CA2029139A1 - High voltage subscriber line interface circuit - Google Patents

Abstract

A HIGH VOLTAGE SUBSCRIBER LINE INTERFACE CIRCUIT

ABSTRACT
A high voltage subscriber line interface circuit is disclosed which is connected via a subscriber loop to a subscriber station. The subscriber line interface circuit of the present invention includes a tip and ring drive amplifier circuits that supply feed current to the subscriber loop. A common-mode amplifier circuit connected to the subscriber loop senses the voltage dropped across the subscriber loop and outputs a control voltage to the tip and ring drive amplifier circuits.
The control voltage offsets the feed current applied to the subscriber loop and develops a balanced longitudinal impedance. A differential amplifier circuit connected to the subscriber loop detects and converts differential transmit voice signals into a single unitary output signal. The unitary output signal is applied to an AC
summing amplifier circuit where it is summed and combined with a receive voice signal that is intended to be transmitted to the subscriber station. The resultant summed output signal is then applied to a phase splitter amplifier circuit. The phase splitter amplifier circuit develops a pair of balanced output signals from the summed voice signals that are equal in amplitude but 180 degrees out of phase with each other. Each of the output signals is connected to a respective one of the tip drive and ring drive amplifier circuits for transmission on the subscriber loop. A tip-party mark detector circuit is also included that produces a logic signal output when a voltage difference between the output signal of the common-mode amplifier circuit and a reference voltage is detected.

Description

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A HIGH VOLTAGE SUBSCRIBER LINE INTERFACE CIRCUIT

CROSS REFERENCE TO RELATED APPLI~ATIONS
Cross Reference is made to the related Canadian Patent Applications entitled: ~A Solid State Telephone Line Circuit,n (Attorney Docket 89-1-032), ~A Circuit For Synthesizing An Impedance Across The Tip And Ring Leads Of A Telephone Line Circuit,n (Attorney Docket 89-1-035), ~A Tip-Ring Short Detector and Power Shut-Down Circuit For A Telephone Line Circuit, n (Attorney Docket 89-1-036), ~A Thermal Protection Circuit For An ~ -Integrated Circuit Subscriber Line Interface,~ (Attorney Docket 89-1-037), nA Thermal Protection Arrangement For An Integrated Circuit Subscriber Line Interface,~
(Attorney Docket 89-1-038), ~A Control Circuit For A
Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-039), and ~A Ring Trip Detector For A Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-040) filed on the same date, and by the same assignee as this Application.

BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to the field of telecommunications and, more particularly, to a high voltage subscriber line interface circuit that provides an interface to a subscriber station apparatus.
2. Description of the Prior Art Telephone line circuits are customarily found in the telephone switchjing system or central office of a telecommunications network. The telephone line circuit interfaces the central office, to a telephone or subscriber station found at a location remote from the central office. The telephone line circuit functions to supply power or battery feed to the subscriber station via a two wire transmission line or subscriber loop and 3 ~
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to couple the intelligence or voice signal to and from the telephone switching system.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and/or inductors which carry up to 120ma dc. This passive circuit has a wide dynamic range, passing noise-free differential ~ignals while not overloading with the ~OHz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths where the current fed to the subscriber station is more than necessary for eqiualization. Active line-feed circuits can be less bulky and require lower total power, but -~
meeting dynamic range and precision balance reqiuirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Devices such as high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL
OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, ~A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface.n These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system.
However, presently known solid state line circuits, still suffer from deficiencies in meeting good transmission performance specifications. These :.

--~` 2~139 deficiencies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy.
Other problems presently encountered are excessive power dissipation at short loops that consume prodigious amounts of central office power and 2 wire input impedance circuits that are complex and that exhibit poor return loss.
Accordingly, it is an object of the present -~
lo invention to provide a new and more effective to a high voltage æubscriber line interface circuit that will effectively and efficiently couple a subscriber station apparatus to a telephone switching system.

DISCLOSURE OF THE INVENTION
The above and other objects, advantages, and capabilities are realized in a subscriber line interface circuit which is connected via the tip and ring leads of a subscriber loop to a subscriber station. The subscriber line interface circuit of the present invention includes a tip drive amplifier circuit connected to the tip lead of the subscriber loop. The tip drive amplifier circuit is arranged to convert feed voItage from a central office battery to tip feed current on the subscriber loop. Similarly, a ring drive amplifier circuit is connected to the ring lead of the subscriber loop. The ring drive amplifier circuit is arranged to convert feed voltage from a central office battery to ring feed current on the subscriber loop.
A common-mode amplifier circuit is included that is connected between the tip and the ring leads of the subscriber loop and to both the tip drive and the ring drive amplifier circuits. The common-mode amplifier ~ ;
circuit is arranged to sense the voltage dropped across the subscriber loop and output a control voltage to the tip drive and ring drive amplifier circuit. The control -voltage allows the tip drive and ring drive amplifier 2~291 39 circuits to offset the tip and ring feed currents applied to the subscriber loop, thereby, developing a balanced longitudinal impedance to ground at the tip and ring leads.
A differential amplifier circuit connected to the tip and ring leads of the subscriber loop is used to detect a voltage difference between the tip lead and the ring lead that is normally associated with the ~-transmission of voice signals from the subscriber lo station. The differential amplifier converts the detected differential transmit voice signals into a single unitary output signal. ~ -The output signal from the differential amplifier ;~
circuit is applied to an AC summing amplifier circuit.
The AC summing amplifier circuit receives the unitary transmit voice signal and also a receive voice signal that is intended to be transmitted to the subscriber station. The AC summing amplifier circuit sums and combines the transmit and receive voice signals and connects the resulting output æignal to a phase splitter amplifier circuit. The summed output signal also aids in ~-developing an input impedance for the subscriber line interface circuit of 900 ohms at 2.16 f at the tip and ring leads. ' ,, The phase splitter amplifier circuit develops a pair of balanced output signals from the summed voice signals that are equal in amplitude but 180 degrees out of phase with each other. Each of the output ~ignals is connected to a respective one of the tip drive and ring drive amplifier circuits for transmission on the subscriber ~ , loop.
The subscriber line interface circuit of the present invention further includes a tip-party mark detector circuit. The tip party mark detector circuit is connected to the common-mode amplifier circuit and to a reference voltage source. The detector is arranged to produce and output a logic signal, when a voltage 20-2~39 difference between the output signal of the common-mode amplifier circuit output and the reference voltage is detected.

BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention may be had from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:
Figure 1 is a block diagram of a solid state telephone line ~ircuit where the high voltage subscriber line interface circuit, in accordance with the present invention, is used to advantage.
Figure 2 is a block diagram of the high voltage subscriber line interface circuit, in accordance with the lS present invention.
Figure 3 is an electrical schematic showing in combination the Tip Drive Amplifier, Ring Drive Amplifier, Common Mode Amplifier, Phase Splitter Amplifier and AC Sum amplifier circuits, in accordance with the present invention.
Figure 4 is an electrical schematic of the XMT
Differential amplifier circuit, in accordance with the present invention.
Figure 5 is an electrical schematic of the Tip Party Mark Detector circuit, in accordance with the present invention.

DESCRIPTION OF A PREFERRED EM~ODIMENT
Directing attention first to Figure 1, a broad level block diagram o~f a solid state telephone line circuitlis shown. The line circuit is shown driving a subscriber station or telephone 10, via a subscriber loop 20. The subscriber loop 20 is comprised of a twisted two wire (2W) loop pair having a tip and a ring lead. The 2W loop is connected from the subscriber station 10 to a High Voltage Subscriber Line Interface Circuit (HVSLIC) 30.

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HVSLIC 30 feeds a -48 V dc voltage to the subscriber loop across the tip and ring leads from a central office battery (not shown). The HVSLIC 30 further functions to superimpose a voice signal on the dc ~eed voltage and also feed ringing current to subscriber loop 20 for signalling. The HVSLIC 30 still further functions to provide the 2W to 4W (four wire) hybrid function of splittinq the balanced signal on the tip and ring leads into separate transmit and receive paths that are ground referenced.
The control circuit 40 works in conjunction with the HVSLIC 30 to provide the dc loop current shaping and the line balance impedance portion of the 2W to 4W hybrid function. The control circuit 40 further controls various detection functions, such as, ring trip detection and loop sense detection, as well as, providing a logic interface to the central controller of the central office switching system. ~
Most modern digital telephone switching systems use ~-Pulse Coded Modulation (PCM) digital data to convey voice traffic through the central office switching system.
Therefore, some method of signal translation is required to convert the analog voice signals received by the interface circuit 30 to PCM digital data. This is typically accomplished by a PCM codec and filter circuit such as shown at 50. These devices are commercially available as a so called CODEC/FILTER COMBO~ from various manufacturers. Such as the TP30XX family of COMBO~
devices manufactured by the National Semiconductor Company. Analog voice data from subscriber station 10 is processed by the~PCM codec 50 and applied to the PCM bus of the central office switching system for transmission to its destination. Similarly, the codec 50 receives PCM
data from the switching system and converts the PCM data into analog signals which are superimposed on the dc feed ~ -voltage of the subscriber loop 20.
: : ;

-`` 202~39 The three solid state circuits 30, 40 and 50 just described, in combination embody a complete solid state line circuit adapted to connect a single subscriber station to a central office switch.
Referring now to Figure 2, a broad level explanation of the functions of HVSLIC 30, in accordance with the present invention, will now be given. The HVSLIC 30 shown in Figure 2, is constructed as a bipolar integrated circuit. All signals requiring high voltages and currents are interfaced by this device. With the addition of a few external discrete components a complete transformerless line interface can be constructed.
The Tip Drive Amplifier circuit 31 and Ring Drive Amplifier circuit 32 function to feed dc and ac voltages and currents to the TIP and RING leads respectively of subscriber loop 20. Voltage input into each amplifier 31 and 32 is converted into output current at the ~IP lead and RING lead respectively, of HVSLIC 30. The gain of each amplifier circuits 31 and 32 is set by using four precisely matched internal resistors and one precision external resistor (not shown). Each amplifier circuit 31 and 32 is capable of sourcing or sinking current depending on the input voltage.
The tip and ring amplifier circuits 31, 32 are connected to a Common-Mode Amplifier circuit 33.
Amplifier circuit 33 is used to sense the voltage across the subscriber loop 20 and to amplify the voltage by a factor of 3.33. The output of amplifier circuit 33 is fed to amplifier circuit 31 and Amplifier circuit 32.
Amplifier circuit 33 is dc biased at one half of the central office battery (Vbat) or VB/2. The Common-Mode Amplifier circuit 33 primarily functions to synthesize a longitudinal (common-mode) input impedance at the TIP and RING terminals of HVSLIC 30 that is a virtual ac ground.
Amplifier circuits 31, 32 and 33 are further connected to a Tip-Ring Short Protection circuit 34.
Circuit 34 functions as a detector, which monitors the :
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output of the Common-Mode Amplifier circuit 33. If the output of amplifier circuit 33 is 5.8 volts or closer to either ground or Vbat, the detector places amplifier circuits 31 and 32 into a high impedance mode. This condition represents a combined short of the TIP and/or RING leads to ground or Vbat. When in this short condition, the Tip Drive Amplifier circuit 31 and Ring Drive Amplifier circuit 32 are turned off to remove drive current from the subscriber loop 20.
10AC Sum Amplifier circuit 35 functions to sum the receive voice signals with the transmit voice signals.
Amplifier circuit 35 provides a ground referenced summing node at the SUMA terminal of HVSLIC 30. The transmit voice signals are fed to amplifier circuit 35 to 15synthesize an input impedance of 900 ohms at 2.16 f at the TIP and RING terminals.
The AC Sum Amplifier circuit 35 is connected to Phase Splitter Amplifier circuit 36. The Phase Splitter Amplifier circuit 36 performs two functions. First, it sums a dc control signal from the control circuit ~o applied to terminal SUMB, with the summed receive and transmit voice signals output by amplifier circuit 35.
Second, it level shifts and balances the now combined signals about VB/2 and applies the signals to two output leads. The signals output from the each of the two output leads of amplifier circuit 36 are equal in amplitude, but 180 degrees out of phase with each other.
One of the output leads of amplifier circuit 36 is connected to the Tip Drive Amplifier circuit 31 and the other to the Ring Drive Amplifier circuit 32~
The XMT Differential amplifier 37 functions to amplify the voltage difference between the TIP lead and the RING lead of the subscribers loop 20 and applied to terminals RPT and RPR, respectively. These voltage ~
differences which are analogous to the tran~mit voice signals are amplified and converted into a single ended output by amplifier circuit 37. The single ended voice ,, -8- `1 `-" 2~2~139 signals output from amplifier circuit 37 are connected to terminal XMTA and applied via a coupling network (not shown) to the SUMA terminal. As explained earlier in the description of the AC Sum Amplifier circuit 35, the transmit voice signals applied to the SUMA terminal are used by amplifier circuit 35 to synthesize the input impedance at the TIP and RING terminals. The transmit voice signals from XMTA are also applied to the control circuit 40, to drive the transmit input of that circuit.
lo A scaled down representation of the signals output from terminal XMTA is output from terminal XMTB. The signals from terminal XMTB are applied to a dc loop control circuit (not shown) on control circuit 40.
A Tip Party Mark Detector circuit 38 is connected to the common mode amplifier 33 and the VBAT/2 battery reference circuit 39. Detector 38 functions to sense a difference voltage between the output of Common-~ode Amplifier circuit 33 and the VB/2 voltage generated by circuit 39. When a differenced is sensed a logic signal is generated by detector 38 and applied to the TPM
terminal. Under normal operating conditions, the output voltage of amplifier circuit 33 equals the VB/2 reference voltage. However, when a tip party mark is placed on line, the tip current will not equal the ring current. ~-When the voltage from amplifier circuit 33 is sufficiently different than VB/2 the detector trips and outputs its logic signals at terminal TPM.
The VBAT/2 circuit 39 generates from the central office battery (not shown) a reference voltage which is approximately one half of the central office battery voltage input at VBAT. The VB/2 output voltage of circuit 39 is used as a reference by the Common-Mode 33, Phase-Splitter 36 and XMT Differential 37 amplifiers, as -well as the Tip-Ring Short Protection circuit 34. A
scaled down voltage output is generated by circuit 39 and output from the SVB terminal. This output is used by the :

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aforementioned dc loop control circuit of control circuit 40.
Finally, a Power Down and Thermal Shut Down circuit 131 is included which controls the power dissipation of the HVSLIC 30. The power down function of circuit 131 includes logic input controls arranged to receive logic input signals at terminal TRHZ. When the logic signal at TRHZ is a logic high or ~ln, all internal HVSLIC 30 amplifiers are put into a high impedance mode. The output drive current and internal bias current to all the amplifiers of HVSLIC 30 is cut off, and thus, the power consumption of HVSLIC 3 0 is reduced to a minimum. When a logic low or ~on is applied to TRHZ the HVSLIC 30 circuit functions normally. The thermal shut-down function of circuit 131 senses the temperature of HVSLIC 30 and cuts off drive current and internal bias current to the HVSLIC
30 amplifiers if the temperature reaches above lO0 degrees C. Therefore, trimming power dissipation.
Turning now to Figure 3, the Tip Drive, Ring Drive, Common-Mode, Phase-Splitter and AC Sum amplifier circuits, of the present invention, are shown in combination as an electrical schematic.
There are only two methods of feeding a subscriber ~ -loop while maintaining control of the dc and ac impedances required by the circuit. The first method uses voltage drive with series current feedback control.
The second, uses current drive with shunt voltage feedback control. The Second feed method avoids potential circuit instability which often occurs when a voltage output operational amplifier has a heavy capacitive load. The feed circuit of the present invention uses this second method.
Additionally, it is understood by those skilled in ~ v the art, that in the design of tip and ring drive operational amplifiers, that are configured as transconductance amplifiers, longitudinal balance of the subscriber loop circuit becomes essentially a matching of ~ 1 0-- ~' ' '' ~ ~ '''''' i ' ' ' `''' ' ;' ~ ', ' '-'.`'' ' ' 2~29~ 3~
certain pairs of resistors. Interdigitization of these matched resistor pairs, plus mirrored image layout techniques for operational amplifiers generally achieve good longitudinal balance.
With renewed reference to Figure 3, of the included drawings a subscriber loop 20 is shown comprising a TIP
lead and a RING lead. The loop 20 further includes resistors 210 through 213. Resistors 210 and 211 are the subscriber line 20 protection resistors (RP). Resistors 212 and 213 combine the TIP and ~ING voltages and feed the common-mode signal to the Common Mode Amplifier circuit 33. Resistors 214 and 215 represent the transmission network of the subscriber instrument. Tip Drive Amplifier circuit 31 in accordance with the present invention comprises operational amplifier (op amp) 310, and resistors 311 through 315. Similarly, a Ring Drive Amplifier circuit 32 comprises op amp 320 and resistors 321 through 325. Resistors 315 and 325 are the feed resistors (RF) of amplifier circuits 31 and 32 respectively. The two symmetrical amplifier circuits 31 and 32 are configured as transconductance amplifiers having a voltage to current gain. Amplifier circuit 31 ~ ;
provides drive current to the TIP lead of subscriber loop 20 and Amplifier circuit 32 provides drive current to the RING lead of the subscriber loop. The transconductance amplifier circuit of each circuit 31, 32 has a differential input and a bi-directional output so that current can be sourced or sunk depending on the differential input voltage applied to the non-inverting (positive) and inverting (negative) inputs of each op amp 310, 3~20.
A Common-mode Amplifier circuit 33 comprising op amp 330 and resistors 331 and 332. Amplifier circuit 33 is used to drive the inverting (negative) inputs of op amp 31 and 32. Resistors 212 and 213 combine the TIP and RING voltages and feed the common-mode signal to the positive input of op Amp 330. The output of amplifier : ~, -11- ;~.''';

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circuit 33 is a function of the bias voltage VB/2, developed by the VBAT/2 voltage reference circuit 39, ;
shown on Figure 2 and the common-mode voltage (Vcm). Vcm ~
can be expressed as: ~ , VTIP + VRING ;
Vcm = -----------When the Amplifier circuit 33 is connected to Amplifier circuits 31 and 32, for a gi~en voltage difference between the positive and negative inputs of op amp 310 and 320 the voltage Vcm will be equal to VB/2, or one half the effective central office battery voltage. The loop resistance of subscriber loop 20 floats between the voltage at the TIP and RING leads. The negative feedback produced by amplifier circuit 33 is such that common voltages induced on the TIP and RING leads will tend to offset in an equal manner the drive at the outputs of op amp 310, 312. This provides a balanced longitudinal impedance to ground. When (RP+RF)(gm)(l+Ra/Rb)=l the outputs of op amp 31 and 32 are at an ac virtual ground for any value of longitudinal current. That is, the voltage output by op amp 31 and 32, will not fluctuate as longitudinal current varies. Therefore, the longitudinal impedance can be simply expressed as (RP+RF) at the TIP
and RING leads. In pri~ciple a large value of longitudinal current that is within the current carrying capacities of amplifiers 31 and 32 can be handled by the -~, Amplifier circuits without incidences of voltage excursions at their outputs.
~ The common-mode eedback developed by amplifier , circuit 33 only effects signals common to the TIP and RING leads of the subscriber loop 20 and has no effect on the differential signals. One advantage to the arrangement shown in figure 3, is that the common-mode feedback is taken after resistors 210 and 211 or the RP -~
resistors. A close matching of RP, over a long period : ~ ~

2~139 can not be guaranteed due to lightning surges. However, common-mode feedback taken after resistors RP allows for circuit balance that is insensitive to RP matching.
Resistors 315 and 325, the RF resistors, exhibit the most sensitivity affecting circuit balance and must be matched to 0.1%.
With renewed reference to Figure 3, the Phase Splitter Amplifier circuit 36 and AC Sum Amplifier circuit 35 is shown. Amplifier circuit 36 includes op amp 360, input resistors 361 and 362 and feedback resistor 363. The positive output of op amp 360 is connected via resistor 312 to the positive input of op amp 310 of amplifier circuit 31. The negative output of op amp 360 is connected to the positive input of op amp 320 of amplifier circuit 32~ As explained above during the discussion of Figure 2, the Phase Splitter Amplifier circuit 36 performs two functions. First, it translates and level shifts a low level dc control signal from the control circuit 40 applied to terminal SUMB and second, it superimposes the summed output of the ac receive voice ~-signal and ac transmit voice signal. The output developed by op amp 360 is level shifted and referenced to VB/2. The positive and negative output signals are balanced around half the central office battery voltage.
The signals output from the two output leads of op amp 360 are equal in amplitude, but 180 degrees out of phase with each other.
The AC Sum Amplifier circuit 35 includes op amp 350 and rasistor 351. The negative input of op amp 350 is connected to terminal SUMA and its single output is connected to the positive input of op amp 360.
Amplifier circuit 35 functions to provide a ground referenced summing node at the SUMA terminal which sums the ac receive voice signals received from the CODEC
circuit 50 and shown on Figure 1, with the ac transmit voice signals output from the XMTA terminal of the XMT
Differential Amplifier circuit 56. The summation of the -2~9~39 ::
receive and transmit voice signals synthesize a 900 ohm 2.16~f input impedance across the TIP and RING leads of the sub~criber loop 20.
Turning now to Figure 4, a schematic representation of the XMT Differential Amplifier circuit 37 is shown.
Amplifier circuit 37 includes an op amp 370 having its positive lead connected to an input resistor 371 and to the RPT terminal. The negative lead of op amp 370 is connected to input resistor 372 and the RPR terminal. The RPT terminal is connected to the TIP lead and the RPR
terminal is connected to the RING lead of the subscriber loop 20. The output of op amp 370 is connected to feedback resistor 373 and to the XMTA terminal. A scaled version of the op amp 370 output is developed by a vo tage divider network comprising resistors 374 and 375 and output from terminal XMTB.
Amplifier circuit 37 functions to amplify the voltage difference between the TIP lead and the RING lead of the subscribers loop 20 and applied to terminals RPT
and RPR, respectively. These voltage differences which are analogous to the transmit voice signals are amplified and converted into a single ended output by op amp 370.
The single ended voice signals output from op amp 370 are connected to terminal XMTA and applied via a coupling ~ , network (not shown) to the SUMA terminal. As explained earlier in the description of the AC Sum Amplifier circuit 35, the transmit voice signals applied to SUMA
are used by amplifier 35 to synthesize the input impedance at the TIP and RING terminals. The transmit voice signals from XMTA are also applied to the control circuit 40, to drive the transmit input of that circuit.
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A sc~led down representation of the signals output from terminal XMTA is output from terminal XMTB. The signals from terminal XMTB are applied to a dc loop control circuit (not shown) on control circuit 40.
Turning now to Figure 5 of the included drawings, the Tip Party Mark Detector circuit 38 of the present -- 2~139 invention is shown. The Tip Party Mark Detector circuit 38 includes an op amp 380 having its positive input connected to the output of op amp 330 via a voltage resistors 381 and 382. The negative input of op amp 380 is connected to VB/2 and via revisors 383 and 381 to op amp 330. The output of op amp 380 is output via the TPM
terminal. Detector 38 functions to sense any out of balance voltages across the TIP and RING leads of subscriber loop 20. A logic signal is output from op amp 380, when op amp 380 senses a difference in voltage between the output of op amp 330, that is scaled down by resistor 381, 382 and 383, and a 83mV threshold developed by circuit 39. Under normal operating conditions, the output voltage of op amp 330 equals the VB/2 reference voltage. However, when a tip party mark is placed on line, the tip current will not equal the ring current.
When the voltage from op amp 330 is sufficiently different than VB/2, the detector circuit 38 trips and outputs its logic signal at terminal TPM.
The high voltage subscriber line interface circuit just described can be manufactured as a single compact large scale integrated circuit using any of the presently known film techniques used to build microcircuits.
Further, control circuit 40 can also be manufactured as a large scale integrated circuit suitable for mounting on a hybrid assembly. The hybrid assembly can thus provide the capabilities of a line circuit which in the past occupied a complete circuit card to a line circuit having a greater functional capability and occupying one sixteenth the same space. The solid state telephone line sircuit if the present invention also benefits from the increased reliability inherent in solid state construction as well the economies in labor cost and manufacture which are enjoyed by solid state devices.
It will be obvious to those skilled in the art that ~;
numerous modifications to the present invention can be made without departing from the scope of the invention as 2~29~ 39 defined by the appended claims. In this context, it should be recognized that the essence of the invention resides in a high voltage subscriber line interface circuit having the advantages and capabilities described herein.

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Claims (10)

1. A subscriber line interface circuit connected to a subscriber station via the tip lead and a ring lead of a subscriber loop, said subscriber line interface circuit comprising:
tip drive means connected to said tip lead arranged to convert feed voltage from a central office battery to tip feed current on said subscriber loop;
ring drive means connected to said ring lead arranged to convert feed voltage from a central office battery to ring feed current on said subscriber loop;
common-mode sensing means connected to said tip and said ring leads and to said tip drive and said ring drive means, said common-mode sensing means arranged to sense the voltage dropped across said subscriber loop and output a control voltage to said tip drive means and said ring drive means, offsetting the tip feed current and said ring feed current produced by said tip drive means and said ring drive means, respectively;
differential sensing means connected to said subscriber loop for detecting the voltage difference between said tip lead and said ring lead associated with the transmission of voice signals from said subscriber instrument;
AC summing means connected to said differential sensing means and to a source of receive voice signals intended to be transmitted to said subscriber station, said AC summing means arranged to sum and combine said receive voice signals and said transmit voice signals detected by said AC summing means; and phase splitter means connected to said AC summing means and to said tip drive and said ring drive means, said phase splitter means arranged to receive said summed and combined transmit and voice receive signals from said AC summing means, and develop a pair of balanced output signals that are of equal amplitude and 180 degrees out of phase with each other, whereby, each output signal of said pair of output signals is connected to a respective one of said tip drive and said ring drive means for transmission on said subscriber loop.

2. The subscriber line interface circuit as claimed in claim 2, wherein said subscriber line interface circuit further includes:
tip party mark detector means connected to said common-mode sensing means and to a reference voltage, said tip party mark detector means arranged to develop and output signal responsive to a difference between said common-mode sensing means control voltage and said reference voltage.

3. The subscriber line interface circuit as claimed in claim 2 wherein, said AC summing means in response to summing said receive and said transmit voice signals further develops an input impedance of 900 ohms at 2.16 f at the tip and ring leads of said subscriber loop.

4. The subscriber line interface circuit as claimed in claim 2, wherein said tip drive means is an operational amplifier circuit operating as a transconductance amplifier converting input voltage to output current gain.

5. The subscriber line interface circuit as claimed in claim 2, wherein said ring drive means is an operational amplifier circuit operating as a transconductance amplifier converting input voltage to output current gain.

6. The solid state line circuit as claimed in claim 5, wherein, said common-mode sensing means further includes a resistor network connected between said tip lead and said ring lead, said resistor network arranged to sense the voltage dropped across said subscriber line and develop a common-mode voltage.

7. The solid state line circuit as claimed in claim 6, wherein said common-mode sensing means comprises an operational amplifier circuit having its positive input connected to said resistor network and arranged to receive said common-mode voltage, and a negative input connected to a bias voltage, whereby said common-mode amplifier circuit develops and outputs to said tip and said ring drive amplifier circuits a negative feedback control voltage which offsets the current drive of said tip and ring drive amplifiers thereby, developing a balanced longitudinal impedance to ground at said tip and ring leads of said subscriber line.

8. In combination:
means for converting voltage to current connected to the tip and the ring leads of a subscriber loop, said means for converting arranged to convert input voltage to feed current on said subscriber loop;
means for sensing the voltage dropped across said subscriber loop, said means for sensing arranged to output a control voltage to said means for converting offsetting said feed current on said subscriber loop;
means for detecting connected to said tip and ring leads of said subscriber loop, said detecting means for detecting the voltage difference between said tip lead and said ring lead associated with the transmission of voice signals on said subscriber loop;

means for summing connected to said means for detecting and to a source of receive voice signals, said means for summing arranged to sum and combine said receive voice signals and said transmit voice signals detected by said means for detecting; and means for phase splitting connected to said means for summing and to said means for converting, said means phase splitting arranged to receive said summed and combined transmit and receive voice signals from said means for summing, and develop a pair of balanced output signals that are 180 degrees out of phase with each other and of equal amplitude, whereby, said pair of output signals are connected to said means for converting for transmission on said subscriber loop.

9. In combination:
a subscriber station connected to the tip lead and the ring lead of a subscriber loop;
first means for converting voltage to current connected to the tip lead of said subscriber loop, said first means for converting arranged to convert input voltage to feed current on said tip lead of said subscriber loop;
second means for converting voltage to current connected to the ring lead of said subscriber loop, said second means for converting arranged to convert input voltage to feed current on said ring lead of said subscriber loop;
means for sensing the voltage dropped across said subscriber loop, said means for sensing arranged to output a control voltage to said first means for converting and said second means for converting offsetting said feed current on said subscriber line, developing a balanced longitudinal impedance to ground at said tip and ring leads of said subscriber line;

means for detecting connected to said tip and ring leads of said subscriber loop, said means for detecting arranged to detect the voltage difference between said tip lead and said ring lead associated with the transmission of voice signals on said subscriber loop;
means for summing connected to said means for detecting and to a source of receive voice signals, said means for summing arranged to sum and combine said receive voice signals and said transmit voice signals detected by said means for detecting;
means for phase splitting connected to said means for summing and to said first and said second means for converting, said means for phase splitting arranged to receive said summed and combined transmit and receive voice signals from said means for summing, and develop a pair of balanced output signals that are 180 degrees out of phase with each other of equal amplitude, whereby, a first output signal of said pair of output signals is connected to said first means for converting and a second output signal of said pair of output signals is connected to said second means for converting for transmission on said subscriber loop; and means for detecting a tip party mark connected to said means for sensing and to a reference voltage, said means for detecting a tip party mark arranged to develop a logic output signal responsive to a difference in voltage between said means for sensing control voltage and said reference voltage.

10. In combination:
a subscriber station connected to the tip lead and the ring lead of a subscriber loop;
first means for converting voltage to current connected to the tip lead of said subscriber loop;
second means for converting voltage to current connected to the ring lead of said subscriber loop;

means for sensing the voltage dropped across said subscriber loop, connected to said first means for converting and said second means for converting, said means for sensing controlling the current connected to the tip lead from said first means for converting and to the ring lead from said second means for converting;
means for detecting connected to said tip and ring leads of said subscriber loop, said means for detecting arranged to detect a voltage difference between said tip lead and said ring lead associated with the transmission of voice signals on said subscriber loop from said subscriber station;
a source of receive voice signals intended to be transmitted to said subscriber station on said subscriber loop;
means for summing connected to said means for detecting and to said source of receive voice signals, said means for summing arranged to sum and combine said receive voice signals and said transmit voice signals detected by said means for detecting; and means for phase splitting connected to said means for summing and to said first and said second means for converting, said means for phase splitting arranged to receive said summed and combined transmit and receive voice signals from said means for summing, and develop a pair of balanced output signals that are equal in amplitude but opposite in a phase relationship with each other, whereby, a first output signal of said pair of output signals is connected to said first means for converting and a second output signal of said pair of output signals is connected to said second means for converting for transmission on said subscriber loop.