Electric network for use in a subscriber's loop

Electric network for use in a subscriber's loop, comprising at least two connecting terminals for supplying thereto a current flowing in the subscriber's loop, the network comprising the series arrangement of a capacitive element and a switching device and a threshold device coupled to the connecting terminals and the switching device for either coupling or not coupling, depending on the polarity and the value of said current, said capacitive element to the terminals of the network. This results in a network whose pattern of behavior is optimum for the various signals which are exchanged in the loop.

The invention relates to an electric network for use in a subscriber's 
loop, comprising at least two terminals for supplying thereto a current 
flowing in the subscriber's loop, the network comprising a capacitive 
component. 
To understand the various requirements such an electric network should 
satisfy the nature of the signals prevailing in a subscriber's loop are 
described here below. 
If the receiver is on the hook, no direct current flows in the subscriber's 
loop and the subscriber's set is part of an open circuit. A ringing 
alternating current can then be transmitted from the central exchange to 
the subscriber's set to put the ringing device of the set into operation 
via a coupling capacitor which is arranged in the subscriber's set in 
series with the ringing device. 
When the receiver is off-hook, a direct current flows in the loop for 
feeding the microphone of the set; Voice-frequency voice currents having a 
comparatively low amplitude of 1 to 2 mA are superimposed on this direct 
current which has, for example, a value of between 30 and 50 mA. 
To form the number of the telephone subscriber he wishes to call up the 
calling subscriber produces by means of the dial of his set, the receiver 
of which he has removed from the hook, successive loop interruptions of a 
suitable number and rhythm, which consequently correspond to successive 
interruptions of the supply current of the set; these interruptions are 
detected by a suitable circuit which is coupled in the central exchange to 
the subscriber's loop. 
From the above description of the signals occurring in a subscriber's loop 
it will be clear that the various electric or electronic components 
provided in the subscriber's loop will generally behave in a different 
manner for the different signals, depending on the frequency of said 
signals. 
Such a component provided in the subscriber's loop, is, for example, formed 
by the windings of a calling relay which is used in the transmission 
bridge of a subscriber's set. The self-induction of each winding must have 
a low impedance for the supply current of the subscriber's set and for the 
currents used for the ringing device which have a frequency of 50 Hz, but, 
on the other hand, it must have a high impedance for the voice currents. 
In addition, the self-induction of said winding must not be so high that 
it opposes the successive interruptions and the reinstatement of the 
subscriber's loop by the dial of the set during dialling. 
So it is evident that the choice of the value of this self-induction is a 
compromise between two different requirements: a high self-induction for a 
proper transmission of the voice currents and a low self-induction to 
prevent a considerable part of the ringing power from being absorbed and 
the dialling pulses from being excessively distorted. 
As will be explained here below with reference to a number of examples it 
can in general be said that the electric networks which may have been 
provided in a subscriber's loop must also have different, suitably chosen 
impedance values for the various kinds of electric signals which are 
exchanged through said subscriber's loop. Because the price of a capacitor 
is much lower than the price of a self inductor, such electric networks 
will often comprise one or more capacitors for effecting different 
impedance values of said electric networks for the different kinds of 
signals considered. It is obvious that for similar reasons as those 
mentioned in connection with the self-induction of a calling relay in a 
transmission bridge, the choice of such capacitive components is based on 
a compromise between two extremes: a high capacitance value to obtain a 
very low impedance for the variable currents (ringing currents or voice 
currents) and a low capacitance value to prevent the dialling pulses from 
being distorted by charging and discharging transition phenomena of said 
capacitive elements during the successive interruptions and the 
reinstatement of the supply current. 
The invention has for its object to provide an electric network of the type 
described in the preamble in which the necessity for such a compromise is 
avoided and wherein the optimum impedance for the various signals is 
automatically obtained. 
The electric network according to the invention is therefore characterized 
in that said network furthermore comprises a switching arrangement 
included in series with the capacitive component and a threshold device 
coupled to the connecting terminals and the switching arrangement for 
coupling or not coupling in dependency on the polarity and the value of 
said current said capacitive component to the terminals of the network. 
Said switching arrangement is preferably a pnp or npn-transistor whose 
collector-emitter path is used for coupling the capacitive component to 
the terminals of the network and whose emitter-base diode or 
collector-base diode is used as control element. 
In this manner it is possible, thanks to the invention, to choose the 
capacitance value of the capacitive component or of the capacitive 
components to be very high, which is necessary for obtaining the impedance 
required for the network for the high-frequency currents, such as the 
voice currents, without the charging and discharging effects of said 
capacitive elements occurring during the dialling pulses, being able to 
distort said dialling pulses: for, when these capacitive components have 
been charged by the supply current, the receiver being off-hook, they 
cannot discharge themselves during the dialling pulses as said components 
are switched off during the loop openings caused by the dial.

FIG. 1 shows by way of example an arrangement provided with electric 
networks to which the invention can be applied. This prior art arrangement 
is described in an article by J. M. Person, entitled "Systeme d'abonnes a 
courant reduit" which appeared in "L'echo des recherches", pages 58 to 65, 
July 1974. Such an arrangement which is intended for use in a subscriber's 
loop enables the adaptation of the admittance which forms the subscriber's 
line for the telephone exchange, relative to the voice current and also 
allows the feeding of the subscriber's set, the current being either 
decreased or not decreased. 
Said arrangement is provided with terminals 1, 1' which are connected to 
the telephone exchange and with terminals 2, 2' which are connected to the 
subscriber's line. The circuit 3 forms a resistive impedance for the 
direct current passing through it and, simultaneously, a negative 
impedance for the voice currents. The circuits 4 and 5 are devices by 
means of which it is possible to tap off a portion of the supply current 
supplied by the telephone exchange in such a manner that said portion 
flows through the circuit 3. The use of the diodes 6, 7, 8 and 9 enables 
such an orientation of said current portion that said portion always flows 
in the same direction through the impedance Z of the circuit 3 if the 
polarity of the voltage which is supplied by the transmission bridge of 
the telephone exchange to the terminals 1, 1' is reversed. 
The circuits 4 and 5 comprise components which are connected in the same 
manner. The circuit 4 (5 respectively) comprises a resistor 10 (20) 
provided between the terminals 1, 2 (1', 2'), a Zener diode 11 (21) and a 
capacitor 12 (22) which are each connected to the terminals of the 
resistor 10 (20). The emitter electrode of the npn transistor 13 (23) is 
connected to the terminal of the resistor 10 (20) which is connected to 
the terminal 1 (1'), the collector electrode is connected to diode 8 (9) 
and the base electrode is connected via resistor 14 (24) to the other 
terminal of resistor 10 (20). 
If the voltage supplied by the transmission bridge of the exchange to the 
terminals 1, 1' has the polarity indicated in FIG. 1 (at 1 and at 1') the 
arrangement operates in the following manner: if the subscriber's loop 
connected to the terminals 2, 2' is closed (receiver off-hook) a current 
flows through resistor 10 which current is such that transistor is 
saturated. A tapped-off current can then flow through diode 7, impedance Z 
of the circuit 3, diode 8 and the collector-emitter path of transistor 13. 
The current which flows through the subscriber's set passes the resistor 
20, however in a direction which is such that the transistor 23 is 
non-conducting. The capacitors 12 and 22 ensure decoupling of the 
resistors 10 and 20 for the variable current. Zener diode 11 and resistor 
14 are used for limiting the current in the base-emitter path of 
transistor 13. As the circuits 4 and 5 are identical, it is obvious that 
if the polarity of the voltage applied to the terminals 1 and 1' is 
reversed the operation of the arrangement in FIG. 1 remains the same, the 
function of the components 4, 7 and 8 and the function of the components 
5, 6 and 9 being interchanged. 
If the subscriber's loop is open (receiver on-hook) and the subscriber is 
called from the exchange, a 50 Hz a.c. voltage appears at the terminals 1, 
1'. In the ringing device of the set this a.c. voltage produces a current 
which flows through the capacitors 12 and 22, the voltage drop across 
these capacitors must then be sufficiently small to prevent the 
transistors 13 and 23 from becoming conducting. To this end the 
capacitance of the capacitors 12 and 22 must be high in order to prevent 
excessive charging of said capacitors during each half cycle of the 50 Hz 
signal. 
The use of capacitors of a high capacitance value has, however, a marked 
drawback during dialling. The capacitor 12, which is charged to a voltage 
of approximately 0.7 V by the passage of the loop current in the resistor 
10 needs a certain period of time to discharge to a voltage which is below 
the threshold voltage at which the transistor 13 becomes non-conducting. 
This means that, in spite of the fact that the loop is open during the 
loop interruption pulse, the transistor 13 will remain conducting during a 
given period of time, so that a direct current can flow through diode 7, 
impedance Z, diode 8 and the collector-emitter path of transistor 13. The 
dialling pulses received in the exchange may then be distorted to an 
annoying degree. 
Alternatively, if the capacitance values of capacitors 12 and 22 are 
sufficiently low to obviate said drawback, the transistors 13 and 23 may 
be made conducting at the peak values of the ringing current. If 
particularly those transistors 13 and 23 do not become conducting in the 
presence of precisely the same main current in the subscriber's loop, the 
peak ringing currents tapped off by the transistors 13 and 23 may be 
different from one another which results in an average direct current 
differing from zero in the transmission bridge of the exchange, so that it 
is possible that the exchange incorrectly interprets said direct current 
as the lifting of the receiver from the hook of the subscriber's set. 
The circuits 4 and 5 may be considered as electric networks provided with 
capacitive components 12 and 22, and connected into a subscriber's loop 
via terminals 1, 2 and 1', 2' and wherein the measures according to the 
invention can be applied to obviate the above-mentioned drawbacks. 
FIG. 2 shows the circuit diagram of, for example, the electric network 4, 
adapted in accordance with the invention. Said circuit diagram also shows 
the resistor 10, one end of which is connected to the terminal 1, the 
transistor 13, the emitter electrode of which is connected to terminal 1 
and whose base electrode is connected via resistor 14 to the other end 15 
of resistor 10, zener diode 11, which is included between the terminals 1 
and 2 and capacitor 12, one end of which is connected to terminal 1. In 
order to connect the other end of capacitor 12 into the network, a 
switching device, constituted by the npn transistor 16 and controlled by 
the current through resistor 17 was added; resistor 17 is provided between 
the ends 15 of resistor 10 and terminal 2. The base electrode of the 
transistor 16 is connected to terminal 2 and the collectrode electrode to 
the end 15, whereas one end of the emitter electrode of said transistor is 
connected to the other end of capacitor 12 and the other end to terminal 2 
through diode 18. 
If a loop current flows from terminal 2 to terminal 1 with such a value 
that the voltage drop across resistor 17 adjusts the transistor 16 to the 
saturation state the emitter-collector path of this transistor constitutes 
virtually a short circuit and everything proceeds as if capacitor 12 were 
connected to the terminals of resistor 10. 
If the subscriber's loop is open the current in the resistor 17 becomes 
equal to zero and transistor 16 is non-conducting, which prevents 
capacitor 12 from discharging into the resistor 10, so that transistor 13 
remains conducting during said time period as indicated in FIG. 1. If the 
loop is reclosed after each interruption, the loop current is immediately 
restored in resistor 17, transistor 16 becomes conducting and capacitor 
12, which has retained its charge during interruption of the loop, is 
again connected to the terminals of resistor 10. 
Thus, owing to the presence of the switching device constituted by 
transistor 16 and of resistor 17 in the network of FIG. 2 it is possible 
to use a capacitor 12 of a very high capacitance without it being possible 
that during dialling the annoying hysteresis effect, caused by the fact 
that the transistor 13 is rendered alternatingly non-conducting and 
conducting can occur. 
In the ringing phase the 50 Hz ringing currents pass for a given current 
flow direction the capacitor 12 and the emitter-base path of transistor 16 
and for the other current flow direction the capacitor 12 and the diode 18 
provided for that purpose. The capacitor 12, whose capacitance is high, 
cannot take up any worthwhile charge in each half cycle of the ringing 
current. If particularly the current flows from terminal 2 to terminal 1 
the voltage between the terminals 2 and 1 is substantially equal to the 
voltage drop in the base-emitter diode of transistor 16, so approximately 
0.6 V. This voltage is too weak to cause the series-arranged 
base-collector diode of transistor 16 and the base-emitter diode of 
transistor 13 to become conducting, so transistor 13 remains cut-off and 
in the circuit according to FIG. 1 no current of 50 Hz frequency is 
discharged to the circuit 3. 
In the circuit shown in FIG. 2 the zener diode 11 is not connected to the 
terminals of the resistor 10 but to the terminals of the series 
arrangement of resistors 10 and 17; in this manner said zener diode has a 
protective function both for the base-emitter diode of transistor 13 and 
for the base-collector diode of transistor 16. 
In order to prove the general usability of the invention, a further example 
of the application of the invention will be described. To one skilled in 
the art it is known that it is possible to realize an active RC-network 
which has about the same characteristics as a self induction. Such a 
network can be used advantageously, for example for realizing the self 
induction in a transmission bridge for the subscriber's loop. 
FIG. 3 shows the conventional circuit diagram of such an "electronic self 
induction". Said diagram shows an electric network comprising two 
terminals 31 and 32 and a transistor 33 whose collector electrode is 
connected to terminal 31, the emitter electrode through a resistor 34 to 
terminal 32 and the base electrode via a resistor 35 to the terminal 31. 
Furthermore a capacitor 36 is provided between the base electrode of 
transistor 33 and terminal 32. 
For direct current said circuit operates as follows: the current I which 
flows from terminal 31 to terminal 32 produces a base current I/(b+1) in 
resistor 35, b being the current gain coefficient of transistor 33. If the 
ohmic value of the resistors 34 and 35 is indicated by R.sub.4 and R.sub.5 
the voltage drop between the terminals 31 and 32 is given by 
##EQU1## 
v.sub.e being the voltage of the forward polarized base-emitter diode. The 
collector electrode of transistor 33 is polarized in a simple manner by 
the voltage drop R.sub.5 /(b+1) I across the resistor 35. If R.sub.4 is 
small and b is large, it is obvious that the d.c. voltage drop between the 
terminals 31 and 32 can be very low, to the order of 2 to 3 volts, even if 
the current I is relatively large. If, for example, R.sub.4 = 33 ohm, b = 
199, R.sub.5 = 6.6 kOhm, v.sub.e = 0.6V and I = 30 mA, the voltage drop 
between the terminals 31 and 32 amounts to approximately 2.6 V, that is to 
say it is comparable to a voltage drop occurring with a similar current 
across the self induction of a transmission bridge. 
Capacitor 36 forms a sort of short circuit for voice frequencies, whereas 
the impedance of the circuit becomes substantially equal to that of the 
pure resistor 35. 
In particular it can be proved that the circuit shown in FIG. 3 
approximately behaves as the equivalent circuit shown in FIG. 4, wherein 
the parallel arrangement of a resistor A with the series arrangement of a 
resistor B and a self induction L is provided between the terminals 31 and 
32. In FIG. 4 the values of the components A, B and L are indicated as a 
function of the values of the several components of FIG. 3: The values 
R.sub.4 and R.sub.5 of the resistors 34 and 35, capacitance C of 
condensator 36 and current gain coefficient b of the transistor 33. 
As indicated above the circuit shown in FIG. 3 can be used to replace the 
self induction in the transmission bridge of a subscriber's loop, as shown 
in FIG. 5. This FIG. 5 shows the prior art arrangement of such a 
transmission bridge wherein the two circuits 40 and 50, which comprise the 
same components which are disposed in the same manner as in the network 
shown in FIG. 3, replace the two conventional self inductions which are 
provided in series on the two wires L.sub.A and L.sub.B of the 
subscriber's line. The two terminals 41 and 42 of the circuit 40 and the 
two terminals 51 and 52 of the circuit 50 correspond to the two terminals 
31 and 32 of the network shown in FIG. 3. The components 43 to 46 of the 
circuit 40 and the components 53 to 56 of the circuit 50 correspond to the 
components 33 to 36 of the network shown in FIG. 3. The terminal 41 of 
circuit 40 is connected to the subscriber's set through the wire L.sub.A, 
the other terminal 42 of the circuit 40 being connected through the 
negative pole of a d.c. voltage source S.sub.D which supplies a 48 V 
supply voltage and whose positive pole is connected to earth. Arranged 
between earth and terminal 51 of the circuit 50 there are a device D for 
detecting the presence of a loop current when the subscriber lifts the 
receiver from the hook and a source S.sub.A for supplying the 50 Hz 
ringing signals. The other terminal 52 of the circuit 50 is connected to 
the subscriber's set through the wire L.sub.B. 
In the diagram the capacitors C.sub.A and C.sub.B for coupling the 
telephone exchange are indicated by means of dashed lines, an electrode of 
said capacitors being connected to terminals 41 and 52 respectively. 
The circuits 40 and 50, whose components have the value specified above 
relative to the circuit shown in FIG. 3, produce a small voltage drop 
(approximately 2.6 V) for a supply current of 30 mA; in contradistinction 
therewith said circuits have an impedance which is sufficiently large 
(approximately 6600 ohm) to avoid that for the voice currents the 
impedance of the subscriber's line, which is approximately 600 ohm, is 
arranged in parallel. Said impedance is obtained by giving the capacitors 
46 and 56 such a high capacitance that the impedance is very small 
compared with the value of the resistors 44 and 54. 
On dialling, the circuits 40 and 50, which in accordance with the 
equivalent diagram of FIG. 4 behave as self inductions will oppose the 
rapid interruption and reinstatement of the loop current, which might 
disturb the operation of the device D. 
In accordance with the invention these self induction effects are avoided 
during dialling by preventing the discharging and recharging of the 
capacitors 46 and 56 in the rhythm of the loop interruptions on dialling. 
FIG. 6 shows the network shown in FIG. 3 after it was changed in accordance 
with the invention and can be used for replacing the circuits 40 and 50 of 
FIG. 5. The network shown in FIG. 6 comprises, just like the network of 
FIG. 3 the transistor 33 whose collector electrode is connected to 
terminal 31, one end of whose base electrode is connected via resistor 35 
to terminal 31 and the other end to an end of capacitor 36 and whose 
emitter electrode is, finally, connected to an end of resistor 34. In this 
diagram, however, the other end of capacitor 36 and the other end of 
resistor 34 are not interconnected directly but via the emitter-collector 
path of the pnp transistor 37 whose emitter and collector are connected to 
the other end of capacitor 36 and the other end of resistor 34 
respectively. A resistor 38, whose two ends are connected to the collector 
and the base respectively of transistor 37 is included between this 
resistor 34 and terminal 32 of the network. It is clear that in this 
manner the release threshold of transistor 37 is controlled by the voltage 
drop across the terminals of resistor 38. 
If the loop current I flows in the network from terminal 31 to terminal 32, 
said current causes a voltage drop across resistor 38. If this voltage 
drop is sufficient to release the transistor 37, this transistor is 
brought to the saturation state and the impedance of this transistor 
between the collector and the emitter becomes very small; everything then 
behaves as if capacitor 36 were connected directly to the terminals of the 
resistor 34. So for the voice currents superimposed on the supply current 
of the set the circuit functions as the "electronic self induction" of 
FIG. 3. 
On dialling and at the moment a loop interruption occurs, the current I 
will, on the contrary, decrease rapidly to a value which is below the 
release threshold of transistor 37. At this moment the capacitor 37 is in 
an insulated position and retains its charge, the network of FIG. 6 then 
corresponding to a small resistance. On reclosing of the loop, the current 
I will adjust substantially immediately to a value equal to the threshold 
value of the release currents of transistor 37 and will slowly increase 
thereafter. 
An optimum threshold value of said current is given by the difference 
between the specified minimum value of the supply direct current of the 
subscriber's set and the specified maximum value of the voice current in 
the subscriber's loop. 
FIG. 7 shows the diagram of the transmission bridge of FIG. 5 wherein, in 
accordance with the invention, each of the networks 40 and 50 is replaced 
by a network of FIG. 6. In FIG. 7 the elements corresponding with the 
elements of FIG. 5 have been given the same reference numerals. In 
addition, the networks 40 and 50 comprise in the diagram of FIG. 7 the 
transistors 47 and 57 and the resistors 48 and 58 which, relative to the 
other elements, are connected in like manner as transistor 37 and 
transistor 38 in the network of FIG. 6. 
In the time diagram of FIG. 8 the solid curve represents the loop current 
as it is measured by the device D in the case of FIG. 5, whereas the 
dashed curve represents the loop current as it is measured by the device D 
in the case of FIG. 7. The horizontal line V.sub.S represents the release 
threshold of the transistors 47 and 57. It is evident that in the first 
case the dialling current pulses are greatly distorted and that in the 
second case, where networks according to the invention are used, said 
current pulses are shown to be almost perfect for interrupting and 
reclosing the loop. 
In a further mode of operation the circuits 40 and 50 of FIG. 7 can be 
subjected to the 50 Hz ringing signals applied by the generator S.sub.A. 
In order to obtain the proper operation of said circuits the network of 
FIG. 6 is used, which is identical to each of the said circuits, it being 
assumed that a ringing signal is supplied to the terminals of said 
network. During half a cycle the ringing current will flow from terminal 
31 to terminal 32 and that, presuming capacitor 36 is discharged, through 
resistor 35, capacitor 36 and the base-emitter diode of transistor 37. If 
capacitor 36 is charged to a voltage which exceeds the voltage at which 
the base-emitter diode of transistor 33 becomes conducting, this 
transistor 33 starts conducting and the impedance of the network of FIG. 6 
will quickly decrease to a value which is equal to the direct current 
resistance of the circuit because the peak ringing current which is much 
smaller than the loop current will not keep the switching device 37 in the 
closed state. Capacitor 36 remains isolated, a voltage of approximately a 
diode voltage, that is to say 0.6 to 0.7 volts, being present between its 
terminal. During the other half cycle, however, the ringing current will 
flow from terminal 32 to terminal 31 and the current will flow through one 
of the following two paths: resistor 38, resistor 34, emitter-base diode 
of transistor 33, which functions as a zener diode, and the base-collector 
diode of transistor 33 or, alternatively, the base-emitter diode of 
transistor 37 which functions as a zener diode, capacitor 36 and the 
base-collector diode of transistor 33. The first of said paths must be 
preferred to said second path, to avoid discharging of capacitor 36 during 
this half cycle. This means that the zener voltage of the emitter-base 
diode of transistor 33 must be smaller than the zener voltage of the 
base-emitter diode of transistor 37, reduced by the voltage picked up by 
the capacitor 36 during the other half cycle. In these circumstances the 
network of FIG. 6 behaves for a current flowing from terminal 32 to 
terminal 31 as a zener diode in series with a diode and two small 
resistors. The operation of the network of FIG. 6 is not the same for 
every half cycle of the current. One skilled in the art will easily 
understand that it can be interesting to arrange the network of FIG. 6 in 
such a way that it operates in an identical manner for both half cycles of 
the ringing current. This can be realized by means of the circuit of FIG. 
9 in which all the elements of the network of FIG. 6 are present, provided 
with the same reference numerals, between the terminals 31 and 32. The 
network of FIG. 9 is provided with two terminals 61 and 62 and comprises 
four diodes 63 to 66 inclusive which are connected in known manner to the 
terminals 31, 32 and 61, 62 for forming a rectifier bridge which forces 
the current to flow in a certain direction between the terminals 31 and 
32, independent of the current flow between the terminals 61 and 62. 
Such a network can be arranged in the transmission bridge circuit of FIG. 7 
instead of the networks 40 and 50, which furnishes the advantage that the 
operation of the circuit of FIG. 7 is independent of the polarity of the 
voltage source S.sub.D. 
The network of FIG. 6 utilizes a pnp transistor 37 by way of switching 
device. For one skilled in the art it will be evident that it is likewise 
possible to use a npn transistor. Of this npn transistor the emitter would 
then be connected to the capacitor 36, the collector to terminal 32 and 
the base to the junction of the resistors 34 and 38. 
It should be noted that with the transistor constituting the switching 
device an "inverse" circuit is preferably used, wherein the role of the 
emitter and the collector are mutually interchanged compared with the 
normal use of a transistor in the saturation state. The advantage thereof 
is that in the saturation state the d.c. voltage between the collector and 
the emitter closely approaches the value zero, whereas this voltage is 
usually less well-defined and can increase for large base-emitter currents 
to 0.3 V or higher. 
The following describes an example of the use of electric networks 
according to the invention with devices in a subscriber's loop, where it 
is desired to incorporate transformer windings in series in the loop. Such 
an arrangement is, for example, the arrangement shown in FIG. 10. The 
object of this arrangement is to provide an amplifier having a negative 
series impedance value in a subscriber's loop. This is realized by means 
of two transformer windings 71 and 72 which are included in the wires 
which are indicated by L.sub.A and L.sub.B respectively of the loop and a 
further transformer winding 73 which is connected to the output of the 
amplifier 74, having a negative impedance Z.sub.1. The winding sense of 
the windings 71 and 72 is opposite and indicated in the customary manner 
by a dot near one end of each winding. As the supply direct current I of 
the loop flows through the two windings in the opposite direction, this 
current I produces a pre-magnetisation in the magnetic circuit of the 
transformer which results in increased dimensions, and, consequently, in 
an increased cost price of the transformer. 
To obviate this drawback each of the two windings and 71 and 72 can be 
replaced by the electric network, the diagram of which is shown in FIG. 
11. This electric network comprises an input terminal 81 and an output 
terminal 82. The direct current I of the loop flows through the network in 
the direction from terminal 81 to terminal 82. This network comprises a 
current mirror 83, a branch 84, which is connected to a terminal 82 and 
through which the current I flows and which has two branches 85 and 86 
which are each passed by currents I/2. In the embodiment shown the current 
mirror 83 comprises a npn transistor 87 the emitter of which is connected 
to branch 84, the collector to branch 85 and the base, through a diode 88, 
to a branch 84. In addition the current mirror 83 comprises an npn 
transistor 89, the emitter of which is connected to the base of transistor 
87, the collector to branch 86 and the base to branch 85. This branch 85 
is connected through a low-value resistor 90 to a transformer winding 91, 
which performs the function of the winding 71 or the winding 72 of FIG. 
10. The branch 86 is connected to the compensating winding 92. The other 
end of the windings 91 and 92 are interconnected and connected to the 
input terminal 89 of the network. Finally, the network comprises a 
capacitor 93 which is provided between the branch 84 of the current mirror 
and the junction of the resistor 90 and the winding 91. 
The proper bias voltage of the collector of transistor 89 is obtained by 
means of the voltage drop produced across the low-value resistor 90. The 
number of turns of the compensating winding 92 is equal to the number of 
turns of the winding 91, but of the opposite winding sense (see the dots 
at both windings). As, due to the current mirror 83, said two windings are 
passed in the same directions by direct currents I/2 which are of the same 
value, it is clear that the resulting magnetic flux in the transformer is 
equal to zero. The current mirror 83 decouples the capacitor 93 for the 
voice currents so that the voice currents flow through a winding 91 only, 
so that the compensating winding 92 is inoperative. 
The drawback of the electric network of FIG. 11 is that it causes a 
distortion of the dialling current pulses owing to the charging and 
discharging effects of the capacitor 93 in the rhythm of loop 
interruptions and the corresponding flux variations in the windings 91 and 
92. This drawback can be obviated by modifying the electric network of 
FIG. 11 in accordance with the invention. This modified network is shown 
in FIG. 12 and comprises between the terminals 81 and 82 all the 
components of FIG. 11, which have been given the same reference numerals. 
In addition it comprises a transistor 94 whose collector-emitter path is 
applied between a terminal of resistor 90 and the capacitor 93. A resistor 
95 is provided between said terminal of the resistor 90 and the winding 
91. Said resistor 95 is provided between the base and the collector of 
transistor 94. In the same manner as in the preceding examples the release 
threshold of the transistor 94 is controlled by the voltage drop between 
the terminals of the resistor 95. 
The electric network of FIG. 12 is furthermore provided with 4 diodes 96 to 
99 which are connected in such a way to the terminals 81 and 82 and to the 
terminals 100 and 101 that the entire circuit of FIG. 12, between the 
terminals 100 and 101 functions independently of the direction of the 
current flowing through it.