possible.

In particular, the present invention is useful for insulation of a video signal input terminal of a television set, and the like, in addition to an optical system using such an optical coupling element and an optical fiber, the light receiving circuit of the present invention is an AC signal. Can be effectively used alone when is incident on the DC bias light such as external light.

<Effects of the Invention> As described above, according to the present invention, it is possible to provide a useful light receiving circuit and an optical coupling circuit which are excellent in linearity in a stable state in a light receiving or optical coupling including a DC component and an AC signal component. .

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG.
FIG. 6 is a circuit diagram showing a concrete example, FIG. 3 is a circuit diagram showing another concrete example, FIG. 4 is a circuit diagram showing another concrete example, FIG. 5 is a circuit diagram showing a conventional example, and FIG. FIG. 6 is a circuit diagram showing another conventional example. B1 ... Light emitting element drive circuit, LED ... Light emitting diode, PD
...... Light receiving element, RD ... Diode, A2 ... Inverse logarithmic amplifier, I1 ... Integrating circuit, A3 ... Differential amplifier.

Claims (2)

[Claims] 1. A light receiving element for receiving light, a diode connected to the light receiving element for logarithmically converting a photocurrent output from the light receiving element, and an inverse logarithmic conversion of a signal logarithmically converted by the diode. A light receiving circuit comprising a logarithmic amplifier and a negative feedback circuit section for detecting a DC component output of the antilogarithmic amplifier and holding the anode potential of the diode constant in terms of DC. 2. A light emitting side circuit comprising a light emitting element and a light emitting element drive circuit section for driving the light emitting element by superimposing an AC signal current on a DC bias current, and receiving light emitted from the light emitting element. A light receiving element,
A diode connected to the light receiving element for logarithmically converting a photocurrent output from the light receiving element; and an antilogarithmic amplifier for antilogarithmically converting a signal logarithmically converted by the diode,
A light receiving side circuit having a negative feedback circuit section for detecting a DC component output of the antilogarithmic amplifier and holding the anode potential of the diode constant in terms of DC.