SUBSCRIBER LINE INTERFACE CIRCUIT

Abstract: A balanced type subscriber line interface circuit (SILC) is ideally to be fabricated as a monolithic IC. A battery feed circuit (300) comprises three pairs of mirror circuits (A0, B0, A1, B1, A2, B2) to eliminate the longitudinal noise and noise induced in the voltage source. Overcurrent protection is done by varying a mirror ratio of current feeding mirror circuits (A0, B0, A1, B1) when the subscriber line current exceeds a threshold value. A 2- to 4-wire converter (400) is compensated in its frequency characteristics by feeding back a current with an impedance network composed of two impedance elements each having respectively an impedance N time as high as that of the load impedance Zo and the termination impedance Z. The supervising circuit (500) provides scan signals detecting the currents running through the sub­ scriber lines. The circuit is provided with a ground separation circuits (600) to separate the grounds of high voltage source and low voltage source.

Description: FIELD OF THE INVENTION The present invention relates to a subscriber line interface circuit (SLIC) for a telephone terminal station having a digital switching system. The invention is applicable to the fabrication of a battery feed circuit, a hybrid circuit and a supervision circuit in monolithic ICs (integrated circuits) to miniaturize their size and to reduce their fabrication cost. A SLIC having a battery feed circuit is shown in pages 1144-1149 of volume SC-17 no.6 of the IEEE Journal of Solid State Circuits, December 1982. The battery feed circuit comprises a pair of current feed circuits, a pair of voltage to current converting means and two pairs of mirror circuits. The subscriber line interface circuit for the telephone station is generally composed of seven fundamental circuits as shown in Fig. 1. Namely, a battery feed circuit for providing DC power for subscriber lines, a hybrid circuit for converting the telephone signal from 2-wire signal to 4-wire signal or vice versa, overvoltage protection circuit, ringing switch circuit for handling a ringing signal, CODEC (coding and decoding circuit) for telephone code, supervision circuit for supervising the operation of the circuit and test access relay circuit. All of them are provided respectively for each of subscribers. Therefore, it is very important to reduce their size, power consumption and cost. First, CODEC has been fabricated in monolithic IC and is now in wide use, because the voltage applies to it is low (about 5 volts) and it was easy to realize in IC. Attempts to fabricate the remaining circuits in an IC are beginning to be applied in practice. These efforts are directed to design ICs for the battery feed circuits, 2- to 4-wire converter and the supervision circuit. The reason is considered that these circuits deal with a medium voltage of 40-60 volts, while remaining circuits deal with several hundred volts or high current to be handled by relays. The subscriber line interface circuit can be considered as equivalent to a circuit as shown in Fig. 2(a). To the terminals A and B are connected a subscriber equipment having an impedance Z₀ which includes the impedances of the subscriber line and telephone equipment, for example. The subscriber line interface circuit (abbreviated as SLIC hereinafter) is provided with an AC termination impedance Z that terminates the subscriber line, and the battery feed circuit supplies a DC voltage to one of the subscriber line A for example, and the other subscriber line B is grounded through a feeding resistance. Telephone signal from the subscriber is taken out from the terminal A and B via a hybrid circuit converting a two-wire signal to a 4-wire signal. Therefore the internal impedance of the battery feed circuit must be as high as possible for the telephone signal which appears in a differential mode having a phase difference between the lines A and B. But in order to prevent noise generated in the subscriber line, the internal impedance for the noise should be as small as possible. Noise is usually generated on the subscriber line in a longitudinal mode that has a same phase for the lines A and B. So, the internal impedance of the battery feed circuit is preferable to be as low as possible for the longitudinal mode signal, but it should be as high as possible for differential mode signal. To realize such internal impedance, various circuits have been proposed. But they can be reduced to an equivalent circuit as shown in Fig. 2(b) for the longitudinal noise voltage. The equivalent circuit is a bridge circuit having two resistances R corresponding to the lines A and B respectively, and internal resistance R A and R B of battery sources for lines A and B respectively. The noise voltage Vn is applied to a junction point of the two resistances R in equal phase for both of the lines. Therefore, two main streams appeared in the design of SLIC to eliminate the noise voltage between the terminals A and B. The one is balancing the bridge circuit, that is to make R A /R = R B /R the other is to make R A and R B as small as possible compared to R. The former is called as balanced type, and the latter is called as non-balanced type SLIC. "A Monolithic Telephone Subscriber Loop Interface Circuit" by W. David Pace, IEEE Journal of Solid State Circuit, Aug. 1981, discloses fundamentals of monolithic IC circuits for the non-balanced types SLIC. Following Japanese Laid Open Patents Tokkai 55-150658; 56-141655; 57-25766; 57-38053; 58-104559 disclose the balanced type battery feed circuits, Tokkai 58-210775; 59-161172 disclose the non-balanced type battery feed circuits, and Tokkai 57-42263 discloses an IC circuit for 2- to 4-wire converter. The non-balanced type battery feed circuit is more progressed in realizing it in a monolithic IC. It is important to make an equivalent value of the resistances R A and R B as small as possible. Using mirror circuits and feedback circuits, the equivalent value of R A and R B have been reduced to less than 1 Ohm, and a longitudinal balance by 45 dB has been attained, when the circuit is trimmed. But the value of the longitudinal balance is the larger the better. In order to attain higher longitudinal balance, it is necessary to balance the bridge circuit of the equivalent circuit of Fig. 2(b). This is the balanced type circuit. But in prior art circuits, it was necessary to to use discrete components such as capacitors, power transistors and so on, which are difficult to fabricate in monolithic IC. Moreover, the balance was liable to be disturbed by the variation of subscriber line impedance, supply voltage, signal level and so on, and when the balance was disturbed the circuit is suffered by noise, so a precise adjustment of the balance is necessary. A general object of the present invention therefore is to fabricate a main part of SLIC by monolithic IC to reduce its size, power consumption and cost. Another object of the present invention is to provide a combined circuit of a battery feed circuit, a 2-to 4-wire converting hybrid circuit and a supervision circuit, which can be fabricated mostly in a monolithic IC. A further object of the present invention is to provide a balanced type SLIC of which adjustment is easy and can be realized mostly in a monolithic IC. According to the present invention, there is provided a balanced type subscriber line interface circuit (SLIC) having a battery feed circuit for supplying DC current for a 2-wire subscriber lines composed of line A and line B respectively connected to terminals A and B, said battery feed circuit comprising: a pair of current mirror circuits of which output terminals being respectively connected to said terminals A and B, for supplying said DC current; a pair of first voltage to current converting means being respectively connected to said terminals A and B; a pair of first mirror circuits of which input terminals being respectively connected to output terminals of said voltage to current converting means; and a pair of second mirror circuits; characterized by: the input terminals of said second mirror circuits being connected respectively to output terminals of opposite side mirror circuits of said first mirrors circuits via said resistors; the output terminals of said second-mirror circuits being connected respectively to input terminals of opposite side mirror circuits of said current feed mirror circuits; a capacitor being connected between output terminals of said first mirror circuits; and a pair of resistors of which one end is connected respectively to said capacitor. The invention will now be described in more detail by way of example with reference to the accompanying drawings in which:- Fig. 1 is a block diagram of a subscriber line interface circuit (SLIC) illustrating its main functions. Fig. 2(a) is a block diagram illustrating impedance relation between SLIC and subscriber's equipment. Fig. 2(b) is an equivalent circuit of SLIC for noise voltage induced on a subscriber line. FIG. 3 is a block diagram illustrating an operation principle of a battery feed circuit according to the present invention. Fig. 4 is a block diagram for illustrating an operation principal of a hybrid circuit for converting 2-wire signal to 4-wire signal or vice versa according to the present invention. Fig. 5 is a block diagram for a supervision circuit of the present invention. Fig. 6 is a block diagram for illustrating a principle of ground separation method applied in the present invention. Fig. 7 is a block diagram illustrating a total figure of SLIC composed of fundamental circuits according to the present invention. Fig. 8 is a block diagram of a battery feed circuit embodying the present invention. Fig. 9 shows circuit diagrams for various types of mirror circuit applicable to the present invention, wherein: (a) is a circuit symbol of a mirror circuit; (b) is a fundamental circuit of a mirror circuit; (c) is another example of a mirror circuit; and (d) is a mirror circuit having two outputs. Fig. 10 is an example of a voltage stabilizing circuit applicable for the voltage source of the battery feed circuit to reduce a noise induced in the voltage source. Fig. 11 is an example of a current limiting circuit applicable for the battery feed circuit of the present invention. Fig. 12 is a voltage-current characteristics of the current limiting circuit of Fig. 11. Fig. 13 is a circuit diagram of an absolute value detector applied in the battery feed circuit of the present invention. Fig. 14 is a circuit diagram of a comparator and driving circuit for an overcurrent protection circuit of the present invention. Fig. 15 is a block diagram showing a relationship between a 2- to 4- wire converter and the battery feed circuit of the present invention. Fig. 16 is a circuit diagram of another embodiment of a circuit application for the 2 to 4-wire convertor of Fig. 15 for correcting its frequency characteristics. Fig. 17 is a circuit diagram for another embodiment of voltage to current converter applicable in the 2- to 4-wire converter of Fig. 15. Fig. 18 is a circuit diagram of a supervision circuit shown in Fig. 5. Fig. 19 is a block diagram illustrating an operation of the ground separation by the present invention. Fig. 20 is a diagram illustrating a voltage supply system for a subscriber line interface circuit. Fig. 21 is a diagram illustrating a circumstances between various voltage sources of the subscriber line interface circuit according to the present invention. Fig. 22 is a total circuit diagram of the SLIC according to the present invention. An embodiment of the battery feed circuit of the present invention will be described referring to a block diagram of Fig. 3. It utilizes three pairs of mirror circuits A0, B0, A1, B1, A2 and B2. They are arranged in cascade to the subscriber lines (lines A and B), and arranged symmetrically for both A and B lines. The output terminals of the current feed mirror circuit pair A0 and B0 are connected respectively to the lines A and B to feed their output currents to each of them as a DC supply source. The input terminals of the current feed mirror circuit pair A0, B0 (input terminal of mirror circuit is discriminated by a small circle) are respectively connected in series to the output terminals of the second mirror circuits pair B2 and A2. The input terminals of the first mirror circuit pair A1 and B1 are connected to the A and B lines via resistors Rb0 and Rb1 respectively, and the output terminals of them are connected in series to the input terminals of the second mirror circuits B2 and A2 via resistors Rc1 and Rc0 respectively. Between the resistors Rc0 and Rc1 is connected a capacitor C AB . When a voltage appears on the terminals A and B, currents flow in the input terminals of the first mirror circuits A1 and B1 corresponding to respective terminal voltages V A and V B , and according to original nature of the mirror circuit, the same amount of current having same direction run through the output terminals of the first mirror circuits A1, B1. If a voltage having equal phase is applied to the terminals A and B, the output currents of the first mirror circuits (A1, B1), are equal to each other. Therefore, the both sides of the capacitor C AB is charged up to the same voltage, that is equivalent that there is no capacitors. So the same currents run through the second mirror circuits (A2, B2), bit the directions of the currents are reversed to those of the first mirror circuits (A1, B1), because their input terminals are connected in series to the first mirror circuits. Namely, if the current flows into B1, the same current flows out from A2 and vise versa. These currents are fed to respective input terminal of the current feed mirror circuits A0 and B0. Since the inputs terminals of the current feed mirror circuits are connected in series to the second mirror circuits, the output currents of the current feed mirror circuits are same but in opposite direction to those of the second mirror circuits. Therefore, the output currents of the current feed mirror circuits have same amplitude and same direction to those of the input currents to the terminals A and B. This means the longitudinal input impedance of this circuit becomes low value which is equal to the feeding resistances. The feeding resistance is determined by the resistances of Rb0, Rb1 and mirror ratio that is the ratio of input and output currents of the mirror circuit. If there is a voltage difference between the terminals A and B, that is the input signal has a differential mode, the current difference appears between the output of the first mirror circuits A1 and B1. But such differential component is bypassed by the capacitor C AB , moreover, the capacitor C AB all together the resistors Rc0 and Rc1 composes a filter circuit for the differential mode signal. So, the differential component is not input to the second mirror circuits A2 and B2. Therefore, there appears no current corresponding to the differential mode input voltage. This means the circuit provides a very high impedance for the differential mode input signal. So, the input telephone signal that is a differential mode signal is fed to the 2- to 4-wire converter without loss. But for a noise having a longitudinal mode, the circuit becomes a low impedance, moreover the circuit is balanced for the lines A and B, so the output voltage of the noise is canceled by each other and there appears no noise signal to the 2- to 4-wire converting hybrid circuit. At a common terminal of the second mirror circuit A2 is supplied a constant voltage from a stabilized voltage source Vz. This protects the circuit from noise induced from battery source V BB . The battery feed circuit according to this embodiment of the present invention is further provided with a current limiting circuit for protecting the battery feed circuit from overcurrent caused by such as fault contact to ground or to V BB of the subscriber line etc. Principle of the current limiting is done as follows. If an overcurrent is detected, resistance of an input resistor to the current feed mirror circuit A0 or B0 is switched to a lower resistance. So, the mirror ratio of the mirror circuit is reduced and the current fed to the A or B lines by the current feed mirror circuit A0 or B0 is limited. Principle of a 2- to 4-wire converting hybrid circuit will be explained with respect to a block diagram of Fig. 4. A 4-wire input signal received by an input terminal 4WR is converted to a current signal by a voltage to current converter (VI) 6, arid fed to one of third mirror circuit pair A3 and B3. A first output of the mirror circuit A3 is fed to the current feed mirror circuit B0 that composes the current feed circuit of Fig. 3, and the second output current is reversed its direction by another third mirror circuit B3 and fed to another current feed mirror circuit A0. Therefore, from the current feed mirror circuit pair A0 and B0 are output a two-wire signal having opposite phase between each other, and fed to the lines A and B respectively. 2-wire signal received by the input terminals A and B is fed to an operational amplifier OP3, and the differential component is detected. In the output of the operational amplifier OP3 is included a component of 4-wire input signal input to the terminal 4WR, because it is output from the current feed mirror circuits A0 and B0 and turns around to OP3 together with the 2-wire signal input to the terminals A and B. Such turn around signal has a reverse phase to that of input signal to the terminal 4WR. So, the output of OP3 is mixed with a signal branched from the terminal 4WR in a proper network to erase the turn around 4-wire signal, and only 2-wire signal is taken out from a 4-wire output terminal 4WS. The supervision circuit to identify on-hook or off-hook state of the subscriber, and detecting the fault contact of the subscriber line to ground or voltage source will be explained referring a block diagram of Fig. 5. The current feed mirror circuits A0 and B0, composing the current feed circuit shown in Fig. 1, includes power transistors Q0, Q1 and series resistors Re0 and Re1 respectively. The current flows I A and I B running respectively in the lines A and B are detected as voltage drops across the resistors Re0 and Re1 respectively. These voltages are compared with threshold voltages Vth1 and Vth2 by first and second comparators CMP1 and CMP2 respectively. At the same time, the detected voltages are supplied to an adder circuit ADD, and this added voltage is also compared with third threshold voltage Vth3 by a comparator CMP3. Output of these comparators are respectively fed to AND gates G1 and G2 as shown in Fig. 5. In normal off-hook state, the currents I A and I B are almost balanced, and their detected voltages are more than Vth1 and Vth2. The detected voltage sum also more than Vth3. Therefore, both gate circuits G1 and G2 output supervising signals SCNA and SCNB respectively. The variation of the currents I A and I B caused by noise is canceled by the adder circuit ADD, since the noise is induced in the lines A and B as a longitudinal mode. So, the output of the adder circuit is kept almost constant regardless to the noise. Therefore the generation of SCNA and SCNB signals are stable, so long as the noise current is not too high to decrease the currents I A or I B to less than the threshold value of Vth1 or Vth2. It the line A or B is grounded by fault, I A becomes large and I B become very small, but the output of the adder circuit is still over the threshold voltage Vth3. Therefore, the SCNB signal is not generated, but SCNA signal is generated, so the status can be discriminated. On the contrary, if the line A or B is contacted to voltage source V BB , SCNB signal is Generated and SCNA signal is not generated. In on-hook state, the currents I A and I B are very small, so neither the scan signal SCNA nor SCNB is generated. Therefore, in any way, the status is discriminated. Still another important feature of this embodiment of the present invention appears in a method of separating grounds in the system. Generally, the SLIC is operated with various kinds of voltage source. For example, the battery supply circuit is operated by a voltage, source between ground G and -48 volts, and logic circuits are operated with Vcc of +5 volt, V EE of -5 volt and ground E. So it is necessary to transmit signals between the systems having different grounds G and E for example. In other words, it is necessary to separate grounds G from B while the circuit is connected to each other. Usually transformer is used for such ground separation, but use of transformer should be avoided in IC circuits. This embodiment of the present invention intends to feed a current from system E (that has a ground E) to system G (that has a ground G), for example, having equal amplitude but opposite direction to that of signal current from system G to system E. By doing so, though the signal current flows between the systems G and E, it is always canceled by the current fed from the system E to G and vice versa, therefore the total current between the two systems is always zero, this is equivalent that the systems G and E are separated from each other. Fig. 6 shows a principle of the inventive ground separation method. An input signal from the system G is applied to an input terminal IN, and it is fed to a mirror circuit M1. One output of M1 is fed to another mirror circuit M2. Another output of M1 is connected to Vcc via a detector DET, and output of M2 is connected to V EE voltage source of the system E. Since the current flowing from system E to G is equal but in opposite direction to that flowing from system G to E, the System G and E are equivalent to be separated from each other, but the same current to that of the input signal at the input terminal IN flows through a detector DET of the system E, so the signal from the input terminal IN of the system G is detected and taken out from an output terminal OUT of the system E. Similarly, signal from the system E is input to an input terminal IN' of the system E, and taken out from an output terminal OUT' of the system G. Utilizing above described features and fundamental circuits, the SLIC is constructed. Fig. 7 is a block diagram illustrating a total configuration of SLIC composed of the fundamental circuits according to an embodiment of the present invention as described above. The circuit corresponds to a portion encircled by a dotted line in a block diagram of Fig. 1. In Fig.7, it will be clear that a portion encircled by a chained line 300 is a battery feed circuit corresponding to Fig.3, a portion encircled by chained line 400 is a 2-to 4-wire converting circuit corresponding to Fig.4, a portion encircled by a chained line 500 is a supervision circuit corresponding to Fig.5, and a portion encircled by a chained line 600 is a ground separation circuit corresponding to Fig. 6 First, an embodiment of circuit parts performing the fundamental functions of the SLIC will be described, and then a total figure of the circuit will be given. Throughout the drawings, similar or like reference numerals or characters designate similar or corresponding parts in the figures. Battery Feed Circuit Fig. 8 is a block diagram of a battery feed circuit embodying the present invention. The figure corresponds to Fig. 3 describing a principle of the battery feed circuit. The circuit comprises three pairs of mirror circuits, A0, B0, A1, B1, A2 and B2. Among of them the current feed mirror circuits A0 and B0 which are encircled by a chained lines 80, feed DC currents of approximately 20-100 mA to the subscriber lines A and B respectively. The remaining mirror circuits are conventional small current devices which handle a current of less than 1 mA. It will be clear that the interconnection between these mirror circuits and resistors Rb0, Rb1, Rc0, Rc1 and the capacitor C AB is similar to that of Fig. 3, so further description of circuit connection is omitted for the sake of simplicity. The circuit is constructed similarly both for the lines A and B, so detailed description will be given mainly with respect to the line B. The current feed mirror circuit B0 is composed of an operational amplifier OP1, a transistor Q1 and resistors Ra1 and Re1. A non-inverting input terminal (designated by a symbol +) of the operational amplifier OP1 becomes the input terminal of the mirror circuit B0, and the collector of the transistor Q1 becomes the output terminal of the current feed mirror circuit B0. An inverting input terminal (designated by a symbol -) is connected to the emitter of Q1. A current ratio that is a ratio of the output current to the input current of a mirror circuit is determined by Ra1/Re1. Since the input current to B0 supplied by the mirror circuit A2 is small, the current ratio of B0 is chosen to be large. But the current ratio of the first and second mirror circuits A1, B1, A2 and B2 are usually set to one. The value of resistors are also limited from other conditions. The feeding resistances Z A and Z B of the battery feed circuit for the lines A and B are respectively given as Z A = Rb0·Re0/Ra0 (1) and Z B = Rb1·Re1/Ra1 (2) These impedances are the lower the better for noise reduction, but on the contrary, the resistance of resistors Rb0 and Rb1 are the higher the better in order to transmit the input 2-wire signal to the 2- to 4-wire converter without loss. So in an embodiment these resistors have been chosen that, Re1 is 50 Ohms, Ra1 is 6 k Ohms, and Rb1 is 50 k Ohms. Therefore, the current ratio is 120. Generally a higher value of resistance is difficult to fabricate in monolithic IC, moreover, it is important to balance these resistors for the A and B lines, from such view points above values of resistance have been determined. The power transistors Q0 and Q1 are Darlington transsistors, and capacitor C AB is 0.1-0.2 µF for example. It is necessary to chose the value of the capacitance and resistance to be Rc1 = Rc0 » 1/jωC AB where ω is the frequency of the input signal. The reason will become clear later. In an embodiment Rc0 and Rc1 were chosen to be few tens of k Ohms. These transistors Q0, Q1, together with resistors Re0, Re1 and capacitors C AB , are connected to the circuit as external circuit elements, because their size and power consumption are large, and resistors Re0 and Re1 are used to trim the circuit balance. Other circuit elements of Fig. 8 are all fabricated in a monolithic IC chip. The remaining mirror circuits A1, B1, A2 and B2 may be conventional ones, some examples are shown in Fig. 9. In the figure, (a) is a circuit symbol of the mirror circuit, and its input terminal is identified by a small circle. C is a common terminal. If a current flows between IN (input terminal) and a common terminal C, a current having a predetermined current ratio and same direction to the input current flows between OUT (output terminal) and a terminal C. In Figs. 9, Qa-Qe are transistors, and R1-R3 are resistors. The circuit of Fig. 9(b) is a basic configuration of mirror circuit, and the circuit of Fig. 9(c) is further stabilized its operation for wider range of input current. The circuit of Fig. 9(d) has two outputs. The current ratio is determined by the ratio of R1/R2 and R1/R3 respectively. But, since the circuit configuration of the mirror circuit is not explicitly related to the invention, further description is omitted. In Fig. 8, if a differential mode signal having a phase difference between the terminals A and B appears, currents corresponding to respective voltage to the ground will run through the input terminals of the first mirror circuit pair A1 and B1. These currents appear on both sides of the capacitor C AB , and they are bypassed through C AB , because as mentioned before, the resistance of Rc0 and Rc1 are large compared to the capacitive reactance of C AB . Moreover, by a filter action of Rc0 and Rc1 combined with C AB , these differential mode signal currents cannot flow into the second mirror circuit pair A2 and B2. Therefore, no differential mode signal is fed back to the current feed mirror circuits A0 and B0, thus no current variation corresponding to the differential mode signal appears in the output terminals of A0 and B0. This is equivalent that the battery feed circuit has a high impedance against the differential mode signal, that is a telephone signal for example. So, the input signal is fed from the terminals A and B to the 2- to 4-wire converter circuit without loss. On the contrary, if a longitudinal mode noise appears on the terminals A and B, the corresponding currents will appear on both sides of the capacitor C AB . But since the phase of these currents are same to each other, the voltage of both sides of C AB are always equal. This is equivalent that there is no capacitor for the longitudinal signal. So, these currents run in the mirror circuits A2 and B2, and fed back to A0 and B0. Since the current ratio of A0 and B0 are large, a large current having a same phase to the input longitudinal signal appears in the circuit. This means the effective longitudinal input impedance of the battery feed curcuit becomes low value which is equal to the battery feed resistance. In fact, the input impedance for the longitudinal mode becomes as shown by the equations (1) and (2) respectively for the lines A and B. As has been described above, the circuit of Fig. 8 satisfies the fundamental requirements of a battery feed circuit. And as mentioned before, the circuit is composed quite similarly for the lines A and B, the balance of impedance for A, B lines are very good. So, the circuit of Fig. 8 is a balanced type battery feed circuit. Moreover, the balance can be precisely adjusted by adjusting the external resistors RE1 and RE0. So, the longitudinal noise can be reduced very much. In an embodiment a longitudinal balance of 55 dB has been achieved. In Fig. 8, the voltage Vz supplied to the common terminal of the second mirror circuit A2 is equivalent to that of the source voltage V BB applied to other mirror circuits, but its voltage is especially stabilized in order to reduce a noise induced from the voltage source V BB . Noise suppression process by the stabilized voltage source Vz is as follows. Firstly, as a general characteristics of a mirror circuit, its impedance seen from its output terminal is very high. So in Fig. 8, if V BB is varied by noise, the voltage variation does not appear in the output terminal of the current feed mirror circuit A0. Therefore, the noise from voltage source of A0 is negligible. Secondly, with regard to the second mirror circuit A2, the noise can be neglected, because its voltage source is stabilized. Thirdly, the voltage variation of V BB causes a noise in the first mirror circuit A1, therefore, it must be checked. If V BB varies, there are two current paths that generate a noise. The first noise current path is composed of V BB , A1, Rb0, terminal A, subscriber equipment, terminal B, Rb1, B1 and ground. But as mentioned before, Rb1 and Rb0 are chosen to be very high compared to the impedance between terminals A and B. So, the noise voltage induced in such first path becomes very low between the terminals A and B. Therefore, such noise is substantially negligible. The remaining noise path is composed of V BB , A1, Rc1, B2 and ground. However, the current runs through the output terminal of A1 runs not only through Rc1, but it runs also through Rc0, because Rc1 and Rc0 are alternatively connected to each other by the capacitor C AB . Accordingly, the noise current is divided into half and both half currents respectively run through the second mirror circuit pair B2 and A2. The current direction is same for B2 an A2. For example, if the current runs into B2, the same current runs into A2 and vice versa. So, the output currents of B2 and A2 are same phase between each other. Such currents are fed to the input terminals of A0 and B0 respectively, therefore, the output of A0 and B0 are always in the same phase for the noise induced from V BB . So, the noise voltage at the terminals A and B becomes equal to each other, and it cannot be picked up by the 2- to 4-wire converter. Above description has been given with respect to A1. But the noise current running through B1 is reduced by a similar manner. Therefore, by only one stabilized voltage source Vz, the noise induced from the voltage source V BB can be eliminated. In an embodiment, V BB is -48 volts, and Vz is -35 volts for example. The stabilized voltage source may be of any kind, for example a series connected zener diodes and resistor may be applicable. But the power consumption should be small, and it is desirable that the internal resistance between the terminal Vz and ground is low, but the impedance between Vz and V BB is high in order to reduce the noise induction in the V BB . An example of such voltage stabilizing circuit is shown in Fig. 10. In the figure, Q2-Q4 are transistors, D₀ is a diode, R4, R5 are resistors of 100 k ohms and 3 k Ohms respectively, and Dn are zener diodes connected in series. It will be understood that the circuit of Fig. 10 composes a modified mirror circuit. A current flows from a ground to a negative voltage source V BB via R4, Q3 and D₀, so the same current flows through Dn, Q2 and R5. A constant voltage Vz determined by the zener diodes Dn is supplied by an emitter follower transistor Q4 to a common terminal of the second mirror circuit A2 shown in Fig. 8. Since the configuration of the stabilized voltage source is not explicitly related to the invention, further description is omitted. Sometimes, it occurs that the supply current of the battery feed circuit increases very high by a reason, for example the subscriber uses a very low impedance device, the subscriber lines is very short, or accidentally short-circuited. In such cases it is desirable to limit the supply current to less than a predetermined value. For such purpose the resistors Rb0 and Rb1 are replaced by a current limiting circuit. An example of current limiting circuit applicable for such purpose is shown in Fig. 11. The symbols of + and - indicate respectively a positive and negative side of the supply voltage. This circuit is composed of transistors Q5, Q6 and resistors r e , and is conventional in the art, so further description is omitted. An example of voltage to current characteristics is given in Fig. 12. The curve can be varied by selecting the resistance of r c and r e . Normally, the circuit is adjusted to operate at a high inclination portion of the curve. But if the supply current increases abnormally, the input currents to B1 and A1 are limited, so the output of the current feed mirror circuits B0 and A0 cannot exceed a value corresponding to the maximum value of Fig. 12. Overcurrent Protection Circuit A battery feed circuit of Fig. 8 is provided with a function to protect the circuit from an overcurrent caused by a fault contact of the subscriber lines to ground or V BB . The overcurrent protection circuit according to the present invention is composed of a resistance switching means which is encircled by a chained line 83 and 83' in Fig. 8 for switching an input resistor of the current feed mirror circuits A0 and B0; a detecting means encircled by a chained line 81 for detecting a difference between the current flows of the first mirror circuits A1 and B1; and a control means encircled by a chained line 82 for comparing the detected current with a threshold value and controlling the switching of the resistance switching means. While the subscriber lines are operated in normal sate, the currents running through the lines A and B are almost balanced, so the currents running through the first mirror circuits A1 and B1 are almost equal. But if one of the line is short-circuited to ground or to the voltage source, the current becomes imbalance. Such imbalance is detected by the detecting means, and it is compared with a threshold value V th . When the imbalance exceeds a predetermined threshold value, the control means controls the resistance switching means to shunt the input resistor Ra0 or Ra1 with another resistor. This reduces the current ratio of the current feed mirror circuit A0 or B0, so the output current of A0 or B0 is reduced. As shown in Fig. 8, the first mirror circuits A1 and B1 are provided with two output terminals, an example of such circuit is shown in Fig. 9 (d). The second output of A1 and B1 are connected to each other and connected to the detecting means 81. By doing so, the detecting means receives a difference current between the output currents of A1 and B1. Because in normal state, the currents of the lines A and B are equal but in opposite direction to each other, since the battery current flows along a circuit composed of ground, B0, terminal B, subscriber's equipment, terminal A, A0 and V BB . The input currents of B1 an A1 are proportional to the output currents of B0 and A0 respectively, so the output currents of B1 and A1 are equal to each other but their direction is opposite to each other. The detecting means 81 is composed of a resistor Rd and an absolute value detector ABS which receives the difference current and detects its absolute value. The output current of ABS is converted to a voltage by the resistor Rd whose resistance being few k Ohms. An example of absolute value detector is shown in Fig. 13. It is composed of two transistors Qf (npn), Qg (pnp) and a mirror circuit CM1 having a mirror ratio of 1. The circled terminals 1-3, correspond respectively to an input terminal, a voltage source terminal, and an output terminal, and they correspond respectively to the circled terminals 1-3 in Fig. 8. In such circuit, if, for example, a current runs into the terminal 1, it runs into an input terminal of the mirror circuit CM1 via the transistor Qg, so the same current runs into CM1 from the terminal 3. On the contrary, if a current runs out from the terminal 1, it runs in from the terminal 3 and runs via the transistor Qf. So, the same amount of current runs into the output terminal 3 regardless to the direction of current running through the input terminal 1. This is equivalent that the current running into the output terminal 3 is the absolute value of the input terminal 1. The absolute value detector may be of any kind, but it is not related explicitly to the invention, further description is omitted. A circuit diagram of the control means is shown in Fig. 14, it is composed of a comparator CMP and a driving circuit DV. The encircled terminals 4-8 correspond respectively to the encircled terminals 4-8 in Fig. 8. The comparator CMP is composed of three transistors Qh (npn), Qi (pnp), Qj (npn) and a constant current source CI. These circuits are conventional ones in the art. The voltage across the resistor Rd (Fig. 8) is fed to the terminal 7 and it is compared with a predetermined threshold voltage Vth (-2 volts for example) supplied to the terminal 8 of the comparator CMP. The driving circuit DV is composed of two transistors Qk, Ql (both pnp) and another mirror circuit CM2 having a mirror ratio 1. The terminals 5 and 6 are connected respectively to the resistance switching means 83' and 83 in Fig. 8. When the output current of the absolute value detector ABS is small, that is the subscriber line is in normal state, the transistor Qh becomes ON state, and transistors Qi and Qj becomes OFF state. So, the transistors Qk and Ql become OFF state and the resistance switching means 83 and 83' does not operate. So the input resistor for the current feed mirror circuits A0 and B0 are respectively Ra0 and Ra1, and A0 and B0 feed a predetermined currents to the subscriber lines A and B respectively. If the output current of ABS becomes large, that is the subscriber line becomes unusual state, and the input voltage of the terminal 7 increases over a predetermined threshold voltage Vth, the transistor Qh becomes OFF state. So, the transistors Qi, Qj, Qk and Ql become ON state, and driving currents appear in the terminals 5 and 6. These currents run through the resistors Rg1 and Rg0 in Fig. 8 respectively, and make the transistors Qr0 and Qr1 of the resistance switching means 83 and 83' conductive. This implies that the input resistors Ra0 and Ra1 are shunted by Rf0 and Rf1 respectively. So the effective resistance drops to a lower value. Since the current ratio of A0 and B0 are determined by Ra0/Re0 and Ra1/Re1 respectively, the mirror ratio becomes small and the output currents of A0 and B0 are suppressed to a low value. This value can be adjusted by selecting the resistance values of Rf0 and Rf1. In an embodiment, the mirror ratio was reduced from 120 to 20, and the current was suppressed to one sixth of the normal current when the subscriber line was short-circuited. 2- to 4-wire converter Fig. 15 shows a block diagram of a 2- to 4-wire converter according to the present invention. Comparing it with the block diagram of Fig. 4 which explains the operation principle of the 2- to 4-wire converter, it will become more apparent the relationship of the converter circuit to the battery feed circuit. Fundamental requirement to the 2- to 4-wire converter is to transmit the 4-wire signal received at a terminal 4WR to a subscriber equipment Zo connected to the terminals A and B, and to transmit a signal sent from the subscriber equipment to an output terminal 4WS of the 4-wire line, and it should not turn around the input signal of 4WR to the output terminal 4WS. There is still another requirement for the 2- to 4-wire converter that above requirement should be satisfied over an operational frequency band of the equipment. Generally, at the input side of the battery feed circuit, the terminals A and B are terminated by a termination impedance Z (denoted 2 in the figure), but the load impedance Zo that includes the impedances of the subscriber equipment and the subscriber line varies by the variation of equipments and the length of the subscriber line, so it is difficult to perfectly terminate the subscriber line with the battery feed circuit. Moreover, the termination impedance Z has some frequency characteristics, so the above requirement becomes more difficult. In Fig. 15, portions encircled by chained lines 5 and 6 correspond respectively to the compensation network 5 and the voltage to current converter 6 in Fig. 4. The 4-wire signal received at a terminal 4WR is input to an inverting input terminal (-) of an operational amplifier OP2 via a resistor Rs1. The non-inverting input terminal (+) is grounded. The output of OP2 is fed back to the input by a resistor Rf5. The pnp transistor Qp is supplied its current from a constant current source 9. Between the output terminal of the constant current source 9 and ground are loaded the compensation network 5 composed of parallel impedance elements 7 and 8. The impedances of these impedance elements are respectively NZ and NZo which are respectively N times as high as those of the termination impedance Z and the load impedance Zo that is the impedance of the subscriber equipment including subscriber line. Therefore, the total impedance of the network is N times as high as the total impedance at the input side of the current feed circuit composed by Z and Zo. If let the parallel impedance of Z and Zo be B, the total impedance of the compensation network 5 beomes B·N. The input signal voltage V 4R at the terminal 4WR is multiplied by Rf5/Rs1 by the operational amplifier OP2, and fed to the base of a pnp transistor Qp. If a base emitter voltage V BE of the transistor Qp is neglected, a current corresponding to the output voltage of OP2 runs through the compensation network 5. So, i p the current running through the transistor Qp becomes where is a total impedance when Zo and Z are connected in parallel. Since the interval impedance of the constant current source 9 is infinitive, this current runs through one of third current mirror circuit pair A3. A3 has two output terminals, one output is fed to the current feed mirror circuit B0 and the other output is inverted its direction by another mirror circuit B3 and fed to A0. Therefore, from these current feed mirror circuits A0 and B0 are sent out currents having an amplitude corresponding to the input voltage and phase opposite to each other. So V AB , the voltage across the 2-wire line terminals A and B, becomes V AB = i p x M x B where M is the current ratio of A0 and B0, so V AB = (Rf5/Rs1)(V 4R /BN) x M x B = (Rf5/Rs1) x (M/N) x V 4R (3) Therefore, V AB the output voltage of the 2-wire signal becomes proportional to input voltage V 4R of the 4-wire signal. It is important in the above equation (3), that V AB is related neither to the load impedance Zo nor the termination impedance Z. This means the output voltage is neither related to Zo nor Z, therefore, the above mentioned frequency characteristics of Z and Zo are all eliminated. By choosing the value of M/N, Rf5 and Rs1, it is possible to adjust the gain of the circuit. In an embodiment the values of M and N have been chosen to respectively 120 and 100, and Rf5 was 5 k Ohms and Rs1 was 10 k Ohms. The value of N has another significance, as has been mentioned above, the impedances of NZ and NZo are N times as high as those of Z and Zo respectively. So, the resistances are N times as higher, and capacitances are 1/N times as smaller as those of Z and Zo. This means their size and current to deal with become smaller. It is preferable to miniaturize their size or fabricating them in a monolithic IC. The 2-wire signal applied to the terminals A and B from the load impedance Zo is fed to a third operational amplifier OP3 via capacitors C A , C B and resistors Rs2, Rs3. The inverting and non-inverting input voltages of OP3 are respectively fed back by a resistor Rf3 or divided by a resistor Rf2. Neglecting the reactive impedances of C A and C B , and choosing the resistances as Rs2 = Rs3, Rf2 = Rf3, gain k of OP3 becomes k = Rf3/Rs3 Let the input voltage of the 2-wire signal from the load Zo be V₂, its voltage between the terminals A and B are given as This voltage is fed to a third operational amplifier OP3 having a gain k, and led to a fourth operational amplifier OP4. The output of OP4 is fed back through an impedance networks 3 and 4 each having impedance of N'Z and N'Zo respectively. Namely, N' times as high as those of the termination impedance Z and Zo the load impedance of the 2-wire lines A and B. Therefore, V 4S the output voltage of the operational amplifier OP4 becomes As can be seen in the equation (4), the 4-wire output voltage V S4 is proportional to the 2-wire input voltage V₂. And since the equation (4) is related to neither Z nor Zo, the problem of the frequency characteristics has been also avoided. The 4-wire signal appeared at the terminals A and B is also fed to the operational amplifier OP3 and becomes a turn around signal. But as can be seen in the circuit of Fig. 15, the 4-wire input signal is also fed to the operational amplifier OP4 via a resistor R R , and its phase is reversed to that of the turn around signal. Therefore, by adjusting the resistance of R S and R R so as to be |V p3 |/R S = |V 4R |/R R it is possible to cancel the both signals. By such a manner, the turn around of the signals from 4-wire input to the 4-wire output has been avoided. Thus, all important features required to a 2- to 4-wire converter has been satisfied. In an embodiment the resistances of Rs1 was 10 k Ohms and Rf5 was 5 k Ohms, Rs2 and Rs3 were 200 k Ohms, Rf2 and Rf3 were 32 k Ohms, R S was 10 k Ohms, R R was 100 k Ohms, and capacitances of C A and C B were 0,1 µF. Some modification of the 2- to 4- wire converter of Fig. 15 are possible. Fig. 16 shows another embodiment of the circuit applicable for correcting the frequency characteristics of the 2- to 4-wire converter of Fig. 15. The circuit is applicable to replace a portion encircled by a chained line 150 in Fig. 15. Compared to that of Fig. 15, the circuit of Fig. 16 is provided with additional resistors Rf4 and Rs4 connected in series between the output terminal of the fourth operational amplifier OP4 and ground. And the impedance element 4 (N'Zo) is connected to the junction point of Rf4 and Rs4. The gain G is given as Since N'Zo and N'Z are very large compared to Rs4 and Rf4, the gain can be written as Therefore, it is possible to vary the gain G by varying the resistance of Rs4 and Rf4 without disturbing the frequency characteristics of the circuit. In an embodiment Rs4 and Rf4 were both chosen to be less than 1 k Ohms. Fig. 17 shows a circuit diagram of another embodiment of the voltage to current converter which can take the place of the portion encircled by chained lines 5 and 6 in the circuit of Fig. 15. In the circuit of Fig. 17, the emitter voltage of the transistor Qp is fed back to the inverting input terminal (-) of the operational amplifier OP2. And the input signal 4WR is fed to the non-inverting input terminal (+) via a resistor Rf6, and the non-inverting terminal is grounded through a resistor Rs1. Such circuit can suppress the variation of the characteristics due to variation of gain of the transistor Qp. So, more stable operation can be expected. It should be pointed out that the phase of the output of this circuit is reversed to that of Fig. 15, so in order to cancel the turn around voltage of 4-wire signal, it is necessary to reverse the input connection of OP3 in Fig. 15. The converted current from the input voltage V 4R by the circuit of Figs. 17 becomes So, like a similar manner as has been described with respect to Fig. 15, the output voltage of 2-wire signal becomes This equation corresponds to the equation (3). So the problem of frequency characteristics has been avoided. As can be seen the circuit may be modified in various manner, but the spirit of the invention is to compensate the frequency characteristics by a parallel impedance provided in the 4-wire signal side having N times higher than that of parallel impedance composed of the load and the terminal impedances at the 2-wire side. Supervision Circuit The supervision circuit to detect the condition of the subscriber line and send a signal corresponding to a state of the subscriber line such as on-hook, off-hook, grounded or contacted to voltage source is shown in Fig. 18. This circuit corresponds to a portion encircled with a chained line 500 in Fig. 7, which shows the entire circuit configuration of the SLIC. In Fig. 18, the transistors Q0, Q1 and the resistors Re0, Re1 belong respectively to the current feed mirror circuits A0 and B0 as shown in Fig. 5 or 8. They supply currents I A and I B to the subscriber lines A and B respectively. These currents are detected as a voltage across Re0 or Re1 by transistors Q11 or Q14 respectively. Since the emitters of these transistors are respectively connected to constant current sources CI1 and CI2, their input impedances become very high. Therefore, the currents I A and I B are detected without disturbance of the detectors. Since the bases of transistors Q12 and Q13 are connected to the emitter of Q11, their voltage become equal to that of the resistor Re0 which has a relatively low resistance (50 Ohms for example). This voltage flows a current in resistors Rc11 and Rc12. Similarly, the emitter voltage of a transistor Q15 of which base is connected to the emitter of Q14 becomes equal to that of the resistor Re1, and flows a current in a resistor Rc13. The current running through the resistor Rc13 runs into a mirror circuit M11 and its one output current is converted to a voltage by a resistor Rs12 and fed to a + side input terminal of a comparator CMP2. Another output current of the mirror circuit M11 runs through a resistor Rs13. In this resistor R13 also runs the current running through the resistor Rc11, therefor the voltage at the + input terminal of the comparator CMP3 corresponds to a sum of I A and I B . Such circuit configuration, therefore, corresponds to the adder circuit ADD in Fig. 7. Namely, the input circuit of the comparator CMP3 is composed of a wired OR connected adder circuit. The current running through a resistor Rs11 is converted to a voltage, then fed to a first comparator comparator CMP1 to be compared with a first threshold voltage Vth1. The one output current of the mirror circuit M11 is converted to a voltage by the resistor R12, and fed to the second comparator CMP2 to be compared with a second threshold voltage Vth2. And the voltage across the resistor Rs13 is compared with a third threshold voltage Vth3 by a third comparator CMP3. A circuit composed of a mirror circuit M12 and a transistor Q16 corresponds to the gate G1 in Fig. 7, and a circuit composed of a mirror circuit M13 and a transistor Q17 corresponds to the gate G2 of Fig. 7. The threshold voltages Vth1 and Vth2 are relatively low voltage, -1 volts for example. So, when the subscriber line is in off-hook state and the currents I A and I B flow, and even if the load resistance R L of the subscriber equipment is relatively high, namely the currents I A and I B are relatively small, the input voltage to CMP1 and CMP2 become larger than Vth1 and Vth2 respectively, then transistors Q16 and Q17 become OFF state. But if the sum current of I A and I B is smaller than the predetermined value corresponding to the third threshold value Vth3, the third comparator CMP3 does not send out signal. This prevents the SLIC form a misjudge caused by a small leakage current in the subscriber line with an off-hook state. And, if the voltage corresponding to the sum current of I A and I B exceeds the threshold voltage Vth3, the output current of CMP3 runs through the mirror currents M12 and M13. So, scan signals SCNA and SCNB are sent out simultaneously from M12 and M13, indicatting that the subscriber line is in an off-hook state. On the contrary, when the subscriber equipment is in on-hook state, I A and I B are very small. So, all of the input voltages to the comparators become lower than respective threshold voltages. Then the transistors Q16 and Q17 become ON state, and the output of CMP3 runs through the transistors Q16 and Q17. So, no current runs through M12 and M13. Therefore, no scan signal appears indicating that the subscriber line is in on-hook state. If the subscriber line is shortcircuited to ground by fault, I A becomes very large and I B becomes very small, but the sum current of I A and I B does not vary so much, and exceeds the threshold Vth3. So, the transistors Q16 becomes OFF and Q17 becomes ON state. Accordingly, the output of CMP3 runs through the mirror circuit M12, but the output of CMP3 runs through Q17 and does not run through M13. Therefore, SCNA signal is generated, but SCNB signal is not generated indicating that the subscriber line is grounded. It will be understood that if the subscriber line is shorted to V BB , I A becomes very small, and I B becomes very large. So, SCNB signal is generated, but SCNA is not generated, indicating that the subscriber line is short circuited to V BB . In such a manner, the situations of off-hook, on-hook or fault short to ground or V BB are discriminated. Further it is necessary to check the stability of the circuit against noise. If the longitudinal noise is superposed on the input current, it increases I A and decreases I B or vise versa. But, the total current of I A and I B are almost kept constant by the adder circuit, and stable output is provided from CMP3. Therefore, so long as the noise current is too high to decrease the input voltage of CMP1 or CMP2 to lower than Vth1 and Vth2, the operations of these comparators are secured. As mentioned before, Vth1 and Vth2 are chosen to a low voltage to assure such operations. In an embodiment, Vth1 and Vth2 were chosen to -1 volts, and Vth3 was chosen to -2 volts. The value of resistance, Rc11, Rc12 and Rc13 were 12 k Ohms, Rs11, Rs12 were 30 k Ohms and Rs13 and 20 k Ohms. The constant current sources and the threshold voltage supply sources are conventional ones in the art, and their current handling capacity may be very small (approximately 50 µA for example). So, further description is omitted. Ground Separation Generally, SLIC is operated using various kinds of voltage sources. For example the battery supply circuit is operated by a voltage source between ground G and -48 volts, and logic circuits are operated with a Vcc of +5 volt, V EE of -5 volt and ground E. In the battery feed circuit there are various noise such as the one induced in the subscriber lines, and a noise generated in battery source. Though the noise voltage is not so high for the battery source circuit, it is very high for a logic circuits, which is operated with a low voltage. So it is necessary to separate these voltage sources form each other in order to assure a stable operation of low voltage circuit, but signal must be transmitted between each other. For example, it is necessary to separate the battery source of the battery feed circuit from the voltage source of the logic circuit, but the signal must be transmitted between them. In other words, it is necessary to separate the ground G from the ground E while the circuit is connected to each other. The present invention intends to transfer a signal current between the systems having different voltage sources. And when a signal is sent from a system having a ground G (abbreviated as system G) to a system having a ground E (abbreviated as system E), for example, it feeds a current from system E to system G having equal amplitude but opposite direction to that of the signal current. By doing so, though signal current flows between the systems G and E, the total current between the two systems are always canceled by the current fed from the system E to G or vice versa. Therefore the total current between the two systems is always zero, this is equivalent that the systems G and E are separated from each other. Fig. 19 is a block diagram illustrating an operation of the ground separation circuit according to the present invention. The figure illustrates a case when a signal such as off-hook signal and the like is sent from a subscriber line through a battery feed circuit BFC is a logic circuit (not shown) via the ground separation circuit. The battery feed circuit BFC has a ground G and a voltage source V BB of -48 volt for example, and the logic circuit is operated by a voltage sources Vcc of +5 volt and V EE of -5 volt and has a ground E for example. The figure corresponds to the upper half of Fig. 6. Common terminal of the mirror circuit M1 is connected to a relatively low voltage source Vss (-5 volts for example) which is generated from V BB , so it belongs to the system G. A vertical chained line divides the system E and the system G. A signal current I₁, that may be a SCNA signal for example, is fed to the first mirror circuit M1. First and second output terminals of M1 are connected respectively to input terminals of second and third mirror circuits M2 and M3. The output terminal of M2 is connected to the voltage source V EE of the system E. If the input current I₁ runs into M1, the same amount of currents I₂, I₃, I₄ and I₅ flow in directions as shown by arrows in Fig. 19. If the direction of I₁ is reversed, directions of all these currents are reversed. So, the input signal current I₁ from the system G is transferred to the system E as a current I₅. But since the direction of the currents I₃ and I₄ are in opposite direction to each other, and the total current between the systems G and E are always zero. This is equivalent to that both systems are separated from each other. Like such manner, the signal current from the system G is transferred to the system E as a signal current I₅ running out from the third mirror circuit M3. The detector DET in Fig. 6 is composed of the mirror circuit M3, a transistor Q21, and resistors R21, R22. The signal current I₅ is converted to a voltage by the resistor R21, detected by a transistor Q21, and taken out from an output terminal OUT. Like a similar manner, other signals such as SCNB, 4WS can be transferred from the system G to the system E. In an embodiment R21 and R22 were respectively 50 k Ohms and 10 k Ohms. In the above description, signals from the system G to the system E have been explained. But as can be seen in the lower half of Fig. 6, the signals from the system E can be transferred to the system G in a similar manner. Such reverse transfer is applicable for example in a transfer of 4-wire signal to 2-wire signal from the terminal 4WR to terminals A and B in Fig. 7. Since such application will be apparent for the one skilled in the art, further description is omitted for the sake of simplicity. Moreover the mirror circuits have essentially a high impedance characteristics, and their currents are not disturbed by the variation of source voltages V BB , Vss, Vcc and V EE , so they separate the G and E systems further from the noise induced in the voltage sources. Fig. 20 illustrates a voltage supply system for a SLIC. A battery (48 volts for example) encircled by a chained line 210 is grounded at its positive terminal. This battery provides the subscriber line current I G via a battery feed circuit (not shown). Their ground and negative terminals are designated by G and V BB respectively. This system composes the system G and its load is designated by R G . Using the battery 210, a DC to DC converter CV generates a voltage Vcc (+5 volts for example) and supplies I E the current of the system E. Negative voltage side output of the DC to DC converter CV is grounded by a terminal E. Another voltage source V EE is provided by similar manner. The DC to DC converter generates a constant voltage without being influenced by noise. Importance of ground separation will be understood by following consideration. Fig. 21 illustrates a circumstance between the voltage sources. The system G is operated by a negative voltage source V BB , and the system E is operated by Vcc and V EE . There is a signal pass of current I between the two systems. The system G is grounded at a terminal G, and the system E is grounded at a terminal E. If the potentials of the points G and E are equal, there is no problem. But it is inevitable to avoid a noise induced, especially in the system G which has a long subscriber line, so there appears a voltage difference ΔV between the points G and E. It is equivalent to insert a generator ΔV between the points E and G. If the grounds of the system are not separated from each other, this voltage ΔV or current corresponding to it will appear in both systems. It is especially harmful for the system E which is operated by low voltage, and it courses malfunctions. But if the ground is separated, in other words, if there is no current flow between the points G and E, even though the voltage difference between the two ground terminals G and E appeared, the circuits of the system G (that is the battery feed circuit etc.) and the circuits of the system E (that is a logic circuit for example) operates normally so long as respective source voltage is kept constant. Such constant voltage is secured by the battery 210 and the DC to DC converter CV. As has been described above, and is shown in Fig. 21, the present invention provides another current pass between the system G and E (denoted by a broken line), and flows a current -I which has same amplitude but opposite direction to the signal current I. So the total current between the two systems are always kept to zero. This is equivalent that the points G and E are separated from each other. This is an inventive ground separation method. In prior art systems such ground separation have been done by using transformers. But it is impossible to fabricate the transformer in an IC circuit, and it is also impossible to transfer a DC signal between the systems. Recently, a photo coupler is beginning to be applied for such purpose, but it is still not difficult to fabricate a photo coupler in an IC. Total Circuit Configuration Fig. 22 shows a total circuit diagram of a subscriber line interface circuit according to the present invention. It will be easy to understand for the one in the art that the portions encircled by chained lines correspond respectively to the portion shown in Fig. 7. The details of the circuit have been described already with respect to each functional circuit. So, further description of the portion is omitted for the sake of simplicity. It should be pointed out that the ground separation circuits encircled with chained lines 601 and 602 correspond to upper half of the circuit of Fig. 6. These circuits all transmit the signal (SCNA and SCNB) from let to right in the figure. For the signal of 4WS the output signal of 4-wire circuit, however, such ground separation is unnecessary, because the output circuit 401 (corresponds to the system E) is separated from the battery feed circuit (corresponds to the system G) by the capacitors C A and C B . Therefore, it is unnecessary to worry about the DC voltage difference between the grounds. And as has been described before, the AC voltage difference (that is the longitudinal noise) is eliminated by the differential amplifier OP3 and does not appear in the output. A circuit corresponding to the lower half of Fig. 6 which transmits a signal from right to left in the figure is used for the input circuit of 4WR which is encircled by a chained line 402. Though it is not shown in the figure in order to avoid complexity, the circuit is inserted between the transistor Qp and the operational amplifier OP2. It will be easy for the one skilled in the art to introduce the ground separation circuit of Fig. 6 at this point. Further, it should be pointed out, that if the voltage of the constant current source 9 is supplied from Vcc, and the supply voltages to the mirror circuits A3 and B3 are replaced by V EE and Vcc respectively, it will be clear that these mirror circuits play parts of the lower half of the ground separation circuit of Fig. 6. The remaining parts of Fig. 22 are similar to those described before, so further description is omitted. The SLIC of the present invention has been designed so as to be fabricated in a monolithic IC, so it is very effective in reducing its size and cost. As has been described above, many modification of the circuit is possible, but they are all in the scope and spirit of the invention.

Claims: A balanced type subscriber line interface circuit (SLIC) having a battery feed circuit for supplying DC current for a 2-wire subscriber line composed of line A and line B respectively connected to terminals A and B, said battery feed circuit comprising: a pair of current feed mirror circuits (A0,B0) of which output terminals are respectively connected to said terminals A and B, for supplying said DC current; a pair of first voltage to current converting means (Rb0,Rb1) being respectively connected to said terminals A and B; a pair of first mirror circuits (A1,B1) of which input terminals are respectively connected to output terminals of said voltage to current converting means (Rb0,Rb1); and a pair of second mirror circuits (A2,B2); characterised by: a capacitor (C AB ) being connected between output terminals of said first mirror circuits (A1,B1); and a pair of resistors (Rc0,Rc1) of which one end is connected respectively to said capacitor (C AB ); the input terminals of said second mirror circuits (A2,B2) being connected respectively to output terminals of opposite side mirror circuits of said first mirror circuits (B1,A1) via said resistors (Rc0,Rc1); the output terminals of said second mirror circuits (A2,B2) being connected respectively to input terminals of opposite side mirror circuits of said current feed mirror circuits (B0,A0). A SLIC according to claim 1, wherein said current feed mirror circuits (A0, B0) respectively comprise : a voltage follower type first operational amplifier (OP0 or OP1), of which non-inverting input terminal (+) becomes the input terminal of said current feed mirror circuit, and is connected to the output terminal of said second mirror circuit (A2 or B2); a transistor (Q0 or Q1) of which collector is connected to one of said terminals A or B, base is connected to output terminal of said first operational amplifier (OP0 or OP1), and emitter is connected to invertinq input terminal (-) of said first operational amplifier (OP0 or OP1); and an input resistor (Ra0 or Ra1) connected between voltage source (V BB or ground) and said non-inverting terminal (+) of said first operation amplifier (OP0 or OP1). A SLIC according to claim 1, wherein said pair of first voltage to current converting means being respectively resistors (Rb0, Rb1). A SLIC according to claim 1, wherein said first voltage to current converting means has a function to limit a current running through it, and is composed. of resistors (r c and r e ) and transistors (Q5 and Q6). A SLIC according to claim 1, wherein the battery side common terminal of said second mirror circuit (A2) is connected to a voltage stabilized source (Vz). A SLIC according to claim 1, further comprising an overcurrent protection circuit, which comprises: a plurality of resistive means (83, 83') which are respectively switchable to be connected in parallel or not to said input resistors (Ra0, Ra1); a detecting means (81) for detecting a difference between the currents running through said input resistors (Ra0, Ra1); a comparing means (CMP) for comparing said difference of current with a predetermined threshold value (Vth); and a driving means (DV) for controlling the switching of said resistive means. A SLIC according to claim 6, wherein said resistive means (83, 83') respectively comprises: a switching transistor (Qr0 or Qr1) of which emitter is connected to a voltage source, base is connected to said driving means, and collector is connected to said non-inverting input terminal (+) of said first operational amplifier (OP0 or OP1) via a second input resistor (Rf0 or Rf1); and a resistor (Rg0 or Rg1) connected between the base of said switching transistor and voltage source (V BB or ground). A SLIC according to claim 6, wherein said detecting means (81) is composed of a resistor (Rd) and an absolute value detector (ABS), said absolute value detector comprises: a pair of pnp (Qg) and npn (Qf) transistors, of which emitters are connected to each other to receive an input signal, the collector of said npn transistor (Qf) becomes the output terminal of the absolute value; and a mirror circuit (CM1) of which input terminal is connected to the collector of said pnp transistor (Qg), and output terminal is connected to the collector of said npn transistor (Qf). A SLIC according to claim 1, further comprising a 2- to 4-wire converter for converting 2-wire signal to 4 wire signal or vice versa, said 2- to 4-wire converter comprises: a second voltage to current converting means (6) for converting voltage of the 4-wire input signal (4WR) to a current; a pair of third mirror circuits (A3, B3) for converting said current of input 4-wire signal to a pair of currents having opposite phases to each other and feeding them respectively to the input terminals of said current feed mirror circuits (A0, B0); a voltage detecting means (151) for detecting a voltage of 2-wire signal appearing between said terminals A and B; and a canceling means (152 for canceling a turn around 4-wire input signal in said output of said voltage detecting means (151) by adding an opposite phase 4-wire input signal to the output signal of said voltage detecting means, and providing an output signal corresponding to the 2-wire input signal. A SLIC according to claim 9, wherein said second voltage to current converting means (6) comprises: a second operational amplifier (OP2) to which converting input terminal is fed the 4-wire signal (4WR) via a series resistor (Rs1), non-inverting side input terminal is grounded, and output terminal is connected to the inverting terminal via a feedback register (Rf5); a transistor (Qp) for converting the output of said second operational amplifier to a current and feeding it to one of said third mirror circuit (A3); and a constant current source (9) feeding a current to said transistor (Qp). A SLIC according to claim 9, wherein said voltage detecting means (151) comprises: a third operational amplifier (OP3), of which non-inverting and inverting input terminals are connected respectively to said terminals A and B via said second or third series resistors (Rs2, Rs3) and capacitors (C A , C B ), between the non-inverting input terminal and ground are connected a second feedback resistor (Rf2), and between the output terminal and inverting input terminal is connected a third feedback resistor (Rf3). A SLIC according to claim 9, wherein said canceling means (152) is composed of sending resistor (R S ) and a receiving resistor (R R ) connected to each other in series, another end of said output resistor (R S ) is connected to the output terminal of said third operational amplifier (OP3), another end of said receiving resistor (R R ) is connected to the input terminal of the 4-wire signal (4WR), and the connection point of these resistors (R S and R R ) provides the output voltage corresponding to the 2-wire input signal, further, the resistance of these resistors being determined by |V p3 |/R S = |V 4R |/R R where |V p3 | and |V 4R | are the output voltage of said voltage detection means (151), and the input voltage of 4-wire signal respectively. A SLIC according to claim 9, further comprising a first compensation network (5) for compensating a frequency characteristics of a signal sent from 4-wire input (4WR) to 2-wire input, said first compensation network composed of a parallel impedance elements of respectively N times as high as those of the load impedance (Zo) of the 2-wire subscriber line at the terminals A and B, and the termination impedance (Z) of the battery feed circuit, where N is a constant. A SLIC according to claim 13, said first compensation network (5) is provided between output terminal of said constant current source (9) and ground. A SLIC according to claim 9, further comprising a second compensation network (150) for compensatting a frequency characteristics of a signal sent from 2-wire input to 4-wire output (4WS), said second compensation network (150) comprises: a fourth operational amplifier (OP4) of which non-inverting input terminal is fed with output signal of said canceling means (152); a first impedance element (3) having an impedance N' times as high as that of said terminal impedance (Z), said first impedance element is connected between the inverting input terminal of said fourth operational amplifier (OP4) and ground; and a second impedance element (4) having an impedance N' times as high as that of said load impedance (Zo), said second impedance element is connected between the output terminal and the inverting input terminal of said fourth operational amplifier (OP4), where N' is a constant. A SLIC according to claim 15, wherein said second compensation network further comprises: a fourth feedback resistor (Rf4) connected between the output terminal of the fourth operational amplifier (OP4) and said second impedance element (4); and a fourth series resistor (Rs4) connected between the ground and the junction point of the second impedance element (4) and the fourth feedback resistor (Rf4). A SLIC according to claim 1, further comprising a supervision circuit for generating signals corresponding to the state of the subscriber line that it is on-hook, off-hook, short-circuited to ground or to a voltage source, said supervision circuit comprises: first and second current detecting means for respectively detecting the currents running through each of said 2-wire lines (I A , I B ); first and second comparing circuits (CMP1, CMP2) for comparing the values detected by said first and second current detecting means respectively with first and second threshold values (Vth1, Vth2); a third comparing circuit (CMP3) for comparing a sum of the values detected by said first and second current detecting means with a third threshold value (Vth3); and first and second AND gates (G1, G2) for respectively providing SCNA and SCNB signals according to respective inputs, the inputs for the first gate (G1) being the outputs of the first and third comparing circuits and the inputs for the second gate (G2) being the outputs of the second and third comparing circuits. A SLIC according to claim 17, wherein said first and second current detecting means being respectively a resistors (Re0 or Re1) inserted in the current pass of the current feeding mirror circuit (A0 or B0). A SLIC according to claim 17, wherein an input circuit for said third comparing circuit (CMP3) comprises a wired OR circuit. A SLIC according to claim 1, further comprising a ground separation circuit for transferring a signal between a systems having different grounds, said ground separation circuit sends out a signal current along a current pass from one system (system G) to other system having a different ground (system E), at the same time said ground separation circuit feeds a current having a same amplitude but having opposite phase to that of the signal current along another current pass between the two systems. A SLIC according to claim 20, wherein said ground separation circuit comprises: a sending mirror circuit (M1) an input terminal thereof is fed with a signal current to be sent out, and an output terminal thereof sends out a signal current to a system having different ground (system E); and a compensating mirror circuit (M2) an input terminal thereof is fed with an output current from another output terminal of said sending mirror circuit (M1), and an output terminal thereof sends out a current having a same amplitude but opposite phase to that of the signal current to a system having different ground (system E). A SLIC according to claim 20, further comprising a third current detecting means (DET) for detecting the signal current sent from said one system (system G) and providing an output signal corresponding to the signal current. A SLIC according to claim 17, further comprising a ground separation circuits (600) provided to each of the output terminals of said first and second gates (G1, G2), each of said separation circuits comprises: a sending mirror circuit (M1 or M1') an input terminal thereof is fed with a signal current to be sent out, and an output terminal thereof sends out a scan signal (SCNA or SCNB); and a compensating mirror circuit (M2 or M2') an input terminal thereof is fed with an output current from another output terminal of said sending mirror circuit (M1 or M1'), and an output terminal thereof sends out a current having a same amplitude but opposite phase to that of the signal current sent out from said sending mirror circuit (M1 or M1').

Inventors: AYANO, MITSUTOSHI, IKETANI, YOZO, SHIBUYA, KIYOSHI, TAKATO, KENJI, TOJO, TOSHIRO
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: H04M19
IPC codes: H04M19, H04Q1