BATTERY FEED CIRCUIT FOR SUBSCRIBER LINE

Abstract: A battery feed circuit for a subscriber line including first and second feed circuit parts (B a ,B b ) each supplying a constant current to a corresponding subscriber line (A,B). The first and second feed circuit parts (B a ,B b ) are provided with a common mode noise suppressor and/or a power source noise suppressor. The common mode noise suppres­ sor includes first and second reference voltage output circuits (RV₁,RV₂) and an intermediate voltage output circuit (IV) to produce the voltage between the first and second subscriber lines (A,B) and superpose the same onto the thus output first and second reference voltages. The power source noise suppressor functions to mutually cancel the outputs, concerning the power source noise voltage, at each output of the first and second feed circuit parts (B a ,Bbγ).

Description: Background of the invention (1) Field of the invention The present invention relates to a battery feed circuit, more particularly, to a battery feed circuit for use as a constant-current battery feed circuit for subscriber lines in a switching system. (2) Description of the related art As well known, battery feed circuits play an important role in feeding a constant DC current, via subscriber lines, to telephone sets, data terminals, and other subscriber units. Battery feed circuits are thus some of the most important parts in a switching system. Recently, the need has been growing for switching network featuring increased quality of transmission signals, decreased power consumption, reduced systems costs, and so on. The need is being met by improvements in hardware, including battery feed circuits. There are several types of battery feed circuits, e.g. a "constant-resistor" type and a "constant-current"type. Constant-resistor types are not very practical from the viewpoint of power consumption. Constant-current types feature lower power consumption, but suffer from the problem of instability and undesired clipping of transmission signals due to common mode noise induced along the subscriber lines. Constant current type circuits can have an additional drawback of amplifier saturation which also results in the signal voltage being clipped WO-A1-84/01250 shows a battery feed circuit intended to prevent the amplifers from becoming saturated when out of phase signals are transmitted. An object of the present invention is to provide an improved battery feed circuit installed in a line circuit for a subscriber line in a switching system, which will work to feed DC current as usual and further suppress noise in the transmission signals. According to the invention, there is provided a battery feed circuit for a subscriber line operative to feed constant currents to a pair of a first subscriber line (A) and a second subscriber line (B) via a first feed circuit part (B a ) and a second feed circuit part (B b ), respectively, the battery feed circuit comprising first and second reference voltage output circuits (RV₁,RV₀) operable to produce first and second reference voltages by dividing the voltage between a ground level (GD) voltage and a power source level voltage (-V BB ) and to apply the first and second reference voltages so obtained to the first (B a ) and second (B b ) feed circuit parts, respectively, for activating same, characterised by: an intermediate voltage output circuit (IV) operable to produce a voltage intermediate those of the first and second subscriber lines (A,B) and to apply the intermediate voltage directly to an internode (M2) between the first and second reference voltage output circuits whereby the intermediate voltage is superposed onto each of the first and second reference voltages simultaneously and common mode noise is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS The above object and features of the present invention will be more apparent from the following description of the preferred embodiments with reference to the accompanying drawings, wherein: Fig. 1 is a general circuit diagram of an example of a conventional subscriber line in, for example, a digital switching system; Fig. 2(a) is a circuit diagram of a conventional constant-resistor type battery feed circuit; Fig. 2(b) is a graph depicting DC voltage-current characteristics of the subscriber lines A and B; Fig. 3(a) is a circuit diagram of a conventional constant-current type battery feed circuit; Fig. 3(b) is a graph depicting DC voltage-current characteristics of the subscriber lines A and B; Fig. 4 is more detailed circuit diagram of the battery feed circuit shown in Fig. 3(a); Fig. 5 is a circuit diagram of a battery feed circuit for a subscriber line according to a first embodiment of the present invention; Fig. 6 is a circuit diagram of a battery feed circuit for a subscriber line according to a second embodiment of the present invention; Fig. 7 is a circuit diagram of a battery feed circuit for a subscriber line according to a third embodiment of the present invention; Fig. 8 is a circuit diagram used for explaining a principle of a fourth embodiment; Fig. 9 depicts schematic diagrams of voltages appearing at major portions in Fig. 8; Fig. 10 is a circuit diagram of a battery feed circuit according to the fourth embodiment of the present invention; Fig. 11 depicts schematic diagrams voltages appearing at major portions in Fig. 10; Fig. 12 is a circuit diagram of a battery feed circuit according to a fifth embodiment of the present invention; and Fig. 13 is a circuit diagram of a battery feed circuit according to a sixth embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments of the present invention, the prior art and problems therein will be described with reference to the related figures. Figure 1 is a general circuit diagram of an example of a conventional subscriber line in, for example, a digital switching system. In Fig. 1, reference characters "BF" represents a battery feed circuit to which the present invention specifically refers, "O" a high-voltage protector (in the drawings, a primary protector O(I) and a secondary protector O(II) are illustrated) used for protecting the line circuit LC from a high voltage surge such as lightning, and "RG" a ringing circuit operative to send ringing signals (given from a continuous ringer CR) to a subscriber unit U (in the drawing, U is a telephone set) connected, via subscriber lines A and B, with the circuit LC. The transmission signal to and from the subscriber unit U is transferred by way of a hybrid transformer H for performing conversion between a two-wire line provided with a DC-cut capacitor C and a four-wire line provided with a balancing network BN. The hybrid transformer H is connected, via a codec (coder-decoder) CD comprised of both an analog/digital (A/D) converter and a digital/analog (D/A) converter, to a digital switching network NW. The codec CD works to effect conversions between a usual voice signal and pulse code modulation (PCM) signal. The continuous ringer CR cooperates with a scanner SCN. The scanner SCN is also connected with a supervisory circuit S. Reference character T denotes a test circuit connected with test configuration lines TST. As seen from Fig. 1, the battery feed circuit BF is connected with the subscriber lines A and B and supplies DC current thereto during an off-hook state. During this state, the battery feed circuit BF provides a high impedance so as to prevent attenuation of the AC transmission signals, e.g., voice signals. Figure 2(a) is a circuit diagram of a conventional constant-resistor type feed circuit, and Fig. 2(b) is a graph depicting DC voltage-current characteristics of the subscriber line A and B. In Fig. 2(a) the subscriber line A and a power source of -V BB are provided with the series-connected resistor R₁ and inductance L₁ therebetween, while the subscriber line B and a ground GD are provided with the series-connected resistor R₂ and inductance L₂ therebetween. The members R₁ and L₁ form a battery feed circuit B a , and the members R₂ and L₂ form a battery feed circuit B b . The inductance L₁ and L₂ represent respective coils for feeding DC current. Their resistance components are represented by the resistor R₁ and R₂, respectively. Since each of the coils L₁ and L₂ provides a sufficiently large inductance for frequencies higher than 300 Hz of the voice signals, a high impedance state can be achieved. The coils exhibit a low impedance against a DC current, defined by the resistors R₁ and R₂. Thus, the battery feeding current I is stabilized, such as shown in Fig. 2(b), wherein line b represents a voltage-current characteristic regarding the subscriber line B and line A represents the same regarding the subscriber line A. The intermediate voltage between the subscriber lines A and B is held at -V BB /2 during the off-hook state of the subscriber unit U (telephone set), wherein a closed loop is formed by way of the lines A and B. According to the constant-resistor type battery feed circuit shown in Fig. 2(a), a DC current of 100 mA or more is necessarily given to the subscriber lines A and B, especially when the lines are short in length. Therefore, a battery feed circuit of such type is not economical due to large size coils. Also, there is a problem of heat generated by the large power. Figure 3(a) is a circuit diagram of a conventional constant-current type battery feed circuit, and Fig. 3(b) is a graph depicting DC voltage-current characteristics of the subscriber lines A and B. The constant-current type battery feed circuit having the parts B a and B b represents an improvement over the battery feed circuit of Fig. 2(a) in power consumption. In Fig. 3(a), the line A and line B side feed circuit parts B a and B b are basically constructed by pairs of resistors (R) and constant-current sources (CS), i.e., (R a , CS a ) and (R b , CS b ), respectively. The resistor (R) and the constant-current source (CS) are connected in parallel. According to the constant-current type battery feed circuit shown in Fig. 3(a), since the related battery feed circuit has a high impedance, voice signals are attenuated slightly thereby. Further, a constant DC current can always be ensured regardless of the length of the subscriber liner A and B. Therefore, the intended low power consumption can be attained. However, as previously mentioned, undesired clipping of the transmission signals takes place. This will be explained with reference primarily to the voltage-current characteristics shown in Fig. 3(b). As understood from Fig. 3(b), during the off-hook state of the related subscriber unit (telephone), the intermediate voltage between the subscriber lines A and B cannot always be set at -V BB /2 due to an imbalance between the constant currents I sa and I sb and/or an imbalance between the resistors R of the line A and line B side feed circuit parts B a and B b . The voltages of the subscriber lines A and B should be within the range of -V BB through -V BB /2 and the range of -V BB /2 through the ground level. However, in the worst case, the voltages of the lines A and B shift too far to -V BB and the ground level, respectively. Such extreme shifts in the line voltage cause saturation in the related transistors and thereby cause the problem of clipping in the transmission signals. In the above case, even if two constant-current sources have exactly the same voltage-current characteristics, undesired clipping is very liable to take place due to external noise on the lines A and B, in the form of common mode noise. This is because, when such common mode noise is given to the lines A and B, the constant currents I sa and I sb also are unbalanced with each other. Therefore, the intermediate voltage concerned deviates from the normal voltage of -V BB /2. Figure 4 is more detailed circuit diagram of the battery feed circuit shown in Fig. 3(a). In Fig. 4, reference characters A₀ and A₁ denote operational amplifers (hereinafter abbreviated as OP-AMP) and T r0 and T r1 denote transistors. Resistors R a0 , R b , and R a1 divide the voltage across the ground GD (0V) and the power source level (-V BB ). The voltages across the resistors R a0 and R a1 are applied, as respective reference voltages, to the non-inverting input terminals (+) of the OP-AMP's A₀ and A₁, respectively. Therefrom, the OP-AMP's operate so as to make the voltages across the resistors R e0 and R e1 equal, respectively, to the voltages across the resistors R a0 and R a1 . In this case, the constant currents I sa and I sb flow through the respective resistors R e0 and R e1 , which constant currents are defined as follows: I sb ={V BB × R a0 R a0 +R b +R a1 }/R e0 I sa ={V BB × R a1 R a0 +R b +R a1 }/R e1 Assuming here that the current amplification factor h FE satisfies h FE >>1, the constant current I sb along the resistor R e0 becomes substantially the same as the current flowing across the transistor T r0 . Similarly, the current I sa becomes the current flowing across the transistor T r1 . If the equations I sb =I sa and R₀=R₁ stand strictly, the intermediate voltage between the subscriber lines A and B should theoretically assume the voltage -V BB /2. In actuality, however, the constant currents I sa and I sb cannot be exactly the same. Accordingly, the voltages of the subscriber lines A and B shift too far to -V BB or the ground level of 0V, as mentioned previously. In order to prevent such a shift in voltage toward -V BB and 0V, resistors R₀ and R₁ are added to the feed circuit parts B b and B a , respectively. The resistors R₀ and R₁ work to compensate for the imbalance between the currents I sb and I sa . The resistors R₀ and R₁ preferably have neither too high nor too low resistance values. If the resistance values of the resistors R₀ and R₁ are too low, the constant-current characteristics are deteriorated and the transmission signal is attenuated thereby. Conversely, if they are too high, the constant-current characteristics are kept normal, but the intermediate voltage between the subscriber lines A and B is very liable to deviate from -V BB /2. Further, when common mode noise is given to the subscriber lines A and B, common mode noise voltages appear on these lines A and B. The voltage amplitudes are proportional to the impedances of the feed circuit parts B b and B a . The noise voltages destroy the balance between the two feed circuit parts. Thus, the aforesaid clipping takes place in the transmission signal or, in some case, the transmission signal is stopped from propagating. In view of the above-mentioned problems, the present invention proposes a battery feed circuit which features, first, low power consumption with normal constant-current characteristics, second, superior noise suppression against common mode noise and/or power source noise, and, third, prevention of undesired clipping in the transmission signals through the subscriber lines A and B. Figure 5 is a circuit diagram of a battery feed circuit for a subscriber line according to a first embodiment of the present invention. The differences here relative to the prior art construction shown in Fig. 4 are as follows. First, an intermediate voltage output circut IV is newly employed. Second, a first reference voltage output circuit RV₀ and a second reference voltage output circuit RV₁ are newly employed, the related resistors R b0 and R b1 being equivalent to two identical resistors obtained by dividing R b shown in Fig. 4. The resistors R a0 , R b0 and R a1 , R b1 function to determine the first and second reference voltages obtained by dividing the voltage between the ground level (GD) and the power source level (-V BB ), in which the following equations are satisfied: R a0 =R a1 and R b0 =R b1 . The intermediate voltage output circuit IV operates to produce an intermediate voltage between the first and second subscriber lines A and B. The output circuit IV is comprised of resistors R c0 and R c1 (R c0 =R c1 ) connected in series and an amplifier A₂. The input of the amplifier A₂ is connected to an intermediate connecting point M₁ of the resistors R c0 and R c1 , while the output of the amplifier A₂ is connected to an intermediate connecting point M₂ of the resistors R b0 and R b1 . The amplifier A₂ is constructed as a voltage follower of gain 1 and is operative to feed the intermediate voltage at the point M₁ back to the intermediate point M₂. In this case, usual transmission signals do not appear at the point M₁ since they form so-called differential mode signals between the lines A and B. Only the common mode transmission signals appear at the point M₁. The common mode signals at the output of the amplifier A₂ are applied to each non-inverting input terminal of the OP-AMP's A₀ and A₁. Due to the inherent functions of the OP-AMP's A₀ and A₁, the common mode signals appear at each inverting input terminal (-) of the OP-AMP's A₀ and A₁. The common mode signals are then inverted by the transistors T r0 and T r1 , whereby the subscriber lines A and B are supplied with signals of opposite phases relative to the aforesaid common mode signals. Accordingly, the impedances of the first feed circuit part B b and second feed circuit part B a , in terms of the common mode signals, are reduced from infinity (∞) to (R a0 +R b0 )×R e0 R a0 and (R a1 +R b1 )×R e1 R a1 , respectively. Due to the reduction of the impedances, the common mode noise is cancelled by the feed circuit parts B b and B a . Therefore, the noise suppression capability for common mode noise is improved by the battery feed circuit of Fig. 5. Further, deviation of the intermediate voltage between the lines A and B due to an imbalance between the feed circuit parts B b and B a can be detected in the form of common mode deviation between the lines A and B. Thus, the intermediate voltage can be stabilized at level close to -V BB /2 by the reduced impedances mentioned above. This makes it possible to eliminate the resistors R₀ and R₁ from the prior art circuit of Fig. 4. As a result, a battery feed circuit for a subscriber line can be realized which features low impedance with respect to common mode signals, while maintaining the inherent constant-current source structure and low power consumption. The first embodiment can be further improved. As seen from Fig. 5, the constant current concerned is determined merely by dividing the voltage between the ground (GD) level and the power source level -V BB . Accordingly, if a noise voltage V N is induced in the power source, the non-inverting input terminals of the OP-AMP's A₁ and A₀ are, respectively, supplied with the voltages V+₁ and V+₀, which are expressed as follows: V+₁= R a0 +R b0 +R b1 R a0 +R b0 +R b1 +R a1 ×V N V+₀= R a0 R a0 +R b0 +R b1 +R a1 ×V N Consequently, the subscriber lines A and B are supplied with noise signals different from each other, which results in differential noise in the subscriber unit U. Figure 6 is a circuit diagram of a battery feed circuit for a subscriber line according to a second embodiment of the present invention. The second embodiment improves the first embodiment with regard to the differential noise. The second embodiment differs from the construction of Fig. 5 in the provision of the following: a resistor R f0 connected between the emitter of the transistor T r0 and the inverting input terminal (-) of the OP-AMP A₀; a resistor R s0 and a DC-cut capacitor Co connected in series between the power source (-V BB ) and the inverting input terminal (-) of the OP-AMP A₀; a resistor R f1 connected between the emitter of the transistor T r1 and the inverting input terminal (-) of the OP-AMP A₁; and a resistor R s1 and a DC-cut capacitor C₁ connected in series between the ground GD and the inverting input terminal (-) of the OP-AMP A₁. In Fig. 6, the values of the resistors R f0 , R s0 , R f1 , and R s1 are determined to satisfy the following equations: R f0 R s0 = R a0 R a0 +2×R b0 and R f1 R s1 = R a1 R a1 +2×R b1 Also, the values of the capacitors C₀ and C₁ are selected to be negligible impedances in terms of AC. Under there conditions, the non-inverting input terminal (+) of the OP-AMP A₁ is supplied with a voltage V+₁, due to the noise voltage V N , expressed as: V+₁= R a0 +R b0 +R b1 R a0 +R b0 +R b1 +R a1 ×V N Here, if the resistance values are predetermined to be as R a0 =R a1 and R b0 =R b1 , the above noise voltage V+₁ can be rewritten as: V+₁ = R s1 R f1 +R s1 V N On the other hand, the voltage V₋₁ at the non-inverting input terminal of the OP-AMP A₁ assumes the following value due to the noise voltage V N : V₋₁= R s1 R f1 +R s1 ×V N As a result, an equation V+₁ =V₋₁ stands, which means that no signal effected by power source noise appears at the output terminal of the OP-AMP A₁. This holds true for the OP-AMP A₀ as well. Therefore, V+₀=V₋₀ stands. This means that no signal effected by power source noise appears at the output terminal thereof. As understood from the above, the battery feed circuit according to the second embodiment is advantageous in that, even if power source noise is induced, differential noise resulting therefrom does not appear in the subscriber lines A and B. The second embodiment can be further improved, however. In Fig. 6, the OP-AMP's A₀ and A₁ continuously attempt to feed the constant currents even when the subscriber unit U is in an on-hook state, i.e., even when no loop current is needed. This is because the non-inverting input terminals of the OP-AMP's A₀ and A₁ are continuously supplied with voltages divided by the related resistors between the ground GD and the power source (-V BB ). Therefore, the transistors T r0 and T r1 do not pass any current and accordingly, no voltage drop is induced across the resistors R e0 and R e1 . Accordingly, the 0V level and -V BB level are given to the inverting input terminals (-) of the OP-AMP's A₀ and A₁, respectively, which levels are not equal to the respective voltage V+₀ and V+₁ appearing at the non-inverting input terminals. The OP-AMP's thus function as comparators, the OP-AMP A₀ reducing its output voltage to the power source level -V BB . Thus, a constant current, the same as that flowing during an off-hook state, flows along the path of resistor R e0 →transistor T r0 →OP-AMP A₀. Alternatively, the OP-AMP A₀ pulls as large a current, at its output terminal, as possible. The above fact also applies to the OP-AMP A₁. Therefore, its output voltage increases as high as possible. Thus, a constant current, identical to that mentioned above, flows. Alternatively, it pulls a current at its output terminal as large as possible. Figure 7 is a circuit diagram of a battery feed circuit for a subscriber line according to a third embodiment of the present invention. The problem of the second embodiment of the uneconomical flow of current is improved. The difference relative to the construction of Fig. 6 lies in the provision of a resistor R d0 for limiting current connected between the output terminal of the OP-AMP A₀ and the base of the transistor T r0 , a zener diode ZD₀ connected between the ground GD and the intermediate connecting point M₂, a resistor R d1 for limiting current connected between the output terminal of the OP-AMP A₁ and the base of the transistor T r1 , a zener diode ZD₁ connected between the power source (-V BB ) and the intermediate connecting point M₂, and a resistor R g connected between the point M₂ and the output terminal of the OP-AMP A₂. The current during the on-hook state can be limited by the resistors R d0 and R d1 . Accordingly, a low power consumption is attained. The resistors are selected to have a maximum resistance value in a range wherein sufficient base currents for the transistors T r0 and T r1 can be maintained during an off-hook state. The resistor R g and the zener diodes ZD₀ and ZD₁ are useful for emergencies such as ground-faults along the subscriber lines A and/or B or a faulty contact. For example, when a ground-fault takes place along the subscriber line A while the subscriber line B is left open, the voltage at the intermediate connecting point M₁ becomes the ground (GD) level and the output of the OP-AMP A₂ changes to the ground (GD) level. As a result, the voltage level at the intermediate connecting point M₁ relative to the potential -V BB increases to twice the voltage normally obtained, and the constant current given from the feed circuit part B a increases two times the normal predetermined current. Accordingly, the power consumption increases four times the power normally consumed, necessitating reconsideration of heat design. The resistor R g and the zener diodes ZD₀ and ZD₁ are useful for clamping a current flowing through the transistor T r1 to a certain magnitude, e.g., 1.4 times, thereby limiting the related increare in power consumption to two times (1.4×1.4=1.96) at most. Incidentally, the amplifier A₂ in each embodiment is not limited to a gain of 1 and may have a gain of over 1. If the gain G is higher than 1, the impedance in terms of the common mode noise induced in the subscriber lines A and B can advantageously be reduced by 1/G. On the other hand, however, such an amplifier A₂ of a gain G (G>1) cannot simply be fabricated, because a usual voltage follower cannot be used. Instead, a special non-inversion circuit is needed. Referring again to the second and third embodiments shown in Figs. 6 and 7, these embodiments not only suppress undesired common mode noise, but also deleterious power source noise (V N ). There is, however, room for improvement in these embodiments in the sense of cost. Figure 8 is a circuit diagram for explaining the principle of a fourth embodiment. The construction of Fig. 8 is substantially the same as Figs. 6 and 7. The intermediate voltage output circuit IV is illustrated simply as a circuit block. The resistors R b0 and R b1 (including the zener diodes ZD₀ and ZD₁) are replaced by a single resistor having a resistance value of R₂. Characters R₁ through R₇, R₈₁, and R₈₂ in parentheses denote respective resistance values. Furthermore, characters P a and P b (in Fig. 10), and V₁ through V₄ are introduced for easy comprehension in relation to Fig. 9. Figure 9 depicts schematic diagrams of voltages appearing at major portions in Fig. 8. In Fig. 9, first, the voltage V₃ is preset as 0V and the voltage V₄ is preset as a level equal to the power source noise voltage V N in order that the condition be satisfied that the voltage across the resistor of R₄ and the voltage across the resistor of R₅ can be made 0V. This is required for the following reason. The voltages appearing at the points P a and P b are defined as V₁ and V₂, expressed below: V₁={R1/(R1+R2+R3)}×V N V₂={(R1+R2)/(R1+R2+R3)}×V N Thus, the voltages V₁ and V₂ are applied to the non-inverting input terminals (+) of the OP-AMP's A₀ and A₁, respectively. Then, identical voltages (V₁ and V₂) appear at their inverting input terminals (-) due to the well-known phenomenon of "imaginary short-circuits" inherent to an operational amplifier. Therefore, noise currents i n1 and i n2 caused by the power source noise voltage V N flow through the resistors of R4 and R5, respectively. The noise currents i n1 and i n2 are expressed as: Where, "0" denotes the ground (GD) level. The noise currents i n1 and i n2 cannot flow if no voltage is generated across each of the resistors (R4 and R5) by the power source noise voltage V N . This results in the aforesaid condition and, at the same time, is another way for proving the power source noise can be suppressed. Returning again to Fig. 9, the voltages V₁ and V₂ at the non-inverting input terminals (+ of the OP-AMP's A₀ and A₁ appear also at the inverting input terminals (-). This is due to the aforesaid "imaginary short-circuits" and is schematically indicated with broken line arrows in Fig. 9, i.e., V₁→V₁ and V₂→V₂. Under this condition, the voltage diagrams shown by Fig. 9 stand if the following proportional equations are satisfied. R1:(R2+R3)=R6:R81 R3:(R2+R1)=R7:R82 Conversely, if the resistance values R6, R81, R7, and R82 are determined as to satisfy the above-recited two proportional equations, the related equations V₃=0 and V₄=V N can be satisfied. Accordingly, since no voltage drops are generated across each of the resistors of R4 and R5, there are no noise currents i n1 and i n2 . This means the power source noise can no longer travel to the subscriber unit. As apparent from Figs. 6 and 7, the number of the resistors and capacitors is relatively large compared with the first embodiment of Fig. 5. From this viewpoint, the battery feed circuit according to the second and third embodiments can be improved in view of cost. Even a single reduction in the resistors and capacitors is beneficial for a switching system from the economical viewpoint, because battery feed circuits are allotted for every subscriber unit. If k subscribers are accommodated in the related switching system, up to k resistors and k capacitors, can be saved. Accordingly, the manufacturing cost may be reduced. Figure 10 is a circuit diagram of a battery feed circuit according to the fourth embodiment of the present invention. In the fourth embodiment, the inverting input terminals (-) of the OP-AMP's A₀ and A₁ are connected to each other via both a resistor R s (resistance value of R8) and a capacitor C₀₁ connected in series. As clear from Fig. 10, two electrical components are reduced compared with Fig. 8. Specifically, the four electrical components R s0 (R81), R s1 (R82), C₀, and C₁ comprising the battery feed circuit of Fig. 8 are replaced in Fig. 10 by two electrical components R s (R8) and C₀₁. This replacement may be made due to the following: The voltages V₁ and V₂, appearing at the points P a and P b in Fig. 10, are expressed by the previously recited equations, i.e.: V₁={R1/(R1+R2+R3)} xV N V₂={(R1 +R2)/(R1+R2+R3)} xV N Voltages identical to V₁ and V₂ also appear, due to "imaginary short-circuits", at the inverting input terminals of the OP-AMP's A₀ and A₁. This being so, the aforesaid noise currents i n1 and i n2 flow through resistors of R4 and R5, respectively. In order to make there noise currents substantially zero, the voltages V₃ and V₄ are, preset respectively, to be 0V and V N , as mentioned before. Figure 11 depicts schematic diagrams of voltages appearing at major portions in Fig. 10. The method of reading the diagrams is similar to that of Fig. 9. The difference with Fig. 9 is that the voltages V₁ and V₂ (refer to GD - V N in upper row of fig. 9) are not used independently, as depicted in the middle row (V₃ - V N ) and in lower row (V₃ - V₄) of Fig. 9, but are commonly used, as depicted by V₃ - V₄ in the lower row of Fig. 11. According to the voltage diagrams of Fig. 11, the intended conditions, i.e., V₃ = 0(GD) and V₄ = V N , can be satisfied by the following proportional equations: R1 : R2 = R6 : R8 R3 : R2 = R7 : R8 These are rewritten as follows: R1 : R2 : R3 = R6 : R8 : R7 or R1/R6 = R2/R8 = R3/R7. If the resistance values R6, R7 and R8 are determined to satisfy either one of these two proportional equations, the conditions V₃ = 0 and V₄ = V N can be satisfied. Therefore, the noise currents i n1 and i n2 can be suppressed. Figure 12 is a circuit diagram of a battery feed circuit according to a fifth embodiment of the present invention. The battery feed circuit of the fifth embodiment an improved version of the second embodiment and features a reduced number of electrical components. Figure 13 is a circuit diagram of a battery feed circuit according to a sixth embodiment of the present invention. This corresponds to a similar improved version of the third embodiment. As explained above in detail, the battery feed circuit according to the present invention is superior in view of one or more of power consumption, suppression of common mode noise, and suppression of power source noise.

Claims: A battery feed circuit for a subscriber line operative to feed constant currents to a pair of a first subscriber line (A) and a second subscriber line (B) via a first feed circuit part (B a ) and a second feed circuit part (B b ), respectively, the battery feed circuit comprising first and second reference voltage output circuits (RV₁,RV₀) operable to produce first and second reference voltages by dividing the voltage between a ground level (GD) voltage and a power source level voltage (-V BB ) and to apply the first and second reference voltages so obtained to the first (B a ) and second (B b ) feed circuit parts, respectively, for activating same, characterized by: an intermediate voltage output circuit (IV) operable to produce a voltage intermediate those of the first and second subscriber lines (A,B) and to apply the intermediate voltage directly to an internode (M2) between the first and second reference voltage output circuits whereby the intermediate voltage is superposed onto each of the first and second reference voltages simultaneously and common mode noise is suppressed. A battery feed circuit as set forth in claim 1, wherein: the first reference voltage output circuit (RV₁) includes a first and a second resistor (R a1 ,R b1 ) connected in series; the second reference voltage output circuit (RV₀) includes a third and a fourth resistor (R b0 ,R a0 ) connected in series; and the first through fourth resistors are connected in series between the ground (GD) and the power source (-V BB ). A battery feed circuit as set forth in claim 2, wherein the intermediate voltage output circuit (IV) comprises: a fifth resistor (R c0 ) and a sixth resistor (R c1 ), connected in series between the first and second subscriber lines (A, B); and an amplifier (A₂), an input of which is (A₂) connected at an intermediate connecting point (M₁) between the fifth (R c0 ) and sixth (R c1 ) resistors and an output of which (A₂) is connected at an intermediate connecting point (M₂) between the second (R b1 ) and third (R b0 ) resistors. A battery feed circuit as set forth in claim 3, wherein: the first feeder part (B a ) includes at least a first operational amplifier (A₁) receiving the first reference voltage at an input terminal, a first transistor (T r1 ) the base of which is connected to the output terminal of the first operational amplifier (A₁), and a seventh resistor (R e1 ) connected between the power source level voltage (-V BB ) and the first subscriber line (A), in series with the first transistor (T r1 ); and the second feed circuit part (B b ) includes at least a second operational amplifier (A₀) receiving the second reference voltage at an input terminal, a second transistor (T r0 ) the base of which is connected to the output terminal of the second operational amplifier (A₀), and an eighth resistor (R e0 ), connected between the ground level voltage (GD) and the second subscriber line (B), in series with the second transistor (T r0 ). A battery feed circuit as set forth in claim 4, wherein: a second input terminal of the first operational amplifer (A₁) is directly connected to an intermediate connecting point between the seventh resistor (R e1 ) and the first transistor (T r1 ); and a second input terminal of the second operational amplifier (A₀) is directly connected to an intermediate connecting point between the eighth resistor (R e0 ) and the second transistor (T r0 ). A battery feed circuit as set forth in claim 4, wherein: a second input terminal of the first operational amplifier (A₁) is connected, via a ninth resistor (R f1 ), to the emitter of the first transistor (T r1 ), the ninth resistor (R f1 ) being further connected in series through a tenth resistor (R s1 ) and a first capacitor (C₁) to the ground (GD); and a second input terminal of the second operational amplifier (A₀) is connected, via an eleventh resistor (R f0 ), to the emitter of the second transistor (T r0 ), the eleventh resistor (R f0 ) being further connected in series through a twelfth resistor (R s0 ) and a second capacitor (C₀) to the power level voltage source (-V BB ). A battery feed circuit as set forth in claim 6, wherein: a thirteenth resistor (R d1 ) is provided between the first transistor (T r1 ) and the first operational amplifier (A₁); and a fourteenth resistor (R d0 ) is provided between the second transistor (T r0 ) and the second operational amplifier (A₀). A battery feed circuit as set forth in claim 7, wherein: a first and a second zener diode (ZD₁,ZD₀) are connected in series between the ground (GD) and the power level voltage source (-V BB ), the junction between the zener diodes being connected to the intermediate connection point (M₂) between the second and third resistors (R b0 , R b1 ); and a fifteenth resistor (R g ) is connected, at one end, to the intermediate connecting point (M₂) between the first and second zener diodes and at the other end to an output of a third amplifier (A₂). A battery feed circuit as set forth in claim 8, wherein the third amplifier (A₂) is a voltage follower, having and an input in electrical communication with the first and second subscriber lines (A,B). A battery feed circuit as set forth in claim 9, wherein the voltage follower has a gain of 1. A battery feed circuit as set forth in any one of claims 6 to 8, wherein the tenth (R s1 ) and twelfth (R s0 ) resistors and the first and second capacitors (C₁,C₀) are all fabricated, as a whole, as a series-connected sixteenth resistor (R s ) and a third capacitor (C₀₁) connected between the second input terminals of the first and second operational amplifiers (A₁,A₀).

Inventors: AYANO, MITSUTOSHI, IIJIMA, YOSHIMI, SERIKAWA, ATSUO, SHIBUYA, KIYOSHI, TAKATO-KENJI
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: H04M19
IPC codes: H04M19