JPH0349224B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a power supply circuit for a digital exchange, and particularly to a constant current power supply circuit for a subscriber circuit.

FIG. 4 shows an example of a general configuration of a subscriber circuit of a digital exchange. In the figure, _B1 is a power supply unit to which the present invention relates, O is a high voltage protection unit for protecting subscriber circuits from large voltages such as lightning (primary and secondary are provided in the figure), R is a ringing circuit that notifies subscriber J of a bell sound;
C is a codec that performs conversion between audio signals and PCM signals, H is a hybrid that performs 2-wire to 4-wire conversion, and T is a test circuit.

Power supply section B is connected to communication lines A and B, and supplies constant current to communication lines A and B when subscriber J is off-hook. In this case, the power feeding section B has a high impedance for AC signals (audio signals) so as not to attenuate the signals.

[Prior art]

The power supply circuit of the conventional subscriber circuit is shown in Figures 5 to 7.
This will be explained using figures.

Figure 5a is a circuit diagram of a conventional constant resistance type power supply circuit.
FIG. 5b is a graph showing the DC voltage-current characteristics of the respective feeder circuits of communication lines A and B in this circuit. In Figure 5a, the communication line A and the power supply -
A resistor R ₁ and an inductance L ₁ are connected in series between V _BB and a resistor R ₂ and an inductance L ₂ are connected in series between the communication line B and ground G. Inductances L ₁ and L ₂ are usually letter coils used in relays, and resistors R ₁ and R ₂ are usually letter coil winding resistances. Since the letter coil generally has a high impedance for audio frequency signals, the audio signal is not drawn into the power supply circuit, and therefore the audio signal passes through the communication lines A and B without attenuation. Furthermore, the letter coil has a low impedance with respect to direct current as indicated by the resistances R ₁ and R ₂ , and the feeding current value is stably determined as shown in FIG. 5b.

That is, in FIG. 5b, the graph shows the voltage-current characteristics of the communication line B, and the graph shows the voltage-current characteristics of the communication line A. When subscriber J goes off-hook and communication lines A and B are connected, the midpoint potential of communication lines A and B is always -
V _BB /2.

In a constant resistance type power supply circuit using a letter coil, a current of 100 mA or more flows when the subscriber line is short, so there is a problem that it is not economical from a mounting point of view in terms of power consumption and heat generation.

Therefore, a constant current type power supply circuit capable of low power consumption, as shown in FIG. 6a, has been considered in the past.
In FIG. 6a, each of the power supply circuits B _a and B _b is constituted by a parallel circuit of a resistor R and a constant current source I _sa or I _sb . According to this circuit, the constant current source itself has high impedance, so the voice signal does not attenuate on the telephone line, and a constant current can always flow regardless of the distance of the subscriber line, resulting in low power consumption. It is possible to aim for However, as can be seen from the voltage-current characteristics shown in Figure 6b, the constant current values I _sa (Figure 2) and I _sb (Figure 1)
Also, due to the unbalance of the resistor R, the midpoint potential between the communication lines A and B when subscriber J is off-hook is -
The potential of A and B is not necessarily V _BB /2, but the voltage of each communication line A and B is between ground and -V _BB /2, and between -V _BB /2 and power supply voltage -V _BB , so the potential of A and B is Power supply voltage -
There is a problem in that there is an extreme shift to V _BB or ground potential G, resulting in saturation of the transistor in the power supply circuit and clipping of the speech signal.

Furthermore, even if the characteristics of the two constant current sources are exactly the same, if external noise is added to the communication lines A and B as common mode noise, the current values of the two constant current sources I _sa and I _sb will become unbalanced. The above midpoint potential still deviates from -V _BB /2. This causes audio signal clipping.

FIG. 7 is a circuit diagram showing a conventional constant current power supply circuit which is a concrete example of the conventional circuit shown in FIG. In the figure, A ₀ and A ₁ are operational amplifiers (hereinafter referred to as operational amplifiers), Tr ₀ and Tr ₁ are transistors, T is a transformer that passes a communication signal, and C is a DC blocking capacitor. The voltage between the ground air (0V) and the battery _-VBB is divided by resistors R _a0 , R _b , and R _a1 . When the voltages across the resistors R _a0 and R _a1 are applied as reference potentials to the non-inverting input terminals of the operational amplifiers A ₀ and A ₁ , respectively, the resistors R _e0 and
The operational amplifiers A _{0 and} A ₁ operate so that the voltage across R _e1 is equal to the voltage across resistors R a0 and R _a1 , respectively, and the resistor R _e0 has I _sb = {V _BB ×R _s0 /R _a0 +R A constant current of _b + R _a1 }/R _e0 flows. Similarly, a constant current of I _sa = {V _BB ×R _a1 /R _a0 +R _b +R _a1 }/R _e1 flows through the resistor R _e1 . If the current amplification factor h _FE of the transistors Tr ₀ and Tr ₁ is h _FE ≫1, the current I _sb flowing through the resistor R _e0 is approximately equal to the current flowing through the transistor Tr ₀ , and the current I _sa flowing through the resistor R _e1 is It is approximately equal to the current flowing through transistor _Tr1 . I _ab and I _sa
If the resistors R ₀ and R ₁ are exactly equal, theoretically the midpoint potential of communication lines A and B should be -V _BB /2, but even if the characteristics of the elements that make up the constant current circuit are slightly different, If they are different, I _sa and I _sb will have different values, and as described above, the communication lines A and B will extremely move to the power supply voltage -V _BB or the ground voltage 0V. To prevent this, resistors R ₀ and R ₁ are added to the constant current source.
The resistors R ₀ and R ₁ work to absorb errors in the two constant current values I _sa and I _sb .

However, if the resistance values of resistors R ₀ and R ₁ are made small, the constant current characteristics will be impaired, and the impedance of the power supply circuit to the speech signal will be lowered, causing the speech signal to be attenuated, so it cannot be made too small. There's a problem. If the resistance value is large, constant current characteristics are maintained, but the midpoint potential between communication lines A and B tends to deviate from -V _BB /2.

Furthermore, when common-mode noise is applied to communication lines A and B, a common-mode noise voltage proportional to the impedance value is generated, which upsets the balance between the two constant current sources and causes the communication signal to become clipped or There is a problem in that it is likely to cause the passage of communication signals to be blocked.

[Problem to be solved by the invention]

SUMMARY OF THE INVENTION In view of the above-mentioned problems with the conventional type, it is an object of the present invention to maintain constant current characteristics and reduce power consumption in a constant current power supply circuit of a subscriber circuit, and to The purpose of this method is to lower the impedance to improve resistance to common-mode noise, and to fix the midpoint potential of the communication line between the power supply voltage and the ground voltage to prevent communication signal clipping.

[Means to solve the problem]

In order to solve the above problems, according to the present invention,
What is provided is a voltage that is divided between the power supply and the ground by two pairs of reference voltage setting elements that are vertically symmetrical with respect to the potential, and the voltage obtained is set as the reference potential, and a constant current is supplied to each of the two communication lines. and a midpoint voltage output means for detecting a midpoint voltage between the communication lines and outputting it to the midpoint of the two pairs of reference voltage setting element groups; It includes an operational amplifier that inputs the voltage divided by the voltage setting element group as a reference voltage, and a voltage-current conversion circuit composed of a resistor and a transistor, and a non-inverting input terminal of the operational amplifier inputs the voltage divided by the reference voltage setting element group. The inverting input terminal is connected to one terminal of the power supply side or the ground side of the transistor, the output terminal is connected to the base of the transistor, and the inverting input terminal is connected to one terminal of the power supply side or the ground side of the transistor. The other terminal is connected to the communication line, and is connected between the inverting input terminal of the operational amplifier and the one end of the transistor, and between the inverting input terminal of the operational amplifier and the power supply or the ground, respectively. , a power supply noise absorbing resistor is connected so that the potential of the non-inverting input terminal and the potential of the inverting input terminal of the operational amplifier are the same potential with respect to the power supply noise, and the midpoint voltage output The subscriber circuit is characterized in that the means includes an amplifier that amplifies the midpoint potential between the communication lines in positive phase, and the output of the amplifier is connected to the midpoint of the two pairs of the reference voltage setting elements. This is a constant current power supply circuit.

According to another aspect of the invention, between power supply and ground;
The voltage obtained by dividing by two pairs of reference voltage setting element groups that are vertically symmetrical with respect to the potential is set as a reference potential,
A constant current circuit that supplies a constant current to each of the two communication lines, and a midpoint voltage output means that detects the midpoint voltage between the two communication lines and outputs it to the midpoint of the two pairs of reference voltage setting elements. The constant current circuit includes an operational amplifier that inputs the voltage divided by the reference voltage setting element group as a reference voltage, and a voltage-current conversion circuit composed of a resistor and a transistor. The inverting input terminal receives the output voltage of the midpoint voltage output means via the reference voltage setting element group, the inverting input terminal is connected to one terminal of the power supply side or the grounding side of the transistor, and the output terminal is connected to one terminal of the power supply side or the ground side of the transistor. The transistor is connected to the base of the transistor, and the other terminal of the transistor is connected to the communication line, and between the inverting input terminal of the operational amplifier and the one end of the transistor, and the inverting input terminal of the operational amplifier and A power supply noise absorbing resistor is connected between the power supply or the ground, respectively, so that the potential of the non-inverting input terminal and the potential of the inverting input terminal of the operational amplifier are the same with respect to power supply noise. further, a current limiting resistor is connected between the output of the operational amplifier and the base of the transistor, and the midpoint voltage output means amplifies the midpoint potential between the communication lines in positive phase. an amplifier, the output of the amplifier is connected to the midpoint of the two pairs of reference voltage setting elements, and a Zener diode for current limiting is connected between the output of the amplifier and the power source or the ground. A constant current power supply circuit for a subscriber circuit is provided.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to FIGS. 1 to 3.

FIG. 1 is a circuit diagram showing a constant current feeding circuit for a subscriber circuit, which is the premise of the present invention. In the same figure,
The difference from the conventional example described above in FIG. 7 is that the midpoint voltage detection circuit ID is used as a means of midpoint potential detection output.
is added, resistors R _{b0 and} R _b1 are provided, which are obtained by dividing the resistor R _b in Fig. 7 into two, and resistors R ₀ and R ₁ in Fig. 7 are removed. be. The resistors R _a0 , R _b0 and R _a1 , R _b1 each constitute a reference potential setting element group. The midpoint voltage detection circuit ID includes resistors R _c0 and R _c1 (R _c0 =R _c1 ) connected in series between communication lines A and B, and an amplifier A ₂ . _Constant current setting resistors R _a0 , R _b0 , R _b1 , R _a1 (R _a0 = R _a1 , R _b0
= R _b1 ) are connected in series. The input of amplifier A ₂ is connected to the connection point M ₁ between resistors R _c0 and R _c1 , and the output is connected to resistor R _b0
and R _b1 are connected to the connection point _M2 . Amplifier A ₂
is a voltage follower with a gain of 1, and the midpoint potential signal of communication lines A and B is connected to constant current setting resistor R _b0 ,
Apply feedback to the midpoint _M2 of R _b1 . At the connection point _M1 between the resistors R _c0 and R _c1 , only the common mode signals on the communication lines A and B are obtained, and since the normal communication signal is a differential signal, it does not appear at the connection point _M1 . The common-mode signal obtained at the output of amplifier A ₂ is applied to the non-inverting input terminal of operational amplifier A ₀ or A ₁ via resistor R _b0 or R _b1 , and due to the operation of operational amplifiers A ₀ and A ₁ , its inverting input The above in-phase signal is obtained at the terminal. This in-phase signal is inverted by transistors Tr ₀ and Tr ₁ , and signals having a phase opposite to the in-phase signal are applied to communication lines A and B. Therefore, the impedances of the constant current circuits CC ₀ and CC ₁ for the common-mode signal are respectively (R _a0 + R _b0 × R _e0 /R _a0 , (Ra1 + R _b1 ) × R _e1 /R _a1 from infinity due to the constant current source). The impedance of the power supply circuit decreases to a low impedance, and common-mode noise is absorbed by the constant current circuit, improving the resistance of the power supply circuit to common-mode noise.

The reason why the common mode impedance is expressed as in the above equation is as follows.

That is, in Fig. 1, a common-mode signal means that voltages of the same phase are applied to both terminals of communication lines A and B, and a differential signal means that voltages of the same phase are applied to both terminals of communication lines A and B. This means that phase voltages are applied.

Here, the midpoint voltage M ₁ is A in the case of a common mode signal,
The voltage of both B will be the same.

Further, in the case of a differential signal, if the resistances R _c0 and R _c1 are equal, the voltage becomes zero.

In the case of a common mode signal, the voltage on M ₁ is equal to the voltage on A and B, but the amplifier A ₂ applies a voltage to the midpoint M ₂ at low impedance (0Ω).

Therefore, equivalent circuits of the power supply circuit can be drawn independently on the G side and the -V _BB side.

Common mode impedance is the impedance for alternating current, and if the alternating current voltage at point A is V _a , then M ₁
The voltage of M 2 is also V _a , and the voltage of M ₂ is also V _a .

Since the non-inverting input of operational amplifier _A1 has infinite impedance, the voltage V+ here is the resistance R _b1
and R _a1 .

V ₊ = V _a × R _a1 / (R _a1 + R _b1 ) The voltage of V ₊ is set to the same voltage as V ₊ on the side connected to the transistor of resistor R _e1 by the operation of the operational amplifier (imaginary short). .

Therefore, the current I _re1 flowing through the resistor R _e1 is I _re1 = V ₊ /R _e1 = V _a × R _a1 / ((R _a1 + R _b1 ) × R _e1 . The transistor Tr ₁ is approximately equal to this I _re1 (Less by 1/H _fe ) current is drawn from point A. Therefore, the common mode impedance can be defined by the current with respect to the voltage at point A, so common mode impedance = V _a /I _re1 = V _a / (V _a ×R _a1 /((R _a1
+R _b1 ) x R _e1 )) = (R _a1 R _b1 x R _e1 /R _a1 . Since the circuit is symmetrical, the same goes for the point B side (R _a0 + R _b0 ) x R _e0 /R _a0. .

Also, communication line A due to unbalance of constant current source,
A shift in the midpoint potential of B is detected as a shift in the same phase of A and B, and at the above low impedance value, -
Since the voltage is stabilized around V _BB /2, the additional resistors R ₀ and R ₁ as in the conventional circuit shown in FIG. 7 are not required.

The circuit shown in FIG. 1 provides a power supply circuit that has a constant current configuration and has low impedance to common-mode signals.

However, the circuit shown in FIG. 1 alone has the following disadvantages. In other words, in the circuit shown in Figure 1, the constant current value is set by simply dividing the voltage between the ground G and the power supply _{-VBB .}
If noise of voltage V _N is mixed into the operational amplifier
The non-inverting input terminal of _A1 has R _a0 +R _b0 +R _b1 /R _a0 +R _b0 +R _b1
A signal of +R _a1 × V _N is input, and a signal of R _a0 /R _a0 +R _b0 +R _b1 +R _a1 × V _N is input to the non-inverting input terminal of operational amplifier _A0 .

Therefore, different noise signals are sent to communication lines A and B, and this becomes differential noise and is transmitted to subscriber J.
will be sent to.

FIG. 2 is a circuit diagram showing a constant current feeding circuit for a subscriber circuit according to a first embodiment of the present invention, which is improved in this respect. In the figure, the differences from Figure 1 are as follows:
A resistor R _f0 connected between the inverting input terminal of the operational amplifier A ₀ and the emitter of the PNP transistor Tr ₀ ,
A resistor R _s0 and a capacitor C ₀ are connected in series between the inverting input terminal of the operational amplifier and the power supply −V _BB , and the inverting input terminal of the operational amplifier _A1 and the NPN transistor are connected in series.
This is because a resistor R _f1 is connected between the emitter of Tr ₁ , and a resistor R _s1 and a capacitor C ₁ are connected in series between the inverting input terminal of the operational amplifier A ₁ and the ground G. In Figure 2, R _f0 /R _s0 =R _a0 /R _a0 +2×R _b0 ,R _f1 /R _s1 =R _a1 /R _a1
Select the resistance values R _f0 , R _s0 , R _f1 , R _s1 so that +2×R _b1 ,
If capacitors C ₀ and C ₁ are chosen large enough so that they can be ignored in terms of AC, the non-inverting input of operational amplifier A ₁ will have
If R _a0 = R _a1 and R _b0 = R _b1 due to power supply noise V _N , then V ₊ = R _a0 + R _b0 + R _b1 /R _a0 + R _b0 + R _b1 + R _a1 ×V _N = R _s1 /R _f1 + R _s1 V _N signals are input.

On the other hand, the voltage V _- at the inverting input terminal of operational amplifier _A1
depends on the power supply noise V _N , and becomes V _- = R _s1 /R _f1 + R _s1 ×V _N.

Therefore, V ₊ =V _- , and no signal due to power supply noise appears at the output of operational amplifier _A1 .

Similarly for operational amplifier _A0 , V ₊ = V _- , and power supply noise does not appear in the output.

The reason why the influence of AC noise input from the power supply is removed in the circuit of FIG. 2 will be explained in more detail.

In FIG. 2, as mentioned above, the operational amplifier A ₀
A noise voltage obtained by dividing the noise voltage generated in the power supply -V _BB by resistors R _a0 , R _b0 , R _b1 , and R _a1 is input to the non-inverting input of. Further, the noise voltage of the power supply −V _BB is input to the inverting input of the operational amplifier A ₀ through the capacitor C ₀ . Here, by selecting the resistance values R _f0 and R _S0 as described above, the inverting input voltage and the non-inverting input voltage of the operational amplifier A ₀ match, and a configuration is created in which no voltage is output from the resistor R _e0 No noise voltage is generated across the terminals. Since no noise voltage is generated in the resistor R _e0 , the transistor Tr ₀ does not allow any noise current to flow, and therefore does not generate noise to the subscriber.

In this way, the circuit shown in FIG. 2 has the advantage that even if noise mixes into the power supply, differential noise will not appear on the communication lines A and B.

However, the circuit shown in FIG. 2 also has the following disadvantages. That is, in the circuit shown in FIG. 2, a voltage V obtained by dividing the voltage between the ground and the power supply by resistance is always applied to the non-inverting input terminals of the operational amplifiers A ₀ and A ₁ . , B, the operational amplifier tries to flow a constant current even though no loop current is required. In other words, when subscriber J goes on-hook and communication lines A and B become open, the transistor
No current flows through Tr ₀ and Tr ₁ , so the resistances R _e0 and R _e1
Since there is no voltage drop across the op amp
The voltage V _- at the inverting input terminal of A ₀ and A ₁ becomes 0V, −V _BB , and is not equal to the voltage V ₊ at the non-inverting input terminal. As a result, the operational amplifier acts as a comparator and A ₀ reduces its output voltage to the supply voltage −V _BB , thereby reducing the resistance R _e0 − transistor Tr ₀ −
Will the same constant current as in the loop flow in the output path of op amp A ₀ , or will the op amp
The current will be drawn up to the output limit current of _A0 .
Similarly, for the operational amplifier _A1 , the output voltage is set to the highest potential of the operational amplifier output, and the current flows out to the constant current value or the output limit current. In this way, even during on-hook, the same current as that during off-hook or up to the limit current of the operational amplifier flows, which is uneconomical.

FIG. 3 is a circuit diagram showing a constant current feeding circuit for a subscriber circuit according to a second embodiment of the present invention, which is improved in this respect. In the same figure, the differences from Figure 2 are as follows:
The output of operational amplifier A ₀ , A ₁ and transistor Tr ₀ ,
Current limiting resistors R _d0 and R _d1 are connected to the base of Tr ₁ .
and the output of amplifier A ₂ and the resistance
A resistor R ₉ was inserted between the connection point M ₂ of R _b0 and R b1, and Zener diodes _{D 0} and D ₁ were inserted between the connection point M ₂ and the ground, and between the connection point M ₂ and the power supply −V _BB , respectively. It is.

The resistors R _d0 and R _d1 limit the current flowing between the base and emitter of the transistors T _r0 and T _r1 during on-hook, which is highly effective in reducing power consumption. The resistance values of the resistors R _d0 and R _d1 may be selected to be the maximum value within a range that can sufficiently supply base current to the transistors Tr ₀ and Tr ₁ in the off-hook state.

The resistor R ₉ and the Zener diodes D ₀ and D ₁ are provided as a countermeasure in case a line failure such as ground connection or cross contact occurs in the communication lines A and B. For example, if the communication line A is grounded and the communication line B is open, the resistance R _c0 ,
The potential at the connection point _M1 of R _c1 is at ground level G, and the output of amplifier _A2 is at ground level G. As a result,
The potential at the connection point _M1 is twice the normal voltage with respect to the power supply voltage, and the set current of the constant current circuit _CC10 including the operational amplifier _A1 is doubled. As a result,
The power consumption in the constant current circuit CC ₁₀ is 4 during normal operation.
This will double the amount, forcing us to reconsider the thermal design. To avoid this, a resistor R ₉ and Zener diodes D ₀ and D ₁ are provided. For example, if the power consumption of the transistor Tr ₁ is a problem,
The current set by a Zener diode, e.g.
By clamping at 1.4 times, the increase in power consumption can be doubled.

In each of the above embodiments, the amplifier _A2 is a voltage follower with a gain of 1, but by making the gain larger than 1, the impedance against common mode noise on the communication lines A and B can be further reduced to 1/gain. It is possible to do so. However, in this case, it goes without saying that a non-inverting circuit other than a simple voltage follower must be used.

〔Effect of the invention〕

As explained above, according to the present invention, by providing the midpoint voltage detection circuit in the constant current feeding circuit of the subscriber circuit, common mode noise on the communication line can be absorbed while maintaining constant current characteristics. , it is possible to realize low power consumption, to obtain economical efficiency and ease of thermal design, and to improve resistance to common mode noise.

Furthermore, by inserting a resistor into the constant current circuit, it becomes possible to absorb noise mixed into the power supply.

Furthermore, unnecessary current during on-hook can be limited to further reduce power consumption.

Furthermore, it becomes possible to suppress the consumption of large amounts of power due to disturbances such as cross-circuiting and disconnection of communication lines, and to facilitate the thermal design of the power supply circuit.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a constant current feeding circuit of a subscriber circuit which is a premise of the present invention, and FIGS. A circuit diagram showing a current feeding circuit, FIG. 4 is a circuit diagram showing a general configuration example of a subscriber circuit of a digital exchange according to the present invention, and FIGS. 5 to 7 explain a conventional power feeding circuit of a subscriber circuit. This is a diagram for A, B...Talking line, ID...Intermediate voltage output circuit,
CC ₀ , CC ₁ , CC ₁₀ ... Constant current circuit, R _a0 , R _b0 ,
R _b1 , R _a1 ... Constant current setting resistor, A ₂ ... Amplifier,
A ₀ , A ₁ ... operational amplifier, Tr ₀ , Tr ₁ ... transistor, G ... ground, -V _BB ... power supply, R _f0 , R _s0 ,
R _f1 , R _s1 ... Power supply noise absorption resistance, R _d0 , R _d1 ...
Current limiting resistor.

Claims (1)

[Claims] 1. Two pairs of reference voltage setting element groups R _ap , R _bp , which are vertically symmetrical with respect to the potential between the power supply −V _BB and the ground G.
Using the voltage obtained by dividing by R _a1 and R _b1 as a reference potential, constant current bottlenecks CC ₀ and CC ₁ that supply constant current to two communication lines A and B, respectively, and the midpoint voltage between the communication lines. and a midpoint voltage output means ID for detecting and outputting to the midpoint _M2 of the two pairs of reference voltage setting element groups, and the constant current circuits CC ₀ and CC ₁ are divided by the reference voltage setting element groups. The operational amplifiers A ₀ , A _{1 are provided with operational amplifiers A 0 , A 1} which input the obtained voltage as a reference voltage, and _a voltage-current conversion circuit constituted by resistors R _ep , R _e1 and transistors Tr ₀ , Tr ₁ . The non-inverting input terminal of _{No. 1} receives the output voltage of the midpoint voltage output means ID via the reference voltage setting element group, and the inverting input terminal receives one of the power supply side or the ground side of the transistors Tr ₀ and Tr ₁ . The output terminal is connected to the bases of the transistors Tr ₀ and Tr ₁ , and the other terminals of the transistors Tr ₀ and Tr 1 are connected to the communication lines A and B, and the output terminal is connected to the bases of the transistors Tr 0 and Tr ₁ . ₀ , the inverting input terminal of _A1 and the transistor
between the one ends of Tr ₀ and Tr ₁ , and the operational amplifier A ₀ ,
A power supply noise absorbing resistor R _f0 is connected between the inverting input terminal of A ₁ and the power supply −V _BB or the ground G, respectively.
R _f1 , R _s0 , and R _s1 are connected so that the potentials of the non-inverting input terminals and the potentials of the inverting input terminals of the operational amplifiers A ₀ , A ₁ are at the same potential with respect to power supply noise. and the midpoint voltage output means ID is for the communication lines A and B.
Equipped with an amplifier A ₂ that amplifies the midpoint potential between the two in positive phase,
A constant current power supply circuit for a subscriber circuit, characterized in that the output of the amplifier _A2 is connected to the midpoint of the two pairs of reference voltage setting element groups {R _ap , R _bp , R _a1 , R _b1 }. . 2 Two pairs of reference voltage setting element groups {R _{ap ,} R _bp ,
R _a1 , R _b1 }, with the voltage obtained by dividing by R a1 , R b1 } as a reference potential, constant current circuits CC ₀ and CC ₁ that supply constant current to two communication lines A and B, respectively, and the midpoint between the communication lines. midpoint voltage output means ID for detecting a voltage and outputting it to the midpoint _{M2 of} the two pairs _of reference voltage setting element groups; The operational amplifier A ₀ , A ₁ includes operational amplifiers A 0 , A 1 that input the divided voltage as a reference voltage, a voltage-current conversion circuit composed of resistors R _ep , _{R e1} , and transistors Tr ₀ , Tr ₁ . A non-inverting input terminal of A ₁ receives the output voltage of the midpoint voltage output means ID via the reference voltage setting element group, and an inverting input terminal of the transistors Tr ₀ and Tr ₁ on the power supply side or the ground side. The output terminal is connected to the bases of the transistors Tr ₀ and Tr ₁ , and the other terminals of the transistors Tr ₀ and Tr 1 are connected to the communication lines A and B, and the output terminal is connected to the bases of the transistors Tr 0 and Tr ₁ . Inverting input terminals of A ₀ and A ₁ and the transistor
between the one ends of Tr ₀ and Tr ₁ , and the operational amplifier A ₀ ,
A power supply noise absorbing resistor R _f0 is connected between the inverting input terminal of A ₁ and the power supply −V _BB or the ground G, respectively.
R _f1 , R _s0 , and R _s1 are connected so that the potentials of the non-inverting input terminals and the potentials of the inverting input terminals of the operational amplifiers A ₀ , A ₁ are at the same potential with respect to power supply noise. Further, current limiting resistors R _dp and R _d1 are connected between the outputs of the operational amplifiers A ₀ and A ₁ and the bases of the transistors Tr ₀ and Tr ₁ , and the midpoint voltage output means ID is Call lines A, B
Equipped with an amplifier A ₂ that amplifies the midpoint potential between the two in positive phase,
The output of the amplifier A ₂ is connected to the midpoint of the two pairs of reference voltage setting element groups {R _ap , R _bp , R _a1 , R _b1 }, and the output of the amplifier A ₂ and the power supply −V _BB or the Ground G
Zener diode D _p for current limiting between
A constant current power supply circuit for a subscriber circuit, characterized in that it is formed by connecting _D1 .