GB2061669A - Transformerless trunk circuit - Google Patents

Abstract

A trunk circuit comprises a first lead pair for carrying two-way signals, a second lead pair for carrying two-way signals, a first circuit for electrically coupling an incoming signal from the second lead pair to the first lead pair, a second transformerless circuit for applying an incoming signal from the first lead pair to the second lead pair, and a third transformerless circuit for preventing signals from the first circuit applied to the first lead pair from being reapplied to the second lead pair and for preventing signals from the second circuit applied to the second lead pair from being reapplied to the first lead pair.

Description

SPECIFICATION Transformerless trunk circuit This invention relates to a bidirectional amplifier, and a more particularly to a bidirectional trunk circuit for coupling a first pair of leads such as a trunk and a second pair such as a junctor.

Trunk circuits are generally used for handling incoming or outgoing calls to or from a central office or PBX. Due to the existence of a wide variety of kinds of central offices and PBXs, certain operational characteristics of trunks have become standardized. Yet because of the requirement for trunks to interface with various different internal circuits of the central offices or private branch exchanges which circuitry often does not match the trunk characteristics, trunk circuits are in addition required to effect the various matching conversions.

Trunk impedances have become standardized at typically either 900 or 600 ohms, and are comprised of balanced tip and ring leads.

Trunk circuits also carry signalling information and typically carry current of either polarity at 48 volts DC. They normally carry signals at a level of about 0 dbm, although the signal levels often vary widely and sometimes are as high as + 6 dbm, which is between 5 and 6 volts peak to peak at the approximate impedance levels noted.

Yet due to the large number of trunks exposed to the weather and due to other environmental factors such as high voltage power lines, poor or changing ground resistance, temperature variations mechanically affecting terminations, etc. sometimes large common mode signals result on the trunks, sometimes as high as + 200 volts AC.

Trunk circuits further must terminate the tip and ring leads with a matching impedance for AC signals, yet must have internal resistance for direct current which has been standardized at less than 250 ohms.

The trunks must further be able to carry either signalling alone (tone duplex or battery reversal) or signalling including ringing signals. They must also provide separate control of the gain of both incoming and outgoing voice or other message signals in order that the signals can be set at the proper working levels for the central office or PBX.

While trunks themselves are normally electrically balanced, their signals are inherently differential in nature, due to the aforenoted common-mode signal problem. Thus a PBX or central office which operates on simple AC audio signals must have means for coupling the audio signals to the trunks in a differential, isolated manner. The unbalanced pair feeding a trunk circuit (i.e. one terminal and ground) can be of whatever impedance is most convenient to the switching office or PBX designer, but is often 600 ohms.

The trunk circuit must therefore translate the unbalanced input circuit to a balanced output circuit and vice versa, must match the respective trunk or unbalanced impedances, must adjust the signal levels to the standard signal level required both on the trunk and unbalanced lead pair, must remove very high common mode signals often carried by the trunk, must present a different AC impedance to the trunk than DC impedance, etc. It is also desirable to provide a trunk circuit having separate trunk input and output circuits, for connection to a 4 wire device such as a Codec (code-decoder).

Trunk circuits have previously used circuits involving hybrid transformers to provide the matching a common mode reduction required.

However with the advent of electronic switching offices and PBXs of considerably reduced size and weight, the use of such transformers is an impediment to miniturization. For example where the circuitry of a PBX is built into a small console, cabinet or desk, within which the circuitry is disposed on printed circuit boards, such transformers add bulk, weight and increasingly unacceptable size to the unit, and as well are to some extent incongruous with printed circuit boards carrying microelectronic components. The bulk of the transformers are of course due to the requirement for a significantly sized core which will not saturate in the presence of high AC and DC trunk currents.

In our patent application Serial No.

33899/78 filed August 18, 1978, we described the requirements of a trunk noted above, but which does not require the use of a hybrid transformer. Physically the circuitry is light and small and can be fabricated on a printed circuit board plug-in card for a PBX or central office.

It has now been found that the signal cancellation effects of the former invention can also be obtained with the use of an alternative and simpler structure by which the photocoupled circuitry is replaced by an inexpensive transformer coupled stage. No hybrid transformer signal coupling is utilized with attendant impedance balancing requirements, and while the inexpensive transformer is utilized to couple the signal passing from one direction to the second, a completely transformerless circuit is used to couple the signal passing in the opposite direction. Furthermore, transformerless means are used to cancel signals originating from either direction from being fed back thereto.

In general the embodiment of the invention to be described below is a trunk circuit comprising a first lead pair for carrying two-way signals, a second lead pair for carrying twoway signals, first circuit means for applying an incoming signal from the second lead pair to the first lead pair, second transformerless cir cuit means for applying an incoming signal from the first lead pair to the second léad pair, and third transformerless circuit means for preventing signals from the first circuit means applied to the first lead pair from being reapplied to the second lead pair and for preventing signals from the second circuit means applied to the second lead pair from being reapplied to the first lead pair.

Preferably the first circuit means is comprised of a circuit including a transformer having a primary winding connected in a low impedance voltage source circuit to the second lead pair, and having a secondary winding connected in a very high impedance to direct current circuit means for applying the incoming signal from the second lead pair to the first lead pair. Due to the resulting very low current levels in the transformer, the effective bandwidth of the transformer is sub stantially increased, allowing use of an inexpensive transformer which can be readily incorporated on a plug-in printed circuit board.

A better understanding of the invention will be obtained by reference to the detailed description below of the preferred embodiment, and to the following figures, in which: Figure 1 is a schematic diagram of the invention in its simplest form, and Figure 2 is a schematic diagram of the preferred form of the invention in its most detailed form.

Fig. 1 is a schematic of the invention in its simplest form. Tip and ring terminals T and R are provided for connection to balanced tip and ring leads, and normally carry both trunk- signals and a direct current. Terminating the tip and ring terminals is a matching resistor 5 in series with a large valued capacitor 6, which series circuit is connected between the tip and ring terminals.

Terminating the tip and ring terminals for direct current is the collector-emitter circuit of transistor 7, which has its collector connected to the tip or ring terminal and its emitter in series with a small valued resistor 8 to-the other of the tip or ring terminal.

The secondary winding of an inexpensive transformer 80 has a high impedance to direct current circuit connected in series therewith. The high impedance circuit with the secondary winding should be connected between the inverting input of an operational amplifier 10 and the junction between resistor 8 and ring lead R.

The circuit which exhibits high resistance to direct current is comprised.of resistor 81 in series with capacitor 82, in series with secondary winding 83 of transformer 80.

Capacitor 82 allows modification of the frequency response of the circuit besides being of virtually infinite resistance to D.C., and its presence is optional. The value of resistor 81 in one model of the present invention was 10,000 ohms.

The primary winding 84 of the transformer 80 is connected in a circuit to'a low impedance voltage source 85. This voltage source preferably is formed by the output circuit of a further operational amplifier which is connected to a second input to the trunk circuit.

In operation, the transformer 80 is driven by signals originating at the input of the further operational amplifier. Since virtually no (or very little) direct current flows through the secondary of the transformer due to the large value of resistor 81 and due to capacitor 82, its frequency and amplitude characteristics are optimized. Since there is practically no loading of the transformer, a size reduction of 2 can be realized, and an additional size reduction of 4-6 can be realized due to there being virtually no direct current flow through the secondary winding.

The signal current flow can be calculated by multiplying the ratio of the signal voltage at the low impedance source and the value of resistor 81 by the ratio of resistors 11 to 8 times the normalized filter transmission characteristics formed by capacitor 82 and resistor 81.

It is preferred that the primary and secondary windings of the transformer are wound so that stray capacitances to ground are not amplified by operational amplifier 10. Accordingly it is preferred that the ends of the primary and secondary windings which are not connected to, or adjacent the ring lead (or low impedance node) should be separated as widely as possible in order to provide lowest capacitance therebetween.

Turning now to Fig. 2, a detailed schematic of the invention is shown. Ignoring for a moment the bridge rectifier which will be referred to later, a signal from the tip and ring leads is applied to tip and ring terminals T and R. This signal is applied to operational amplifier 20 through input resistors 21 and 22 and DC blocking capacitors 23 and 24. It is preferred that the value of each of resistors 21 and 22 should be at least 100 times the AC impedance of the tip and ring balanced lead pair. It is also preferred that the gain of this stage should be approximately 0.1 or 0.11. A feedback resistor 25 connects the output of operational amplifier 20 to its inverting input terminal in the conventional manner.

Amplifier 20 is thus connected as a standard operational amplifier, to remove common mode signals which may appear on the tip and ring leads. The gain of amplifier 30 is low to prevent saturation thereof in the presence of large common mode signals.

The output of operational amplifier 20 is connected through resistor 26 to the noninyerting input of an operational amplifier 27.

This amplifier has a conventional feedback resistor 28 between its output and inverting input (bypassed by a small capacitor), and a resistor 29 to common or ground. It is pre ferred that with the present circuit the gain of this stage should be approximately 8.

The output of operational amplifier 27 is connected through resistor 30 to the unbalanced terminal U. Resistor 30 should be of value to match the unbalanced external impedance of the unbalanced pair at terminal U.

For example, where this trunk circuit is used in a PBX, the impedance at terminal U would be typically 600 ohms while the tip and ring balanced impedance would typically be 900 ohms.

A gain of 0.1 in operational amplifier 20, followed by a gain of 9 in operational amplifier 27 provides a total loss of 0.1db. This is an example of a circuit for coupling of a 900 ohms trunk to a 600 ohms termination at terminal U. A zero dbm power level in 600 ohms is of course at a lower voltage than zero dbm power level in 900 ohms. The signal level at terminal U is reduced to half at the output of operational amplifier 27 if terminated by a resistor equal in value to resistor 30 (i.e. 600 ohms).

Other impedance conversions can be made by adjusting the gains of the amplifier and circuit impedance.

The tip and ring signal level as applied to the circuit of operational amplifier 20 is reduced by a factor of 10, then raised by a factor of 8 at the output of operational amplifier 27. A 5 volt input signal is therefore transformed to approximately 4 volts at the unbalanced output of amplifier 27. The signal level at terminal U would then be 2 volts assuming it has a matching termination.

These signal levels have been chosen since they are the internal central office or PBX and external trunk signal levels normally standardized by telephone companies.

The level of the signal as applied to terminal U can be selectively increased, to accommodate reduced levels which would occur during internal conferencing within the central office or PBX which includes a call on the instant trunk. Resistors 31 and 32 individually bypass resistor 29 through externally actuated individual switch contacts 33 and 34. The gain of the stage including operational amplifier 27 can thus be increased by switching resistors 31 or 32 in parallel with resistor 29.

The three gains provided by this amplifier are preferred to be about 8, 11 and 1 5.

For signals passing in the return direction, operational amplifier 35 has its noninverting input connected to terminal U.

An inexpensive transformer has its primary winding 84 connected between the output of operational amplifier 35 to ground. Secondary winding 83 has one lead connected to the ring lead terminal R, and its other lead connected through the series circuit of capacitor 82 and resistor 81 to the inverting input of an operational amplifier 44.

The output of operational amplifier 44 is connected through resistor: 48 to the base of transistor 49, which is of the type which can carry a relatively heavy current through its collector emitter circuit. Transistor 49 has its collector-emitter circuit connected between the tip and ring leads, through small valued resistor 50. The junction of the emitter of transistor 49 and resistor 50 is connected through capacitor bypassed resistor 51 to the inverting input of operational amplifier 44.

A terminating resistor 52 having a value matching the balanced tip and ring lead impedance (i.e. typically 900 ohms) is connected in series with a large valued capacitor 53 (for providing an AC short circuit), which series circuit is connected between the tip and ring leads.

With the application of an output signal to terminal U, which signal is translated in operational amplifier 35, it passes through the low impedance input circuit to the primary winding 84 and the high impedance circuit connected to the secondary winding 83 of the transformer, and is applied between terminal R and the inverting input of operational amplifier 44. The presence of capacitor 82 effectively bars the passage of direct current through the secondary winding, and also can be used to limit the low frequency response of the circuit path, with resistor 81. However, even were capacitor 82 omitted, the high resistance of resistor 81 would limit the flow of direct current to a very low level.

The signal voltage applied to the tip and ring leads is the product of the impedance across the tip and ring leads and the current applied thereto. The impedance across the tip and ring leads is the termination impedance in parallel with the impedance applied to the tip and ring lead. Assuming, for example, that the tip and ring line impedance is 900 ohms and the termination impedance (resistor 52) is 900 ohms, and assuming the resistance of resistor 51 is 20,000 ohms, resistor 50 is 20 ohms and resistor 36 is 374,000 ohms, the voltage applied to the line is 450 x 20,000 = = 1.20.

374,000 x 20 This value, it will be recognized, is the voltage ratio required to transfer a signal of the same power from a 600 ohms line to a 900 ohm line.

The circuit also provides means for safeguarding against sidetone, or positive feedback of the signal arriving from either the tip and ring balanced pair of the U terminal and being fed back to the source.

It will be recalled that signal is applied from operational amplifier 27 through a 600 ohm resistor 30 to the U terminal. This signal also appears on the noninverting input of operational amplifier 35. The same signal, but with a different amplitude is applied through resistor 54 to the noninverting input of amplifier 35. Resistor 55 is connected between the output of operational amplifier 35 and the noninverting input. Resistor 54 is chosen of such value sufficient that the gain of amplifier 35 multiplies the signal from the output of operational amplifier 27 applied to the noninverting input of operational to a different extent than the same signal applied to its inverting input such that the signal is cancelled.

It should be noted that signals at the output of operational amplifier 27 should be at full balanced line level and are reduced by half in resistor 30 for application to the U terminal.

Signal originating from the U terminal are applied to the noninverting input of operational amplifier 35. Accordingly a differential signal is produced at the input of operational amplifier 35 from signals originating at the U terminal, but signals originating on the balanced tip and ring terminals and which appear the output of operational amplifier 27 are cancelled in the circuit of operational amplifier 35. The latter signals are accordingly not applied to the transformer and do not appear as part of the signals coupled from the transformer to the circuit which provides and output to the tip and ring leads.

Signals originating at the U terminal appear at the output of operational amplifier 35.

Connected thereto is an AC coupling circuit including capacitor 55 and series resistor 56 which are coupled to the inverting input of operational amplifier 57. Feedback resistor 58 connects the output of operational amplifier 57 to its inverting input. Its noninverting input is connected to common or ground. The output of operational amplifier 57 is connected through resistor 59 to the noninverting input of differential amplifier 20.

Signals arriving from the U terminal at the output of operational amplifier 35 are coupled via capacitor 1 55 and resistor 56 to the input of operational amplifier 57. From there they are coupled to the noninverting input of operational amplifier 20. Preferably the gain of the stage which includes operational amplifier 57 is 1 /8, for the presence case of 900 ohm conversion. The gain of the stage including operational amplifier 20 to signals from operational amplifier 57 should be approximately 1/10 (with the value of resistor 59 about 1/10 the value of resistors 21 and 22), and the gain of operational amplifier 20 should be approximately - 1 /10 outgoing signals on the tip and ring lead applied by transistor 49. If the signals are identical, they will cancel within operational amplifier 20.

Signals translated to the tip and ring lead originating from the U terminals are applied to operational amplifier 20 via resistors 21- and 22, and are also applied thereto at the same level via operational amplifier 57. These signals therefore cancel, and do not pass back through amplifier 27 back to the U lead.

Accordingly the circuitry so far described has translated signals from the balanced tip and ring leads to the unbalanced U lead, at half the full original signal level. It has also translated signals originating on the unbalanced U lead at said half signal level and has applied them to the balanced tip and ring leads at full level. Yet the circuit has safeguarded against signals originating on the tip and ring leads being fed back through the signal path of the U lead to the tip and ring leads, and has safeguarded against signals originating on the U lead and applied to the tip and ring leads from being fed back from the tip and ring leads to the U lead. Only a single small inexpensive transformer has been used, bulky hybrid transformers are eliminated and the entire circuit can be provided on a printed circuit card.

The circuit has also provided an AC terminating impedance which matches the tip and ring impedance, and as well provides a DC impedance (through transistor 49) which is lower than the AC impedance.

The noninverting input of operational amplifier 44 has been noted as being connected to resistor 46 which is connected to the ring lead, to which a direct current is normally applied from the trunk. The non inverting input is also connected to the tip lead terminal of capacitor 53 via resistor 60. Resistor 60 can typically be 10 times the value of resistor 46.

The ratio of the two latter resistors, connected across the tip and ring leads provides a bias level to operational amplifier 44 preferably to cause it to have its operation point near the minimum expected trunk power supply potential level, for example 2 volts. Its DC current draw is determined by the resulting voltage across resistor 50. If desired, the noninverting input could be connected directly to the ring lead, which causes the operational amplifier 44 to act as a current source.

As described in our referenced patent application a power supply circuit for operational amplifier 44 is also provided, comprising transistor 61 which has its collector-emitter circuit connected between a power input terminal of operational amplifier 44 and the tip lead terminal of capacitor 53. The other power input of operational amplifier 44 is connected to the ring lead. The base of transistor 61 is connected to the junction of a bias voltage divider comprising resistor 62 connected to ground and in series with a light sensitive solid state device such as a phototransistor 63, the latter being connected be-tween the base and the tip lead end of capacitor 53.

Preferably the trunk circuit also includes a zener diode 75 which is connected across capacitor 53, to which the operational amplifier 44 power circuit is connected. Zener diode 75 acts as a voltage protector for the power circuit.

The tip and ring terminals also preferably are connected to the tip and ring leads through a bridge rectifier comprising diodes 76a, 76b, 76c and 76d, connected in conventional form. As is well known, the bridge rectifier will apply the correct voltage polarity for operation of this circuit to the T and R terminals, no matter what polarity is the DC voltage across the tip and ring leads. The circuit is therefore insensitive to polarity reversing signalling on the balanced trunk comprising the tip and ring leads.

The circuit described above is insensitive to common mode AC signals on the unbalanced pair, since they are cancelled in differential amplifier 20. It provides a balanced to unbalanced input, and an unbalanced to balanced input bidirectional amplifier, with safeguarding against sidetone or positive feedback of signals incoming from either trunk or the switching office. Complete control of the translated signal levels and of the AC and DC impedances is provided, as well as means for increasing the level of the incoming signal level in the event this is required due to conferencing on the unbalanced U terminal.

The signal applied to the tip and ring is also controllable by means of the variation of the ratio of a pair of resistors.

It will become clear that this circuit is highly useful as a universal trunk for a PBX or other telephone switching office. However its bidirectional attributes, impedance control and signal level control makes it equally applicable for use in other environments.

It should be noted that the secondary winding 83 of the transformer can be connected in a circuit to the input of a coder-decoder or other 4 wire apparatus, rather than to operational amplifier 44. The output of the coderdecoder can be connected to the T and R terminals, removing the requirement for diodes 76A, 76B, 76C and 76D, and the input of the coder-decoder can be connected in a circuit to the transformer secondary winding 83. In this structure the need for one of the circuit means to cancel signals from the unbalanced pair from feeding back thereto can be deleted. Hence operational amplifier 57.

resistors 56, 58 and 59 and capacitor 1 55 can be removed. The impedance of the T and R terminals and the output circuit connected to the secondary winding 83 can be adjusted in a manner known to persons skilled in the art to match the output and input circuits respectively of a coder-decoder or other 4 wire apparatus.

It may also become clear to a person skilled in the art understanding this invention that numerous other embodiments or variations may be made. All are considered within the sphere or scope of this invention, as defined in the appended claims.

Claims (25)

1. A signal translation circuit comprising: (a) tip and ring terminals for connection to tip and ring leads, for carrying signals including a direct current, (b) an a.c. terminating impedance connected between the tip and ring terminals, (c) a high impedance signal source connected between the tip and ring terminals, having a circuit path for conduction of direct current therebetween, (d) means for controlling the signal source including a first transformer having a low impedance input circuit connected to its primary winding, and a high resistance to D.C.

output circuit connected to its secondary winding, which is connected to the signal source for controlling the passage of current through said circuit path as a function of a voltage applied to the primary winding of the transformer, so as to apply a desired signal to the tip and ring terminals, (e) means for connecting a lead pair, including a terminal for carrying trunk signals, (f) an operational amplifier means having its output connected to the low impedance circuit, and its input connected in a signal path to the terminal for carrying trunk signals, whereby the tip and ring terminals are caused to carry signal currents corresponding to a signal current appearing at said terminal for carrying trunk signals, (g) first transformerless circuit means adapted to apply an incoming signal from the tip and ring terminals to said lead pair, (h) second transformerless circuit means adapted to cancel trunk signals from the said lead pair applied to the tip and ring terminals from being reapplied to said lead pair, and (i) third transformerless circuit means adapted to cancel signals from said first transformerless circuit means applied to said lead pair from being reapplied to the tip and ring terminals.

2. A trunk circuit as defined in claim 1 9 in which the high impedance signal source is comprised of the collector-emitter circuit of a transistor having a low resistance resistor connected in series therewith, the base electrode of the transistor being connected to the output of a second operational amplifier means, the high impedance input circuit of the transformer being connected between one of the inputs of the second operational amplifier means and the tip or ring lead, and also including a further resistor connected between said one of the inputs of the second operational amplifier means and the junction between the low resistance series resistor and the transistor, and further circuit means providing a low AC impedance path from said other input of the second operational amplifier means to said tip or ring lead.

3. A trunk circuit as defined in claim 2, in which the high impedance input circuit of the transformer is connected between the inverting input of the second operational amplifier means and the ring led, and the low resistance resistor is connected between the emitter of the transistor and the ring lead.

4. A trunk circuit as defined in claim 1, 2 or 3, in which the first transformerless circuit means includes a first differential amplifier means having a pair of inputs, each connected respectively through a high impedance to the tip and ring terminals, and an output connected via a cricuit path through a matching impedance to said terminals for carrying trunk signals; and the second transformerless circuit means includes a second differential amplifier having one input connected to said terminal for carrying trunk signals and another input connected through a first balancing impedance to the circuit path connected to the output of the first differential amplifier; the second differential amplifier being connected in series with a circuit path through a second balancing impedance to one of the inputs of the first differential amplifier comprising the third transformerless circuit means, the first balancing impedance being of such value as to apply signal to the second differential amplifier at a level such as to cancel signals therein arriving from the tip and ring leads, and the second balancing impedance being of such value as to apply signals from the second differential amplifier at a level such as to cancel signals therein arriving from the unbalanced terminal of the unbalanced lead pair.

5. A trunk circuit comprising: (a) a pair of terminals for connection to a tip and ring lead, (b) a first resistor for matching the impedance of a telephone line which may be connected to said terminals, connected in series with a capacitor having large capacitance for providing a low impedance to voice frequency signals, the series circuit being connected between said terminals, (c) an operational amplifier having a pair of inputs, one input being connected for AC through a high impedance circuit in series with the secondary winding of a transformer, (d) a transistor having its emitter connected through a low valued resistor to one of said terminals, its collector connected to the other terminal, and its base in a circuit path to the output of the operational amplifier, (e) a high valued resistor connected between said one input of the operational amplifier and the junction between the low valued resistor and the emitter, (f) a further operational amplifier, having its output connected in a low impedance circuit to the primary winding of the transformer, (g) means for-applying an unbalanced signal between the input terminals of the further operational amplifier, (h) first transformerless circuit means adapted to apply an incoming signal from the tip and ring terminals to said lead pair, (i) second transformerless circuit means adapted to cancel trunk signals from the said lead pair applied to the tip and ring terminals from being reapplied to said lead pair, and (j) third transformerless circuit means adapted to cancel signals from said first transformerless circuit means applied to said lead pair from being reapplied to the tip and ring terminals.

6. A trunk circuit as defined in claim 5 further including means for connecting power supply and bias leads of the first operational amplifier across the terminals of the capacitor and for supplying operating and bias voltage for the first operational amplifier thereby.

7. A trunk circuit as defined in claim 5, in which the means for applying an unbalanced signal is comprised of a third terminal for carrying a signal relative to common or ground, a third operational amplifier having an input connected to the third terminal and an output connected in a signal path to an input of the further operational amplifier, and comprising means for applying a trunk signal from said pair of terminals to said third terminal, and means for cancelling signals passing through the further operational amplifier from said pair of terminals, from being translated by said means for applying a trunk signal and thus being reapplied to said third terminal.

8. A trunk circuit as defined in claim 7 comprising means for cancelling signals passing through said means for applying a trunk signal from being translated by the second operational amplifier and thus being reapplied to said pair of terminals.

9. A trunk circuit as defined in claim 8 in which said means for applying a trunk signal and both said means for cancelling signals is comprised of a first differential amplifier having each of its inputs connected through resistors to corresponding ones of said pair of terminals, the value of each of the resistors being at least 100 times the impedance of the telephone line, a signal path connected from the output of the first differential amplifier to said third terminal, a signal path for applying a first predetermined portion of the output signal of the first differential amplifier connected to a second inverting input of the third operational amplifier, a fourth operational amplifier connected in a signal path with its input to the output of the third operational amplifier and its output connected to one of the inputs of the first differential amplifier for applying a second predetermined portion of the output signal of the fourth operational amplifier to the first differential amplifier, wherein said first predetermined portion is sufficient to cancel signals input to said pair of terminals from being translated in the third operational amplifier and said second predetermined portion is sufficient to cancel signals input to said third terminal from being translated by said first differential amplifier.

1 0. A trunk circuit as defined in claim 9, further including a fifth operational amplifier connected between the output of the first differential amplifier and the signal path to said third terminal, a resistor of value to match an external impedance between said third terminal and ground connected in series circuit between the output of the fifth differential amplifier and said third terminal, said signal path for applying a first predetermined portion of the output signal comprising a resistor connected to the output signal of the fifth operational amplifier.

11. A trunk circuit as defined in claim 10, in which the gain of the first differential amplifier is about 1 /10, the gain of the fifth operational amplifier is at least about 2, the gain of the third operational amplifier is sufficient to bring the level of a signal originating on said third terminal to the level of the signal originating at the pair of terminals, and the gain of the fourth operational amplifier is about 1/8.

1 2. A trunk circuit as defined in claim 10 or 11, further including means for selectively increasing the gain of the fifth operational amplifier, whereby the level of the signal applied to said third terminal can be increased.

1 3. A trunk circuit as defined in claim 5, 7 or 11 further including a bridge rectifier connected to the pair of terminals and a tip and ring lead.

14. A trunk circuit as defined in claim 5, 7 or 11 further including a break contact of a relay connected between said first resistor and the capacitor, circuit means connected to said third terminal for operating the relay, a power feeding circuit including a light sensitive solid state device which is adapted to cause opening of the power feeding circuit connecting the first differential amplifier to one terminal of the capacitor, and means for illuminating a light emitting device optically coupled to the light sensitive solid state device for causing illumination of the latter light emitting diode and thus causing the opening of the power feeding circuit.

1 5. A trunk circuit comprising: (a) a first lead pair for carrying two-way signals, (b) a second lead pair for carrying two-way signals, (c) first circuit means for applying an incoming signal from the second lead pair to the first lead pair, (d) second transformerless circuit means for applying an incoming signal from the first lead pair to the second lead pair, and (e) third transformerless circuit means for preventing signals from the first circuit means applied to the first lead pair from being reapplied to the second lead pair and for preventing signals from the second circuit means applied to the second lead pair from being reapplied to the first lead pair.

16. A trunk circuit comprising: (a) a first lead pair, (b) a second lead pair, (c) first circuit means for applying an incoming signal from the second lead pair to the first lead pair, (d) second transformerless circuit means for applying an incoming signal from the first lead pair to the second lead pair, (e) third transformerless circuit means for cancelling signals from said first circuit means applied to the first lead pair from being reapplied to the second lead pair and (f) fourth transformerless circuit means for cancelling signals from said second circuit means applied to the second lead pair from being reapplied to the first lead pair.

1 7. A trunk circuit as defined in claim 16, in which the first circuit means includes a transformer having its primary winding connected to a low impedance input circuit which is connected in a circuit to the second lead pair, and having its secondary winding connected in a circuit having very high resistance to direct current to an output circuit connected to the first lead pair.

18. A trunk circuit as defined in claim 1 7 in which the very high resistance circuit includes a capacitor connected in series therewith.

1 9. A trunk circuit as defined in claim 18, further including a current source, adapted to conduct current in response to a signal translated by the transformer, connected between the leads of the first lead pair.

20. A trunk circuit as defined in claim 1 9 in which the current source is comprised of the collector-emitter circuit of a transistor, the input of the transistor being connected to the output of an operational amplifier having an input to which the very high resistance circuit is connected.

21. A trunk circuit as defined in claim 16, 1 8 or 20, further including means terminating the first lead pair with a first matching A.C.

impedance and a second resistance means for conducting D.C. between the leads.

22. A trunk circuit comprising: (a) a first lead pair, (b) a second lead pair, (c) a third lead pair, (d) first circuit means for applying an incoming signal from the second lead pair to the third lead pair, (e) second transformerless circuit means for applying an incoming signal from the first lead pair to the second lead pair, and (f) third transformerless circuit means for cancelling signals from said second circuit means applied to the second lead pair from being applied to the third lead pair, in which the first circuit means includes a transformer having its primary winding connected to a low impedance input circuit which is connected in a circuit to the second lead pair, and having its secondary winding connected in a circuit having very high resistance to direct current to an output circuit connected to the third lead pair.

23. A trunk circuit as defined in claim 22 in which the very high resistance circuit includes a capacitor connected in series therewith.

24. A trunk circuit as defined in claim 22, further including means terminating the first lead pair with a first matching A C. impedance.

25. A trunk circuit as defined in claim 22 in which the first lead pair is adapted to be connected to an encoder and the third lead is adapted to be connected to a decoder.