CA1105162A - Telephone line circuit - Google Patents
Telephone line circuitInfo
- Publication number
- CA1105162A CA1105162A CA296,137A CA296137A CA1105162A CA 1105162 A CA1105162 A CA 1105162A CA 296137 A CA296137 A CA 296137A CA 1105162 A CA1105162 A CA 1105162A
- Authority
- CA
- Canada
- Prior art keywords
- tip
- differential amplifier
- output
- input
- amplifier means
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M3/00—Automatic or semi-automatic exchanges
- H04M3/40—Applications of speech amplifiers
Abstract
TELEPHONE LINE CIRCUIT
Abstract of the Disclosure A transformerless bidirectional amplifier useful in a PBX4/2 wire line circuit, which blocks common mode signals and allows full control of signal levels, impedances, etc. The amp-lifier is comprised of a balanced tip and ring lead for connec-tion to a telephone set, and an unbalanced line output lead, first differential amplifier circuit having its input terminals connected through individual equal valued resistors to the tip and ring leads, the values of each of the resistors being at least 10 times the offhook impedance of the telephone set, a buffer amplifier having its input connected in a first circuit path to the output of the first differential amplifier circuit and its output connected in a second circuit path to a line out-put lead, and second differential amplifier circuit having its in-put terminals connected respectively in circuit paths to the first and second circuit paths. The output of the second differ-ential amplifier circuit is connected to one of the tip and ring leads and is also connected by a further resistors to the other input of the first differential amplifier circuit which is con-nected to the other of the tip or ring leads. The value of the further resistors is selected to apply sufficient output signal of the second differential amplifier circuit to the other input of the first differential amplifier circuit as to substantially cancel signal within the first differential amplifier circuit appearing at its input terminals from the one of the tip or ring lead from the output of the second differential amplifier cir-cuit.
Abstract of the Disclosure A transformerless bidirectional amplifier useful in a PBX4/2 wire line circuit, which blocks common mode signals and allows full control of signal levels, impedances, etc. The amp-lifier is comprised of a balanced tip and ring lead for connec-tion to a telephone set, and an unbalanced line output lead, first differential amplifier circuit having its input terminals connected through individual equal valued resistors to the tip and ring leads, the values of each of the resistors being at least 10 times the offhook impedance of the telephone set, a buffer amplifier having its input connected in a first circuit path to the output of the first differential amplifier circuit and its output connected in a second circuit path to a line out-put lead, and second differential amplifier circuit having its in-put terminals connected respectively in circuit paths to the first and second circuit paths. The output of the second differ-ential amplifier circuit is connected to one of the tip and ring leads and is also connected by a further resistors to the other input of the first differential amplifier circuit which is con-nected to the other of the tip or ring leads. The value of the further resistors is selected to apply sufficient output signal of the second differential amplifier circuit to the other input of the first differential amplifier circuit as to substantially cancel signal within the first differential amplifier circuit appearing at its input terminals from the one of the tip or ring lead from the output of the second differential amplifier cir-cuit.
Description
01 This invention relates to a transformerless bidirectional 02 ~ wire/~ wire line circuit which is usefully employed in a PsX.
03 Line circuits which are used, for example in a PBX provide 04 a facility for feeding direct current via a subscriber's line to 05 the local ~elephone sets to which they are connected, and as well 06 are required to block common mode signals while transmi-tting 07 voice frequency signals in bo-th directions. Common mode signals 08 often arise as a result of the subscriber's lines passing close 09 to a source of interference, such as house or building wiring which carries 60 Hz mains current. The voice frequency signals 11 which are otherwise carried by the telephone lines are often 12 found deeply modulated by the interfering common mode signals.
13 Slight imbalance between the tip and ring leads of a long line 14 would thus result in the common mode signal frequencies being ~15 differentially received. It is therefore important to stop the 16 transmission of common mode signals within the line circuit.
17 Common mode signals are usually stopped by the use of a 18 transformer in the line circuit. The tip and ring leads are con-19 nected to carefully balanced opposingly wound windings, which cancel the common mode signals. Since voice signals to 21 be transmitted are differential in nature with respect to the tip 22 and ring leads, these siynals do not cancel and as a result are 23 induced in the secondary winding and are applied to further ~24 switching or other circuitry.
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However, since direct current must be fed to the local sub-~i26 scriber's telephone set, it is normally passed through the 27 windings of the line circuit transformer~ This direct current 28 tends to saturate the transformer, requiring a relatively heavy ~29 core which does not saturate at normal line currents.
It is aIso hi~hly desirable to apply ringing current to 31 the local subscriber's llne from -the line circuit, and to be able ~ ~`? ~
*~
~;
~::
:
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~ ~5~ 2 Ol to se~se the state of tne subscriber's telephone set should the 02 telephone set go o~f hook.
03 For use particularly with an electronic PBX ~7hich uses t~o 04 wire switching (that is, one wire connected to common ground and 05 one wire actually switched), it is necessary to convert the 06 balanced subscribers line into an unbalanced line easiLy Eor 07 switching, while at the same time raising the amplitude oE a 08 received signal from other telephone sets to a level required by 09 the local subscribers ~telephone set. It is also pre~erable to be able to change the output signal levels at will in order that the ll switched signal levels might be equalized in the case of a 12 conference call controlled by the PBX.
13 The invention described in U.S. patent 4,007,335 dated 14 February 8, 1977, assigned to Bell Telephone Laboratories, lS utilizes an integrated circuit which is connected between the tip l~ and ring leads and is intended to reduce common mode signal.s~
17 This circuit exhibits a high impedance to diferential voice 18 signals appearing on the tip and ring leads, and exhibits a low 19 impedance between the tip and ring leads and ground for common mode interference signals. While the described circuit would 2l operate satisEactorily for the application to which it ;9 22 intended, by its nature it cannot transEorm a balanced line to an 23 unbalanced line (which~otherwise would be done by a transEormer 24 circuit). Accordingly it cannot be used in a two wire switching network, unless a following trans~ormer is added.
26 Further, there is no facility for sensing the direct ~27 current level in the subscriber's line for determining the on or 28 off hook s~tatus thereof. There is further no acility for 29 changing tbe signal levels applied to the switching matrix.
Neither is there a facility for applying ringing current to the 3l subscribers line nor for cutting it oEf when the subscriber has ~32 2 :,: .: , ~ . . . .
01 c~one of~ hoolc.
02 The p~esent invention, on the otner hand~ is a circuit 03 ~7hich is comprised of a four to two wire bidirectional ampli-0~ Eier. The present circuit transEor~s a balanced tip and ring 05 cirsuit to an unbalanced line, and at the same time trans:Eorms an 06 unbalanced line to a balanced lead pair. Accordingly it can be 07 used to feed a 2 wire switching matrix directly without requiring 08 an additional transEormer. The hidieectional amplifier per~orrns 09 without in-troducing positive feedback or undesired sidetone. ~t the same time the circuit substantially bloclcs common mode sig-11 nals which may appear on the balanced pair from bei~g transfered 12 -to the unbalanced output line.
13 The circuit also provides means for increasing the ampli-1~ tude oE the outgoing signal to the unbalanced line in order that the reduced impedance, which may appear from the line to common 1~ ground due to the connection thereacross of a plllrality of con-17 ferencing telephone sets which would otherwise.result ln a reduc-18 tion in signal amplitude~ is compensated for.
l9 At the same time, the circuit provides means for detecting the on or off hook status of the subscribers line, for applying 21 ringing to the su~scribers line, and for causing ringing to be 22 cut oE upon the local subscriber's telephone set going off hook.
23 The advantages of the invention are obtained by the provi-24 sion of a telephone line circuit comprising a balanced tip and ring lead for connection to a telephone set9 and an unbalanced
03 Line circuits which are used, for example in a PBX provide 04 a facility for feeding direct current via a subscriber's line to 05 the local ~elephone sets to which they are connected, and as well 06 are required to block common mode signals while transmi-tting 07 voice frequency signals in bo-th directions. Common mode signals 08 often arise as a result of the subscriber's lines passing close 09 to a source of interference, such as house or building wiring which carries 60 Hz mains current. The voice frequency signals 11 which are otherwise carried by the telephone lines are often 12 found deeply modulated by the interfering common mode signals.
13 Slight imbalance between the tip and ring leads of a long line 14 would thus result in the common mode signal frequencies being ~15 differentially received. It is therefore important to stop the 16 transmission of common mode signals within the line circuit.
17 Common mode signals are usually stopped by the use of a 18 transformer in the line circuit. The tip and ring leads are con-19 nected to carefully balanced opposingly wound windings, which cancel the common mode signals. Since voice signals to 21 be transmitted are differential in nature with respect to the tip 22 and ring leads, these siynals do not cancel and as a result are 23 induced in the secondary winding and are applied to further ~24 switching or other circuitry.
.. . .
However, since direct current must be fed to the local sub-~i26 scriber's telephone set, it is normally passed through the 27 windings of the line circuit transformer~ This direct current 28 tends to saturate the transformer, requiring a relatively heavy ~29 core which does not saturate at normal line currents.
It is aIso hi~hly desirable to apply ringing current to 31 the local subscriber's llne from -the line circuit, and to be able ~ ~`? ~
*~
~;
~::
:
'`: .
~ ~5~ 2 Ol to se~se the state of tne subscriber's telephone set should the 02 telephone set go o~f hook.
03 For use particularly with an electronic PBX ~7hich uses t~o 04 wire switching (that is, one wire connected to common ground and 05 one wire actually switched), it is necessary to convert the 06 balanced subscribers line into an unbalanced line easiLy Eor 07 switching, while at the same time raising the amplitude oE a 08 received signal from other telephone sets to a level required by 09 the local subscribers ~telephone set. It is also pre~erable to be able to change the output signal levels at will in order that the ll switched signal levels might be equalized in the case of a 12 conference call controlled by the PBX.
13 The invention described in U.S. patent 4,007,335 dated 14 February 8, 1977, assigned to Bell Telephone Laboratories, lS utilizes an integrated circuit which is connected between the tip l~ and ring leads and is intended to reduce common mode signal.s~
17 This circuit exhibits a high impedance to diferential voice 18 signals appearing on the tip and ring leads, and exhibits a low 19 impedance between the tip and ring leads and ground for common mode interference signals. While the described circuit would 2l operate satisEactorily for the application to which it ;9 22 intended, by its nature it cannot transEorm a balanced line to an 23 unbalanced line (which~otherwise would be done by a transEormer 24 circuit). Accordingly it cannot be used in a two wire switching network, unless a following trans~ormer is added.
26 Further, there is no facility for sensing the direct ~27 current level in the subscriber's line for determining the on or 28 off hook s~tatus thereof. There is further no acility for 29 changing tbe signal levels applied to the switching matrix.
Neither is there a facility for applying ringing current to the 3l subscribers line nor for cutting it oEf when the subscriber has ~32 2 :,: .: , ~ . . . .
01 c~one of~ hoolc.
02 The p~esent invention, on the otner hand~ is a circuit 03 ~7hich is comprised of a four to two wire bidirectional ampli-0~ Eier. The present circuit transEor~s a balanced tip and ring 05 cirsuit to an unbalanced line, and at the same time trans:Eorms an 06 unbalanced line to a balanced lead pair. Accordingly it can be 07 used to feed a 2 wire switching matrix directly without requiring 08 an additional transEormer. The hidieectional amplifier per~orrns 09 without in-troducing positive feedback or undesired sidetone. ~t the same time the circuit substantially bloclcs common mode sig-11 nals which may appear on the balanced pair from bei~g transfered 12 -to the unbalanced output line.
13 The circuit also provides means for increasing the ampli-1~ tude oE the outgoing signal to the unbalanced line in order that the reduced impedance, which may appear from the line to common 1~ ground due to the connection thereacross of a plllrality of con-17 ferencing telephone sets which would otherwise.result ln a reduc-18 tion in signal amplitude~ is compensated for.
l9 At the same time, the circuit provides means for detecting the on or off hook status of the subscribers line, for applying 21 ringing to the su~scribers line, and for causing ringing to be 22 cut oE upon the local subscriber's telephone set going off hook.
23 The advantages of the invention are obtained by the provi-24 sion of a telephone line circuit comprising a balanced tip and ring lead for connection to a telephone set9 and an unbalanced
2~ line output lead, first diferential amplifier means having its 27 input terminals connected through individual equal valued resis-28 tor means to the tip and ring leads, the values of each o the .29 resistor means being at least lO times the offhook impedance of ~ the telephone set:, a buEfer amplifier having its input connected ~3~1 in a firsc circuit path~to the output of the first differential 32 : 3 :
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01 ampllfier means and its output connected in a second circuit path 02 to a line output lead, and second diEferential amplifier means 03 having its input terminals connected respectivel~ in circuit 04 paths to the first and second circuit paths. The output oE the 05 second differential amplifier means is connected to one of the 06 tip and ring leads and is also connected by a further resistor 07 means to the other input of the Eirst differential amplifier 08 means which is connected to the other of the tip or riny leads 09 The value of the further resistor means is selected to apply sufficient output signal from the second differential amplifier 11 means to the other input of the first differential amplifier 12 means which is connected to said other of the tip or ring leads 13 such as to substantially cancel a signal within the first 1~ differential amplifier means which is applied to its inpu-t terminals via the one of the tip or ring leads from the output of 16 the second differential amplifier means.
17 It is preferred that the first differential amplifier 18 means should have a gain of less than one, and that the buffer 19 amplifier should have a gain of about the reciprocal of the gain of the first differential amplifier means. It is also preferred ~21 that the second differential amplifier means should be comprised ;22 of amplifying means having gain witch is different for different 23 signals applied to each of its inputs from the Eirst circuit path 24 and from the second circuit path respectively. The gains are selected such as to translate the individual signals applied from 26 the output of the first differential amplifier means to each of 27 the inputs of the second differential amplifier means to 28 amplitude levels which are substantially equal so as to 29 ;substantially cancel within the second differential amplifier 30 means.
31 A better understanding of the invention will be obtained 32 by reference to the detailed description below, and to the .. . , .. i . . .
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01 followiny drawings, in which:
02 Figure 1 is a block schematic of the most gener~l form o 03 the invention, 04 Figure 2 is a detailed schematic oE the invention, and 05 Figure 3, which appears on the same sheet as Fiyure 1, is a 06 detailed schematic of a dual gain ampliEier used in the invention.
07 Turning first to Figure 1, a general block schematic of 08 the invention i5 shown. A balanced line comprising tip and ring 09 leads is connected via equal valued resistors 1 and 2 to differ-ential amplifier means 3. The output of differential amplifier 11 means 3 is connected to the input of a buffer amplifier 4 and ~o 12 the non-inverting input of a differential amplifier means 5. The 13 output of differential amplifier means 5 is connected to one of 14 either the tip or ring leads (shown here as the tip lead), and ~15 through a resistor 6 to the input of differential amplifier means ~16 3 to which the other of the tip and ring leads is connected 17 through one of the aforenoted equal valued resistors. Unbalanced 18 line output 7 is connected to the output of buffer amplifier 4, 19 and is also connected to the inverting input of differential amp-lifier means 5.
~21 In opera~ion, a subscriber's telephone set will be con-22 nected between the tip and ring leads. Signals are generated 23 ~hereby, and appear as a differential signal across the tip and 24 ring leads. Differential amplifier means 3 therefore translates the signal~ and converts it at the same time to an unbalanced ~2~6 outpu~ signal between its output and ground. This signal is 27 applied to buffer amplifier 4 whlch applies it to the unbalanced 28 line output 7.
;29 Common mode interference signals appearing on the tip and ~30 ring leads, however, are in phase on both leads with respect to 31 ground, and are applied with equal amplitude and phase ~o the in-32 puts of differential amplifier means 3~ Accordingly since there
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01 ampllfier means and its output connected in a second circuit path 02 to a line output lead, and second diEferential amplifier means 03 having its input terminals connected respectivel~ in circuit 04 paths to the first and second circuit paths. The output oE the 05 second differential amplifier means is connected to one of the 06 tip and ring leads and is also connected by a further resistor 07 means to the other input of the Eirst differential amplifier 08 means which is connected to the other of the tip or riny leads 09 The value of the further resistor means is selected to apply sufficient output signal from the second differential amplifier 11 means to the other input of the first differential amplifier 12 means which is connected to said other of the tip or ring leads 13 such as to substantially cancel a signal within the first 1~ differential amplifier means which is applied to its inpu-t terminals via the one of the tip or ring leads from the output of 16 the second differential amplifier means.
17 It is preferred that the first differential amplifier 18 means should have a gain of less than one, and that the buffer 19 amplifier should have a gain of about the reciprocal of the gain of the first differential amplifier means. It is also preferred ~21 that the second differential amplifier means should be comprised ;22 of amplifying means having gain witch is different for different 23 signals applied to each of its inputs from the Eirst circuit path 24 and from the second circuit path respectively. The gains are selected such as to translate the individual signals applied from 26 the output of the first differential amplifier means to each of 27 the inputs of the second differential amplifier means to 28 amplitude levels which are substantially equal so as to 29 ;substantially cancel within the second differential amplifier 30 means.
31 A better understanding of the invention will be obtained 32 by reference to the detailed description below, and to the .. . , .. i . . .
, . . .. .
. ,-. , . - . : ,, . . . :
.. .
01 followiny drawings, in which:
02 Figure 1 is a block schematic of the most gener~l form o 03 the invention, 04 Figure 2 is a detailed schematic oE the invention, and 05 Figure 3, which appears on the same sheet as Fiyure 1, is a 06 detailed schematic of a dual gain ampliEier used in the invention.
07 Turning first to Figure 1, a general block schematic of 08 the invention i5 shown. A balanced line comprising tip and ring 09 leads is connected via equal valued resistors 1 and 2 to differ-ential amplifier means 3. The output of differential amplifier 11 means 3 is connected to the input of a buffer amplifier 4 and ~o 12 the non-inverting input of a differential amplifier means 5. The 13 output of differential amplifier means 5 is connected to one of 14 either the tip or ring leads (shown here as the tip lead), and ~15 through a resistor 6 to the input of differential amplifier means ~16 3 to which the other of the tip and ring leads is connected 17 through one of the aforenoted equal valued resistors. Unbalanced 18 line output 7 is connected to the output of buffer amplifier 4, 19 and is also connected to the inverting input of differential amp-lifier means 5.
~21 In opera~ion, a subscriber's telephone set will be con-22 nected between the tip and ring leads. Signals are generated 23 ~hereby, and appear as a differential signal across the tip and 24 ring leads. Differential amplifier means 3 therefore translates the signal~ and converts it at the same time to an unbalanced ~2~6 outpu~ signal between its output and ground. This signal is 27 applied to buffer amplifier 4 whlch applies it to the unbalanced 28 line output 7.
;29 Common mode interference signals appearing on the tip and ~30 ring leads, however, are in phase on both leads with respect to 31 ground, and are applied with equal amplitude and phase ~o the in-32 puts of differential amplifier means 3~ Accordingly since there
3~3 is no difference between the signals across the input of the amp-~ $
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01 lifier means, the signals are not amplifie~ and clo not appear at 02 the output of differential amplifier means 3.
03 Accordingly the circuit so far described amplifies voice 04 frequency signals in one direction, while changing the balanced 05 subscriber's line into an unbalanced line output. At the same 06 time common mode signals are stopped.
07 As noted earlier~ however, the inventive circuit is a bi-08 directional amplifier, and also includes means for amplifying 09 signals arriving from the unbalanced line and translating them into signals applied to the balanced line. Differential ampli-11 fier means 5 has a non-inverting input connected to the first 12 circuit path between the output o:E differential amplifier means 3 13 and the input of buEfer amplifier 4. The inverting inpu-t oE dif-14 ferential amplifier means 5 is connected to the second circuit path from the output of buffer amplifier 4 to unbalanced line ~16 output 7.
17 The signal arriving from -the line output 7 is thus applied 18 for amplification to differential amplifier means 5. However 19 this signal would also include a signal arising from the first described tip and ring leads as amplified and passed through :21 differential amplifier 3 and buffer amplifier 4O The signal from 22 the output of differential amplifier means 3 is applied to both ~23 inputs of differential amplifier means 5 in such manner that, 24 with the respective gains of amplifier means 5 applied to the 25 ~ signals from the respective inputs, the signals from differential ~ -Z6 amplifier 3 are caused to cancel, and are not reapplied from the ~27 output of differential amplifier 5 back to the tip and ring :28 leads. Accordingly positlve feedback, excessive side tone or the 29 like is avoided. The manner in which the specific cancellation occurs will be described in more detail in conjunction with 31 figure 2.
3~2 It should be noted that bu~fer amplifier 4 should there-:
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01 fore be of the non-invertiny t~pe, for the polarity of the tip 02 and ring signal as applie~ to ~ifferential amplifier means 5 to 03 be correct. Furthermore, buffer ampliEier ~ Eunctions as an 04 isolator, restricting signals arriving from line output 7 from 05 being applied to the non-inverting input of differential 06 amplifier 5. Accordingly signals arriving from line output 7 are 07 differentially applied to differential amp]ifier means 5 an~ are 0~ thus amplified and applied to the tip lead.
09 It will be understood that the signal applied to the tip lead is as a result also applied as a differential signal to the 11 input of differential amplifier 3. Resistors 1 and 2 which are 12 connected in series with the respective tip and ring leads and 13 the inputs o difEerential amplifier means 3 should be of high 14 resistance, each preferably at least 100 times the offhook ~15 impedance of the telephone set. This reduces the differential 16 signal level appearing at the input of differential amplifier 17 means 3 from the output of differential amplifier means 5 (the 18 signal arriving from the line output3 by at least 200 times.
19 This lower amplitude signal is cancelled by the applica-20 tion of signal from the output of differential amplifier 5 via 21 resistor 6 to the other input of differential amplifier 3. Re-22 sistor 6 should therefore be at least 200 times the off-hook impe-23 dance of the telephone set, and should be of value such as to 24 apply the exact amount of signal to the differential amplifier 3 ~25 to cancel the residual reduced signal appearing thereat from the 26 output of differential amplifier means 5.
27 Accordingly signals appearing on the unbalanced line 7 are 28 applied to the balanced the tip and ring leads, with virtually no 29 sidetone feedback back to the line.
To inhibit the formation of unbalanced currentsl it is pre-~.
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, 01 ferred that resistors 1 and 2 each should have tolerance within 02 0.]~.
03 Figure 2 is a schematic diagram showing the inventive 04 telephone line circuit in detail. The tip and ring leads are 05 connected through resistors 1 and 2 to the respective input 06 terminals of differential operational amplifier 10. The 07 operational amplifier has a feedback resistor 11 connected 08 between its output and its inverting input in the conventional 09 manner. ~s noted with respect to Figure 1, resistors 1 and 2 should each be at least 100 times the impedance of the off-hook 11 telephone set which is to be connectecl to the tip and ring leads 12 (although it could in some circumstances and special designs, be 13 less). Resistors 12 and 13 connect the invertiny and the 14 noninverting input terminals of operational amplifier 10 to ground.
~16 The output oE operational amplifier 10 is connected 17 through a DC blocking capacitor 14 to the input of a switch 15.
18 Preferably switch 15 is an externally enabled CMOS switch.
19 The output of switch 15 is connected both to the non-in-verting input of a second differential amplifier 16 and to the 21 noninverting input of a third differential amplifier 17, (which 22 corresponds to buffer amplifier 4). The output of third differ-23 ential amplifier 17 is connected through resistor 18 to the line 24 output terminal 19.
Line output terminal 19 is connected through resistor 20 ~26 to the inverting input of second diEferential amplifier 16. The 27 inverting and noninverting inputs thereof are connected through 28 respective resistors 21 and 22 to ground.
29 It is preferred that resistor 18 be of similar impedance as the line which is connected to line output terminal 19, for 31 matching purposes, since the output impedance of differential .~ .
01 amplifier 17 wou:Ld otherwise be very low.
02 The inverting input of third differen-t:ial amplifier 17 i5 03 connected to ground through resistor 21, and :i.s also connected 04 via a feedback resistor 22 to the output of differential ampli-05 fier 17.
06 The inverting input of third di.fferential amplifier 17 is 07 also connected through individual resistors 23 and 24 in series 08 with make contacts 25 and 26 to grouncl. Accordingly should 09 either contacts 25 or 26 be closed, resistors 23 or 24 are con-nected in parallel with resistor 21. Since the gain oE amplifier 11 17 for signals applied to the non-inverting input is determined 12 by the ratio oE the value of resistors 22 to 21, the gain of this ~13 stage can be effectively changed by connecting either resistor 23 14 or resistor 24 in parallel with resistor 21. Switches 25 and 26 ~;15 are closed by an externally operated switching matrix~ no~ shown.
16 The output of second differential amplifier 16 is connect-17 ed through a low valued resistor 125 of approximately line .18 impedance to the input of a power amplifier stage comprising 19 transistors 126 and 127. The bases of NPN transistor 126 and PNP
~20 transistor 127 are connected together to resistor 125. The 21 collector of transistor 126 should be connected to a positive 22 source of direct current, such as +8V, and the collector of ~23~ transistor 127 should be connected to a source of negative 24~ potential, such as -lOV. The emitters are connected together, ;25 and clearly are at close to 0 volts DC.
~26~ The emitters of transistors 126 and 127 are connected ;27 through resistor 28 to the inverting input of differential ampli-.28~ fier 16, which resistor is bypassed by a small high frequency .
~29~ rolloff capacitor 29.
30 ~ The operation of differential amplifier 16 with resistor ~31 125 and transistors ]L26 and 127 as well as feedback resistor 28 32~ is simi1ar to a~differential amplifier with a normally connected ~33 9 :: :
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01 ~Eeedback loop, but haviny a high current capacity output stage 02 The output stage is required to fee~ line current to the tip and 03 ring leads as will be described below4 04 The emitters of transistors 126 and 127 are connected OS through a resistor 30 to the tip lead, and the ring lead is con-06 nected through a resistor 31 to a source of potential such as -4 07 volts.
08 As was described with respect to resistor 6 in Figure 1, 09 the emitters of transistors 126 and 127 are connecte~ through re-sistor 32 to the non-inverting input of operational amplifier 10.
11 Resistors 30 and 31 should each have the value of 1/2 the 12 tip and ring line impedance, the latter being approximately the 13 value of the off-hook telephone station set impedance. It is im-14 portant that resistors 30 and 31 should be very closely matched in value, for instance to 0.1%.
16 Further, resistors 12 and 32 should be also as closely 17 matched in value, and should be at least twice the value of re~
~18 sistors 1 and 2, and should be at least ln times the line impe-19 dance, i.e. 10 times the impedance of the off-hook telephone set. Resistor 13 should be the same value as resistor 11, with `21 the aforenoted 0.1% tolerance, for the purpose of ensuring that 22 ~he tip and ring leads are not subjected to imbalance.
23 In operation, a subscriber's telephone set is assumed to 24 go off-hook. Battery voltage is supplied through transistors 126 25 ~ and 127 to the tip lead through resistor 30, and passes through 26 the telephone set, the ring lead, and through resistor 31 to the 27 other terminal of th~ battery supply, -48 volts It is important 28 that the battery feed circuit appear as a low impedance voltage 29 source.
The ratio of the sum of the resistances of resistors 1 and 31 2 to the line impedallce provides a voltage division of any common 1: .
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01 mo~e signcll, which may appear on t'ne tip and ring le~ds, as 02 applied to ampliEier 10. While equal valued common mode signals 03 beiny applied to the input terminals oE differential amplifier 10 04 would cancel in any event it is preferred to reduce their amp-05 litude in case there is a diferential inter~erence componen~
05 modulated on the common mode signals~ and in the case o~ differ-07 ences in the values of resistors } and 2, which would give rise 0~ to a differential comppnent.
09 It is preferred that the amplific~tion oE the circuit which includes di~ferential amplifier 10 s'nould be about 1/10.
11 Accordingly if resistor ~ is lOOrOOO ohms for instance, resistor 12- 11 should be about 10,000 ohms.
13 DiEferential voice frequency signals are also sub~ected to l4 a reduction in amplitude o 1/10. These signals are applied through capacitor 14, through switch 15, to the non-inverting in-16 put of differential amplifier 16. They are also passed through 17 buffer 17, resistor 18 and resistor 20 to the ~nverting input o-~
-18 differential amplifier 16. The ratio o~ resistors 22 to 21 pre-19 ferably -is 10 to 1, which provides an amplification oE 10 of -the voice signals applied to buffer amplifer 17. This compensates 21 completely of the signal loss in the circuit oE amplifier 10, and -~ ~ 22 the original dlfferential voice signal amplitude is restored.
23 However, resistor 18, as was noted earlier, should have 24 a similar impedance as the unbalanced line at line output term-~25 inal 19, and accordingly the output signal at terminal 19 is 1/2 26 the input voice frequency signal across the tip and ring leads.
~27 The signal at output line terminal 19 is applied to a 2B crosspoint matrix 33, and is thus connected t~rough the PBX to a 29 similar terminal 19 of another line circuit o~ similar struc-ture. Th~ line level of a signal~at ~he line terminal 19 arriv .
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01 ing from another line circuit is therefore at 1/2 the tip and 02 ring signal amplitude level. Both the signal from the local tip 03 and ring leads passing through buffer amplifier 17 and the incom-04 ing signal are applied through resistor 20 to the inverting input 05 of differential ampliEier 16.
06 It should be noted that in the expressions to be given 07 below with respect to gain, the entire circuit including output 08 transistors 126 and 127 as well as resistor 125 and differential 09 amplifier 16, and their accompaning directly connected resistors including feedback resistor 28 are considered as a single ampli-11 fier circuit 34.
12 Amplifier 34 has been designed to ampliEy different sig-13 nals with different predetermined amounts of gain. An example of 14 its operation will be given, using typical values for the assoc-iated resistors. As an easily calculated example, resistor 20 16 can be 40.2K ohms, resistor 21 can be 9.53K ohms, and resistor 28 ~17 can be 200K ohms.
18 Accordingly~ for signals appearing at the non-inverting 19 input of amplifier 34 which arrived from -the output of differen-tial amplifier 10, the gain of the stage is $he ratio of the 21 value of resistor 28 to resistors 20 and 21 in parallel (since 22 resistor 20 is connected to the line input, which is at relative-23 ly low impedance to ground, typically 600 ohms).
24 For the example shown, the gain is 200K/9-53K_X 40.2K
~25 ~ (9.43X + 40.2K) 26~ which e~uals approximately 26 (non-inverting).
27 For signals arrived from the line terminal l9r the gain of 28 the stage is the ratio of resistors 28 and 20. Accordingly the 29 gain in this case is 200K/40.2K which equals approximately 5 (inverting).
31 As was noted earlier, the emitters of transistors 126 and ~32~ 12 . . . , , :
- .- ., , ~, . . - : .
01 127 are connected through resi.stor 30 to the tip lead, the ring 02 lead being connected through resistor 31 to eEfective AC ground 03 (but which resistor in reality forms the DC line feed). The 04 impedance between the tip and ring lead with the subscri.ber's 05 telephone set off-hook is assumed to be 600 ohms, and the 06 resistors 30 and 31 should be 300 ohms each.
07 ~ccordingly for a voice frequency signal on the tip and 08 ring leads applied to diferential amplifier 10, there will be a 09 10 times reduction at its output, and the output signal will be l/10 the input signal. This is the signal which is applied to ll the non-inverting input of amplifier 16.
12 The signal is further ampli~ied 10 times by the circuit of .13 differential amplifier 17, re-establishing the original level at 14 the output of the latter diferential ampliier. However due to matching resistor 18 to the line impedance at output terminal 19, ~16 the output signal level at output terminal 19 is l/2 the original 17 siynal level.
~18 This signal is applied to the inverting input of differen-~19 tial amplifier 16. However due to the aforenoted difference in gain by amplifier circuit 3~ to the signals applied to its non--21. inverting and inverting inputs, the l/lO original si~nal level is ~22 multiplied by about 26 to provide an internally processed signal 23 level of about 2.6 the original signal, while the signal 24 applied to the inverting input is multiplied by -S, for an in-:~25 ternal signal level of -5/2 or -2.5. The differential amplifier 26~ compares the two signals of -~2.6 and -2.5, and provides and out-27 put signal of 0.1 times the input signal, wiich is virtually 28 negligible, but which may be desireable in some instances to 29 avoid the appearance of a "dead" line.
~30 Clearly a sellection of the values of resistors 20, 21 and ~ .
~31 28, as well as resistors 22 and 21 (establishing the gain of di-~32 13 : :
' . i ';
.
' '' , : . ..
5~
01 Eerential amplifier 17), or by providing one or more trimmer re-02 sistors, an exact cancellation o~ any si~nal ~rorn the tip and 03 ring leads Erom Eeeding back through differential amplifier 16 04 can be effected.
05 Considering now a signal arriving at line terminal 19 from 0~ the switching matrix 33, this is applied at halE the orlginal 07 source level to amplifier 34~ However due to the undirectlona:L
0~ nature of bufEer amplifier 17, it is restricted from being 09 applied to the non-inverting input oE diEferential amplifier 16~
Amplifier circuit 34 increases the amplitude of the signal by the 11 gain establisned by the ratio of resistors 28/20, noted earlier -12 as -5. The output signal oE ampli~ier circuit 34 is thus ~S
13 times the hal level input signal, or -2.5 times the input 14 signal. This is applied through resistor 30 to the tip lead.
Due to the voltage dividing action of resistors 30 and 31, there is a gain reduction, for the resistor values given, oE 1/2 of 17 signal amplitude, and the signal level applied to the tip and 18 ring leads are about 2.5/2 times the original signal level, or 19 1.25 times the original signal level, whicn is sli~htly but negligibly greater than the original. Clearly an increase in the 21 value of resistors 30 and 31 (i.e., to 350 ohms, Eor example) 22 will cause the signal level applied to the tip and ring leads to 23 be equal to the original signal, should the slightly increased 2~ level obtained as a result o~ use oE the values given by example, be objectionable.
26 As was noted earlier, the signal applied from 27 amplifier 34 via resistor 30 to the tip lead also appears at the 28 inverting input of differential amplifier 10, reduced in 23 amplltude by resistors 1 and 2, ~hich ~signal would be translated by differential amplifier circuit 10 as a normal differential 31 signal~ Accordingly resistor 32 o~ a-t least twice the resistance ~32 of the total of ~33 14 ~ .
.. , ~ . . . .
01 resistors 1 and 2 carries the output signal of amplifi~r circuit 02 34 to the non-inver-ting input oE differen-~ial amplifier 10. This 03 signal provides an equal amplitude signal to the non-inverting 04 input as appears on the inverting input, in order that exact can-05 cellation of the signal output of amplifier 34 should be obtained 06 at the input of differential amplifier 10, to avoid feedback and 07 sidetone which would otherwise be reapplied back to line output 08 terminal 19.
09 In case the switching matrix 33 has connected a number of subscribers lines together in a conference call, the impedance of 11 the line at terminal 19 will be less than originally 12 contemplated, i.e. less than 600 ohms, for instance. The output 13 signal voltage level will then be less than for a two party 14 call. For a eonferenee call, one or both of switehes 25 or 26are elosed by external means, causing resistor 23 and/or 24 to be 16 connected in parallel with resistor 21. The gain of buffer 17 amplifer 17 is thus inereased inereasing the output signal level 13 to terminal 19 to the two-party eall level. Compensation can 19 thus be made for a conferenee eall with a few or a great many`~20 partieipants.
21 Aeeordingly, a bidireetional amplifier is provided, with a 22 battery line feed and a translation of a balaneed line to an 23 unbalaneed line. The signal levels are eompletely controlled and ~24 the faeility provides seleetable signal level compensation for eonferenee ealls.
26 Before deseribing the remainder of the eircuit in detail, ;27 reference is made to figure 3, which shows more generally the28 amplifier eireuit whieh provides two levels of gain for two dif-~29 ferent input signals~ A first signal source 40 is connected to ~30 the non-inverting input of a differential operational amplifier ~31 ~41. The same signal source is also conneeted through a buffer `32 amplifier 42, and through an input resistor 43 to the inverting `33 15 ~ ~ .
01 input of difEerential amplifier 41. The same inverting input is 02 connected to ground through resistor 44. Since the output impe-03 dance of buffer amplifer 42 is virtually zero, resistor 46 is 04 connected in series.
05A second signal source 45 is connected to resistor 43 in 06 the signal path between the output of buffer amplifier 42 and re-07 sistor 43 where the latter resistor joins resistor 46. The 08 second signal source 45 is at external line impedance. Resistor 09 46 should be of such value as to match the line impedance which 10typically might be 600 or 900 ohms. A feedback resistor 47 is 11 connected between the output terminal 48 of the operational 12 amplifier and the inverting input.
13It will be seen that a signal received from source 40 is 14 applied to the non-inverting input of amplifier 41 as well as to the inverting input, the latter through buffer amplifier 42 and 16resistors 46 and 43. Signals from source 45 are applied only to 17 the inverting input of amplifier 41, and are blocked from being 18 applied to the non-inverting input due to unidirectional buffer ~19 amplifier 42.
20The gain of this amplifier for signals from source 40 is ~21given by the e~pression R47/tR44 x R43) , where each of the ~22tR44 + R43) 23 parameters is the resistance of the numerically identified re-24 sistor, in ohms.
25On the other hand, the gain for signals from source 45, ~26 (assuming R46 is of a low resistance relative to resistors R43 27and R47) is R47/R43.
28It should be noted that should signal source 45 not be of 29~ low impedance relative to resistor 43, its impedance should be ~`` -~30 16 .
. . .
~ ::3 . .
.~ ~ ~ . . . .
~ ' , ' ' , ' ~a.~ f~
01 a~ded to resistors 43 as an internal part thereof for the gain 02 calculation.
03 ~s an example, assume resistor 46 is of low impedance, for 04 instance 600 ohms, resistor 43 is 50K ohms, resistor 44 is 10K
05 ohms and resistor 47 is 250K ohms. Assume also that two differ-06 ent signals, each oE 1 volt amplitude appear at sources 40 and 07 45.
08 The galn of this stage Eor the signal at source 45 will be 09 250K/50K or 5, and the amplitude of the output signal from source 45 will be -5 x 1.0 = 5.0 volts. Since the signal had been AC, ll the 5.0 volt output signal has its phase inverted 180 degrees 12 from the input signal.
13 On the other hand the signal from source 40 is applied to 14 the circuit and has a gain of 250K /(50~ x10K) 60 = 30. The output ~16 signal amplitude therefore is 30 x 1.0 = 30 volts.
17 The above is dependant, of course, on the power supply 18 used for the amplifier (not shown) being sufficient to allow the 19 vol-tage levels noted to be obtained. Nevertheless it is exemplary of a circuit which can equalize different signal 21 levels, or can amplify, in one stage, two different signals to 22 different degrees. Indeed, should various ones of the resistors ~2~3 be made variable, i~e., by means of a signal variable resistor 24 such as a field effect transistor, an optical isolator, or the ~25 like, one or both of the input signals can be subjected to ~26 various gain modifications or can be modulated at will ~rom an ~;27 outside source. Interaction or various degrees of addition of 28~ the two input signals can thus be readily obtained.
`29 Returning now to figure 2, it had been noted earlier that 30~ the circuit contains means for applying ringing voltage to the 31 ~subscriber's line. A ringing relay 50 has the usual back EMF
32~ 17 ~, : ; ~ ;' : ~ :: ~ ~ . - ..
: ~ : : , ~ . .. .
01 protection diode 51 connected thereacross. It is connected to a 02 source of voltage -~8v and to the collector oE a PNP transistor 03 52, which itself has its emitter connected to ground.
04 A source of ring actuate signa:L is applied from a ring 05 actuate lead throu~h resistors 53 and 54 to the base of transis-06 tor 52, while a source of potential +V is connected through re-07 sistor 55 to the ring actuate lead as bias for transistor 52.
08 A signal is applied to the OFH~ lead Eor the provision of 09 remote ringing shut off. 'rhe OFHK lead is connected via diode 56 10 through resistor 5~ to the base of transistor 52.
11 In operation, a source of low potential is applied to the 12 ring actuate lead. This drops the potential level at the base, 13 and causes operation of transistor 52, which conducts and opera-t-14 es ringing relay 50.
In the meantime the OFHK lead has a source of low potent--16 ial amplified, and diode 56 is back biased. In the event of an 17 indication of the local subscriber going off-hook, from a remote ~18 circuit (not shown), the level on OFHK lead changes and carries a ~19 high potential, causing diode 56 to c~onduct, raising the level at ~20 the junction of resistors 53 and 54 and thus the base of transis-~21 tor 52, to high potential. Transistor 52 accordingly ceased con-22 duction, open circuiting the current path through ringing relay , , .
23 50, which is shut off.
~2~ It is preferred to connect a light emitting diode 57 through resistor 58 to a negative source of potential and to the ~26 OFHK lead. Accordingly when high level potential appears on the 27 OFHK lead causing shutoff of ringing relay 50, light emitting ~28 diode S7 operates, providing a visual indication to an operator ,, .
~29 or serviceman that the circuit has functioned.
~30 Ringing relay 50 is comprised of a single pole double ~31 throw set of contacts, having a moving pole contact 59, a brea~
' :~. .~ - - .
01 contact 60 and a ma]ce contact 61. r~ake contact 61 is connected 02 through resistor 62 to a terminal for connect:ion as a source of 03 ringing current 63.
04 When ringing relay 50 is engergized, break contact 60 05 opens and moving pole contact 59 contacts make contact 61, which 06 applies ringing current to the tip and ring terminal 64 for 07 application to the ringer of the loca:L subscribers telephone 08 set connected to terminals 64 and 65. Once ringing has been 09 tripped by the subscriber going off-hook, ringing relay 50 is caused to open, causing moving contact 59 to contact break con-11 tact 60, and thus stopping ringing current from being applied to 12 terminal 64 and the subscriber's telephone set, and re-establish-13 ing the circuit from the telephone set to the line circuit.
14 It will be noted that when the ringing relay has operated, ;15 the subscriber's line becomes unbalancedr and ringing signals as ~16 well as an altered DC level passes through and appears at the out-;17 put of differential amplifier 10. Both the ringing and DC signal 18 are applied through resistor 66 to a low pass active filter 67 of 19 well known contruction. Active filter 67 is fabricated so as to ~20 block the 20 Hz ringing signals, but to pass signals of lower 21 frequency, including the DC off-hook signal level from the output 22 of operational amplifier 10. It is also preferred that active 23 filter 67 should pass 10 Hz signalsr the frequency of rotary dial 24 pulsesr although this is not essential for the present circuit.
The output of low pass active filter 67 is connected to 26 the noninverting input of differential amplifier 68r the output :27 of which is connected through resistor 69 and diode 70 to the 28 noninverting input of differential amplifier 71~ The output of 29 diode 70 is connected to ground through an integrating filter ,, .
;30 circuit comprising copacitor 72 in parallel with resistor 73 con-~31 19 `' ~ ''`
.. . .
01 nec~ed to a negative source o~ potential. The inverting input 02 oE diffe~enti~l am~liEier 71 is connected to grount~, and the in-03 verting ;nput of differential amplifier 68 is connected to a 0~ source off-'nook bias via an OFHlC BIAS lead. The DC level o~ this 05 lea~ sets the sensitivi~y oE the circuit for sensing the DC
06 oEf-hoolc level from the output oE diEEerential ampli~ier 10.
07 In operation, the DC potential level output o~ dif~eren-03 tial operational ampli~ier 10 is applied through resistor 66 to 03 low pass ac-tive filter 67. Here all signals below ringing fre-ln quency are passed throug'n the Eilter, and ringing signals are re-11 -jected or greatly attenuated. The resulting DC level is applied 12 to diEEerential amplifier 63, which provides an output signal 13 once the DC level is higher than the potent'ial level on the 14 o~E-hook bias lead. This signal is rec-ti-~ied and Eiltered to DC
in ~iode 70~ capacitor 72 and resistor 73, the resulting DC
lG signal being applied to the inpu-t of diEferential ampliEier 71.
17 When an off-hook condition exists, the output o~ di~erent-1~ ial amoliEier 71 provides a high potential level, causing diode 19 5~ to become forward biased, and cutting off operation oE trans-istor 52. As described earlier wit'n respect to a high level 21 signal on the OF~IK lead, ringing relay 50 is caused to open, re-22 moving ringing signal.s which had been applied to the subscribers 23 line.
24 In the meantime a signal on the RING ACTUATE lead had been removed, and ~he +V potential, applied through resistors 53 and ~ . , 26 54 are operative to cause the emitter-base junction oE transistor 27 52 to be reverse biased, thus holding ringing relay 50 inopera-28 tive.
2~ Switch lS, whlch preEerably is a CMOS switch also is oper- -atec~ by a~signal on the RING ACTUATE lead. Accordingly when 31 ringing is applied to~the subscriber's line upon the actuation of 32 ~ ~ 20 -'~
-~ `' ':.''" '' ' ' , , 01 ringing rela~ 50, switch 15 is caused to open. Ringing signals 02 thus appearing at the output of differential amplifier 10 are 03 stopped from being applied to the output line via output line 04 terminal 19, as well as from returning through amplifier circuit 05 34.
06 It should be noted that with the conduction of diode 56, 07 light emitting diode 57 also conducts and illuminates, providing 08 an indication that the subscriber has gone off-hook. A
Og serviceman at the PBX examining the line circuit printed circuit boards thus can obtain an indica-tion of which subscriber's lines ~11 are busy and which have remained on hook.
12 As noted earlier, the present line circuit can be used as 13 a standardized circuit for interfacing with standard telephone 14 sets. Two circuits connected with adjoining output terminals provide a four wire to two wire two way amplifier conversion.
`16 Each circuit converts a balanced tip and ring pair to an unbal-17 anced pair, whereby single wire switching can be used. The cir-18 cuit also contains provision for externally controlled gain in-19 crease for conferencing or the like.
~20 As one of the important elements of the invention, a novel ~21 amplifier has been used which amplifies two different signals -22 with different amounts of gain.
23 The circuit also provides means for applying ringing sig-24 nal to subscribers telephone set, and for detecting a subsequent off-hook condition when the subscriber has answered the tele-~26 phone, and for ring tripplng. Facility is also provided for ex-~27 ternally tripping the ringing, as might be provided, for in-~28 stance, by an external timing circuit.
`29 It will now become clear to a person skilled in the art understanding this invention that varlous variations and modifi--~31 cations could be made. All are considered within the scope of 32~ the present invention as defined in the appended claims.
:
01 ~U P EM NTARY DI C OStJRE
02 Figure 4 shows a schematic of an alternative form of 03 the invention.
04 It should be noted that the subject line circuit can 05 be connected to a tip and ring lead at one port, to an incoming 06 signal circuit at another port/ and to an outgoing signal 07 circuit, at a third port, the second and third ports being 08 adapted for connection to such apparatus as a Codec. The input 09 to differential amplifier 16, rather than being connected to terminal 19, can be connected to a circuit ~or receiving an 11 input signal, e.g., the output circuit of the Codec. In 12 addition, the output of operational amplifier 17 can be 13 connected to an input circuit, e.g., the input circuit o~ the 14 Codec. The unbalanced output circuit of Figure 2 thus can be converted to two ports, an input port and an output port, from a 16 single bidirectional port.
17 The circuit of Figure 4, which shows various ~: 18 modifications to the circuit of Figure 2, depicts how the 19 circuit can be converted to have two ports on one side, and a bidirectional port on the other.
21 ~ The two inputs to differential amplifier 16 are Z2 connec-ted via respective resistors 75 and 76, one to ~;23 bidirectional unbalanced line input/output terminal 19, and the ~` 2~4~ other to;the output of differential amplifier 17. The inverting 25 ~; input of operational amplifier 16 is also connected through ;~26 reSlstQr ~0 to ground.
~27 ~lowever, for connection to a Coder-decoder (Codec) or ~ - -~28 other such 4 wlre apparatus, the conductive path to terminal 19 2~9 ~ is broken at polnt 77 ~and resis:tor 75 is connected instead to ;30 input terminal~ 78 instead. The conductive path to the output of :
., - . .::
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.. . . .. .. . ..
~ ~?~
01 differential amplifier 17 is broken at point 79 and resistor 7 02 is connected instead to ground; both of the alternative 03 connections are shown in dashed line. In this case amplifier 16 04 need not be of differential type.
05 To simplify the circuit of Figure 2, ra-ther than 06 utilizing transistors 26 and 27 in the output circuit of 07 differential amplifier 16, a sing:Le transistor 81 i5 used in -the 08 circuit of Figure 4. This allows connection of the collector 09 of transistor 81 to a simplified power supply, e.g. to a -~5 vo]t source, rather than to a split vo:Ltage power supply. The 11 emitter is connected through resistor 30 to the tip lead, in 12 this embodiment. Resistor 82 is connected between the base and ~13 the emitter of transistor 81.
14 Similar to the emitters of transistors 26 and 27 in Figure 2, the emitter of transistor 81 is connected through 16 resistor 30 to the tip lead, and through resistor 32 to the 17 noninverting input of differential amplifier 10.
18 The circuit of Figure 4 has been modified for 19 operation with PBXs utilizing a lower voltage (e.g. -24 volts) than is normally used by a central office, which has the 21 availability of a -48 volt supply. The present circuit is ;22 adapted to use power supplies having filtering which is somewhat 23 poorer than might otherwise be desired, as would be economically 24 advantageous in a low-cost PBX.
~25 Since a low cost power supply would often contain ;-~ 26 significant amounts of A.C. ripple, the power supply leads are 27 applied to the tip and ring leads in a manner similar to the ~8 signals received ~rom ~ifferential amplifier 16. The -24 volt ~;~29 supply is connected (optionally through switch 83, which switch ~ - , is also connected to a source of ringing current~ to the ring ;: :
, ~ :
..
. ,- -. . . , - ~ . :
. : .- - - ~ .: - -01 lead through r~sistor 84 and to the inverting input of 02 diEferential amplifier 10 through resistor 85. The resistance 03 of resistor 8~ should be the same as that of resistor 30, e.g., 04 one half the line resistance (or about 300 ohms) and resistor 85 05 should be of similar resistance as resistor 32 (e.g., about 06 200,000 ohms). Resistor 86 is connected from the noninverting 07 input oE differential amplifier 10 to ground, and can be, e.g., 08 about 10,000 ohms.
09 It may be seen that with the above noted di~erence in resistance between resistors 84 and 85, the major portion of 11 D.C. and A.C. ripple current from the -24 volt supply passes 12 through resistor 84 to the ring lead. The current is applied 13 with substantially reduced amplitude to the inverting input of 14 differential amplifier 10. The effect of the reduced current being applied to the inverting input, and the high curren-t being 16 applied (with reduced amplitude due to resistor 2) to the 17 noninverting input causes cancellation of the A.C. ripple within 18 the differential amplifier, and no significallt ripple appears at 19 the output of differential amplifier 10 which could be passed to output terminal 19.
21 A.C. ripple which appears between the tip and ring 22 terminals 64 and 65 can be cancelled by the application of the 23 -24 volt supply through capacitor 87 in series with resistor 88 24 to the noninverting input of differential amplifier 16, which blocks the -24 volt D.C. but applies the A.C. ripple to the 26 differential amplifier. me values of capacltor 87 and resistor 27 88 should be adjusted so that there is unity gain to the tip 28 lead, or more precisely so that the applied A.C. ripple cancels 29 whatever ripple appears bet~een the tip and ring leads.
In a simplified line circuit in ~hich no conferencing .
; ~ .
. . . - - : . . - .
, . , . . . .. -:
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aq~
01 function is required, the output of dlfferential amplifier 10 0~ can be connected directly through capacitor 14 to the input of 03 differential amplifier 17. However, to facilitate remote muting 04 of the outgoing signal, a pair of resistors 89 and 90 are 05 connected in series with the output circuit o~ differential 06 amplifier 10. The junction of resistors 89 and 90 is connected 07 to the collector of a transistor 91, which has its emitter 08 connected to the ~5 volt supply. A lead from a muting logic 09 circuit, not part of this invention/ is connected to the base of transistor 91.
11 Upon the muting logic circuit applying a low level 12 signal to the base of transistor 91, the ~5 volt supply is 13 connected through the emitter-collector circuit of transistor 91 14 to the junction of resistors 89 and 90. This effectively shunts the output of differential amplifier 10 to the supply, cutting 16 off the transmission of signal to the input of differential 17 amplifier 17.
18 In addition, an indication of an off-hook condition on ~19 the tip and ring leads can be obtained by connecting one input ~`20 of an operational amplifier 92 to the output of differential 21 amplifier 10. The other input is connected to a voltage divider ~22 comprising resistors 93 and 94 connected between the -24 volt 23 power supply lead and ground. With a change in the D.C. voltage ~24 across the tip and ring leads, the output voltage level of ~25 differential amplifier 10 changes, causing operational ~26 amplifier 92 to conduct once the threshold set by the voltage 27 divider has been exceeded. The output lead OFHR of operational 28 ~ amplifier 92 provides an output signal which is indlcative of 29 the on or off-hook ccndition of the tip and ring leads.
~30 ~25 :
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01 lifier means, the signals are not amplifie~ and clo not appear at 02 the output of differential amplifier means 3.
03 Accordingly the circuit so far described amplifies voice 04 frequency signals in one direction, while changing the balanced 05 subscriber's line into an unbalanced line output. At the same 06 time common mode signals are stopped.
07 As noted earlier~ however, the inventive circuit is a bi-08 directional amplifier, and also includes means for amplifying 09 signals arriving from the unbalanced line and translating them into signals applied to the balanced line. Differential ampli-11 fier means 5 has a non-inverting input connected to the first 12 circuit path between the output o:E differential amplifier means 3 13 and the input of buEfer amplifier 4. The inverting inpu-t oE dif-14 ferential amplifier means 5 is connected to the second circuit path from the output of buffer amplifier 4 to unbalanced line ~16 output 7.
17 The signal arriving from -the line output 7 is thus applied 18 for amplification to differential amplifier means 5. However 19 this signal would also include a signal arising from the first described tip and ring leads as amplified and passed through :21 differential amplifier 3 and buffer amplifier 4O The signal from 22 the output of differential amplifier means 3 is applied to both ~23 inputs of differential amplifier means 5 in such manner that, 24 with the respective gains of amplifier means 5 applied to the 25 ~ signals from the respective inputs, the signals from differential ~ -Z6 amplifier 3 are caused to cancel, and are not reapplied from the ~27 output of differential amplifier 5 back to the tip and ring :28 leads. Accordingly positlve feedback, excessive side tone or the 29 like is avoided. The manner in which the specific cancellation occurs will be described in more detail in conjunction with 31 figure 2.
3~2 It should be noted that bu~fer amplifier 4 should there-:
:~ ~ : : . . .
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01 fore be of the non-invertiny t~pe, for the polarity of the tip 02 and ring signal as applie~ to ~ifferential amplifier means 5 to 03 be correct. Furthermore, buffer ampliEier ~ Eunctions as an 04 isolator, restricting signals arriving from line output 7 from 05 being applied to the non-inverting input of differential 06 amplifier 5. Accordingly signals arriving from line output 7 are 07 differentially applied to differential amp]ifier means 5 an~ are 0~ thus amplified and applied to the tip lead.
09 It will be understood that the signal applied to the tip lead is as a result also applied as a differential signal to the 11 input of differential amplifier 3. Resistors 1 and 2 which are 12 connected in series with the respective tip and ring leads and 13 the inputs o difEerential amplifier means 3 should be of high 14 resistance, each preferably at least 100 times the offhook ~15 impedance of the telephone set. This reduces the differential 16 signal level appearing at the input of differential amplifier 17 means 3 from the output of differential amplifier means 5 (the 18 signal arriving from the line output3 by at least 200 times.
19 This lower amplitude signal is cancelled by the applica-20 tion of signal from the output of differential amplifier 5 via 21 resistor 6 to the other input of differential amplifier 3. Re-22 sistor 6 should therefore be at least 200 times the off-hook impe-23 dance of the telephone set, and should be of value such as to 24 apply the exact amount of signal to the differential amplifier 3 ~25 to cancel the residual reduced signal appearing thereat from the 26 output of differential amplifier means 5.
27 Accordingly signals appearing on the unbalanced line 7 are 28 applied to the balanced the tip and ring leads, with virtually no 29 sidetone feedback back to the line.
To inhibit the formation of unbalanced currentsl it is pre-~.
. ~, .
, 01 ferred that resistors 1 and 2 each should have tolerance within 02 0.]~.
03 Figure 2 is a schematic diagram showing the inventive 04 telephone line circuit in detail. The tip and ring leads are 05 connected through resistors 1 and 2 to the respective input 06 terminals of differential operational amplifier 10. The 07 operational amplifier has a feedback resistor 11 connected 08 between its output and its inverting input in the conventional 09 manner. ~s noted with respect to Figure 1, resistors 1 and 2 should each be at least 100 times the impedance of the off-hook 11 telephone set which is to be connectecl to the tip and ring leads 12 (although it could in some circumstances and special designs, be 13 less). Resistors 12 and 13 connect the invertiny and the 14 noninverting input terminals of operational amplifier 10 to ground.
~16 The output oE operational amplifier 10 is connected 17 through a DC blocking capacitor 14 to the input of a switch 15.
18 Preferably switch 15 is an externally enabled CMOS switch.
19 The output of switch 15 is connected both to the non-in-verting input of a second differential amplifier 16 and to the 21 noninverting input of a third differential amplifier 17, (which 22 corresponds to buffer amplifier 4). The output of third differ-23 ential amplifier 17 is connected through resistor 18 to the line 24 output terminal 19.
Line output terminal 19 is connected through resistor 20 ~26 to the inverting input of second diEferential amplifier 16. The 27 inverting and noninverting inputs thereof are connected through 28 respective resistors 21 and 22 to ground.
29 It is preferred that resistor 18 be of similar impedance as the line which is connected to line output terminal 19, for 31 matching purposes, since the output impedance of differential .~ .
01 amplifier 17 wou:Ld otherwise be very low.
02 The inverting input of third differen-t:ial amplifier 17 i5 03 connected to ground through resistor 21, and :i.s also connected 04 via a feedback resistor 22 to the output of differential ampli-05 fier 17.
06 The inverting input of third di.fferential amplifier 17 is 07 also connected through individual resistors 23 and 24 in series 08 with make contacts 25 and 26 to grouncl. Accordingly should 09 either contacts 25 or 26 be closed, resistors 23 or 24 are con-nected in parallel with resistor 21. Since the gain oE amplifier 11 17 for signals applied to the non-inverting input is determined 12 by the ratio oE the value of resistors 22 to 21, the gain of this ~13 stage can be effectively changed by connecting either resistor 23 14 or resistor 24 in parallel with resistor 21. Switches 25 and 26 ~;15 are closed by an externally operated switching matrix~ no~ shown.
16 The output of second differential amplifier 16 is connect-17 ed through a low valued resistor 125 of approximately line .18 impedance to the input of a power amplifier stage comprising 19 transistors 126 and 127. The bases of NPN transistor 126 and PNP
~20 transistor 127 are connected together to resistor 125. The 21 collector of transistor 126 should be connected to a positive 22 source of direct current, such as +8V, and the collector of ~23~ transistor 127 should be connected to a source of negative 24~ potential, such as -lOV. The emitters are connected together, ;25 and clearly are at close to 0 volts DC.
~26~ The emitters of transistors 126 and 127 are connected ;27 through resistor 28 to the inverting input of differential ampli-.28~ fier 16, which resistor is bypassed by a small high frequency .
~29~ rolloff capacitor 29.
30 ~ The operation of differential amplifier 16 with resistor ~31 125 and transistors ]L26 and 127 as well as feedback resistor 28 32~ is simi1ar to a~differential amplifier with a normally connected ~33 9 :: :
~..~. .
01 ~Eeedback loop, but haviny a high current capacity output stage 02 The output stage is required to fee~ line current to the tip and 03 ring leads as will be described below4 04 The emitters of transistors 126 and 127 are connected OS through a resistor 30 to the tip lead, and the ring lead is con-06 nected through a resistor 31 to a source of potential such as -4 07 volts.
08 As was described with respect to resistor 6 in Figure 1, 09 the emitters of transistors 126 and 127 are connecte~ through re-sistor 32 to the non-inverting input of operational amplifier 10.
11 Resistors 30 and 31 should each have the value of 1/2 the 12 tip and ring line impedance, the latter being approximately the 13 value of the off-hook telephone station set impedance. It is im-14 portant that resistors 30 and 31 should be very closely matched in value, for instance to 0.1%.
16 Further, resistors 12 and 32 should be also as closely 17 matched in value, and should be at least twice the value of re~
~18 sistors 1 and 2, and should be at least ln times the line impe-19 dance, i.e. 10 times the impedance of the off-hook telephone set. Resistor 13 should be the same value as resistor 11, with `21 the aforenoted 0.1% tolerance, for the purpose of ensuring that 22 ~he tip and ring leads are not subjected to imbalance.
23 In operation, a subscriber's telephone set is assumed to 24 go off-hook. Battery voltage is supplied through transistors 126 25 ~ and 127 to the tip lead through resistor 30, and passes through 26 the telephone set, the ring lead, and through resistor 31 to the 27 other terminal of th~ battery supply, -48 volts It is important 28 that the battery feed circuit appear as a low impedance voltage 29 source.
The ratio of the sum of the resistances of resistors 1 and 31 2 to the line impedallce provides a voltage division of any common 1: .
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01 mo~e signcll, which may appear on t'ne tip and ring le~ds, as 02 applied to ampliEier 10. While equal valued common mode signals 03 beiny applied to the input terminals oE differential amplifier 10 04 would cancel in any event it is preferred to reduce their amp-05 litude in case there is a diferential inter~erence componen~
05 modulated on the common mode signals~ and in the case o~ differ-07 ences in the values of resistors } and 2, which would give rise 0~ to a differential comppnent.
09 It is preferred that the amplific~tion oE the circuit which includes di~ferential amplifier 10 s'nould be about 1/10.
11 Accordingly if resistor ~ is lOOrOOO ohms for instance, resistor 12- 11 should be about 10,000 ohms.
13 DiEferential voice frequency signals are also sub~ected to l4 a reduction in amplitude o 1/10. These signals are applied through capacitor 14, through switch 15, to the non-inverting in-16 put of differential amplifier 16. They are also passed through 17 buffer 17, resistor 18 and resistor 20 to the ~nverting input o-~
-18 differential amplifier 16. The ratio o~ resistors 22 to 21 pre-19 ferably -is 10 to 1, which provides an amplification oE 10 of -the voice signals applied to buffer amplifer 17. This compensates 21 completely of the signal loss in the circuit oE amplifier 10, and -~ ~ 22 the original dlfferential voice signal amplitude is restored.
23 However, resistor 18, as was noted earlier, should have 24 a similar impedance as the unbalanced line at line output term-~25 inal 19, and accordingly the output signal at terminal 19 is 1/2 26 the input voice frequency signal across the tip and ring leads.
~27 The signal at output line terminal 19 is applied to a 2B crosspoint matrix 33, and is thus connected t~rough the PBX to a 29 similar terminal 19 of another line circuit o~ similar struc-ture. Th~ line level of a signal~at ~he line terminal 19 arriv .
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01 ing from another line circuit is therefore at 1/2 the tip and 02 ring signal amplitude level. Both the signal from the local tip 03 and ring leads passing through buffer amplifier 17 and the incom-04 ing signal are applied through resistor 20 to the inverting input 05 of differential ampliEier 16.
06 It should be noted that in the expressions to be given 07 below with respect to gain, the entire circuit including output 08 transistors 126 and 127 as well as resistor 125 and differential 09 amplifier 16, and their accompaning directly connected resistors including feedback resistor 28 are considered as a single ampli-11 fier circuit 34.
12 Amplifier 34 has been designed to ampliEy different sig-13 nals with different predetermined amounts of gain. An example of 14 its operation will be given, using typical values for the assoc-iated resistors. As an easily calculated example, resistor 20 16 can be 40.2K ohms, resistor 21 can be 9.53K ohms, and resistor 28 ~17 can be 200K ohms.
18 Accordingly~ for signals appearing at the non-inverting 19 input of amplifier 34 which arrived from -the output of differen-tial amplifier 10, the gain of the stage is $he ratio of the 21 value of resistor 28 to resistors 20 and 21 in parallel (since 22 resistor 20 is connected to the line input, which is at relative-23 ly low impedance to ground, typically 600 ohms).
24 For the example shown, the gain is 200K/9-53K_X 40.2K
~25 ~ (9.43X + 40.2K) 26~ which e~uals approximately 26 (non-inverting).
27 For signals arrived from the line terminal l9r the gain of 28 the stage is the ratio of resistors 28 and 20. Accordingly the 29 gain in this case is 200K/40.2K which equals approximately 5 (inverting).
31 As was noted earlier, the emitters of transistors 126 and ~32~ 12 . . . , , :
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01 127 are connected through resi.stor 30 to the tip lead, the ring 02 lead being connected through resistor 31 to eEfective AC ground 03 (but which resistor in reality forms the DC line feed). The 04 impedance between the tip and ring lead with the subscri.ber's 05 telephone set off-hook is assumed to be 600 ohms, and the 06 resistors 30 and 31 should be 300 ohms each.
07 ~ccordingly for a voice frequency signal on the tip and 08 ring leads applied to diferential amplifier 10, there will be a 09 10 times reduction at its output, and the output signal will be l/10 the input signal. This is the signal which is applied to ll the non-inverting input of amplifier 16.
12 The signal is further ampli~ied 10 times by the circuit of .13 differential amplifier 17, re-establishing the original level at 14 the output of the latter diferential ampliier. However due to matching resistor 18 to the line impedance at output terminal 19, ~16 the output signal level at output terminal 19 is l/2 the original 17 siynal level.
~18 This signal is applied to the inverting input of differen-~19 tial amplifier 16. However due to the aforenoted difference in gain by amplifier circuit 3~ to the signals applied to its non--21. inverting and inverting inputs, the l/lO original si~nal level is ~22 multiplied by about 26 to provide an internally processed signal 23 level of about 2.6 the original signal, while the signal 24 applied to the inverting input is multiplied by -S, for an in-:~25 ternal signal level of -5/2 or -2.5. The differential amplifier 26~ compares the two signals of -~2.6 and -2.5, and provides and out-27 put signal of 0.1 times the input signal, wiich is virtually 28 negligible, but which may be desireable in some instances to 29 avoid the appearance of a "dead" line.
~30 Clearly a sellection of the values of resistors 20, 21 and ~ .
~31 28, as well as resistors 22 and 21 (establishing the gain of di-~32 13 : :
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01 Eerential amplifier 17), or by providing one or more trimmer re-02 sistors, an exact cancellation o~ any si~nal ~rorn the tip and 03 ring leads Erom Eeeding back through differential amplifier 16 04 can be effected.
05 Considering now a signal arriving at line terminal 19 from 0~ the switching matrix 33, this is applied at halE the orlginal 07 source level to amplifier 34~ However due to the undirectlona:L
0~ nature of bufEer amplifier 17, it is restricted from being 09 applied to the non-inverting input oE diEferential amplifier 16~
Amplifier circuit 34 increases the amplitude of the signal by the 11 gain establisned by the ratio of resistors 28/20, noted earlier -12 as -5. The output signal oE ampli~ier circuit 34 is thus ~S
13 times the hal level input signal, or -2.5 times the input 14 signal. This is applied through resistor 30 to the tip lead.
Due to the voltage dividing action of resistors 30 and 31, there is a gain reduction, for the resistor values given, oE 1/2 of 17 signal amplitude, and the signal level applied to the tip and 18 ring leads are about 2.5/2 times the original signal level, or 19 1.25 times the original signal level, whicn is sli~htly but negligibly greater than the original. Clearly an increase in the 21 value of resistors 30 and 31 (i.e., to 350 ohms, Eor example) 22 will cause the signal level applied to the tip and ring leads to 23 be equal to the original signal, should the slightly increased 2~ level obtained as a result o~ use oE the values given by example, be objectionable.
26 As was noted earlier, the signal applied from 27 amplifier 34 via resistor 30 to the tip lead also appears at the 28 inverting input of differential amplifier 10, reduced in 23 amplltude by resistors 1 and 2, ~hich ~signal would be translated by differential amplifier circuit 10 as a normal differential 31 signal~ Accordingly resistor 32 o~ a-t least twice the resistance ~32 of the total of ~33 14 ~ .
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01 resistors 1 and 2 carries the output signal of amplifi~r circuit 02 34 to the non-inver-ting input oE differen-~ial amplifier 10. This 03 signal provides an equal amplitude signal to the non-inverting 04 input as appears on the inverting input, in order that exact can-05 cellation of the signal output of amplifier 34 should be obtained 06 at the input of differential amplifier 10, to avoid feedback and 07 sidetone which would otherwise be reapplied back to line output 08 terminal 19.
09 In case the switching matrix 33 has connected a number of subscribers lines together in a conference call, the impedance of 11 the line at terminal 19 will be less than originally 12 contemplated, i.e. less than 600 ohms, for instance. The output 13 signal voltage level will then be less than for a two party 14 call. For a eonferenee call, one or both of switehes 25 or 26are elosed by external means, causing resistor 23 and/or 24 to be 16 connected in parallel with resistor 21. The gain of buffer 17 amplifer 17 is thus inereased inereasing the output signal level 13 to terminal 19 to the two-party eall level. Compensation can 19 thus be made for a conferenee eall with a few or a great many`~20 partieipants.
21 Aeeordingly, a bidireetional amplifier is provided, with a 22 battery line feed and a translation of a balaneed line to an 23 unbalaneed line. The signal levels are eompletely controlled and ~24 the faeility provides seleetable signal level compensation for eonferenee ealls.
26 Before deseribing the remainder of the eircuit in detail, ;27 reference is made to figure 3, which shows more generally the28 amplifier eireuit whieh provides two levels of gain for two dif-~29 ferent input signals~ A first signal source 40 is connected to ~30 the non-inverting input of a differential operational amplifier ~31 ~41. The same signal source is also conneeted through a buffer `32 amplifier 42, and through an input resistor 43 to the inverting `33 15 ~ ~ .
01 input of difEerential amplifier 41. The same inverting input is 02 connected to ground through resistor 44. Since the output impe-03 dance of buffer amplifer 42 is virtually zero, resistor 46 is 04 connected in series.
05A second signal source 45 is connected to resistor 43 in 06 the signal path between the output of buffer amplifier 42 and re-07 sistor 43 where the latter resistor joins resistor 46. The 08 second signal source 45 is at external line impedance. Resistor 09 46 should be of such value as to match the line impedance which 10typically might be 600 or 900 ohms. A feedback resistor 47 is 11 connected between the output terminal 48 of the operational 12 amplifier and the inverting input.
13It will be seen that a signal received from source 40 is 14 applied to the non-inverting input of amplifier 41 as well as to the inverting input, the latter through buffer amplifier 42 and 16resistors 46 and 43. Signals from source 45 are applied only to 17 the inverting input of amplifier 41, and are blocked from being 18 applied to the non-inverting input due to unidirectional buffer ~19 amplifier 42.
20The gain of this amplifier for signals from source 40 is ~21given by the e~pression R47/tR44 x R43) , where each of the ~22tR44 + R43) 23 parameters is the resistance of the numerically identified re-24 sistor, in ohms.
25On the other hand, the gain for signals from source 45, ~26 (assuming R46 is of a low resistance relative to resistors R43 27and R47) is R47/R43.
28It should be noted that should signal source 45 not be of 29~ low impedance relative to resistor 43, its impedance should be ~`` -~30 16 .
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01 a~ded to resistors 43 as an internal part thereof for the gain 02 calculation.
03 ~s an example, assume resistor 46 is of low impedance, for 04 instance 600 ohms, resistor 43 is 50K ohms, resistor 44 is 10K
05 ohms and resistor 47 is 250K ohms. Assume also that two differ-06 ent signals, each oE 1 volt amplitude appear at sources 40 and 07 45.
08 The galn of this stage Eor the signal at source 45 will be 09 250K/50K or 5, and the amplitude of the output signal from source 45 will be -5 x 1.0 = 5.0 volts. Since the signal had been AC, ll the 5.0 volt output signal has its phase inverted 180 degrees 12 from the input signal.
13 On the other hand the signal from source 40 is applied to 14 the circuit and has a gain of 250K /(50~ x10K) 60 = 30. The output ~16 signal amplitude therefore is 30 x 1.0 = 30 volts.
17 The above is dependant, of course, on the power supply 18 used for the amplifier (not shown) being sufficient to allow the 19 vol-tage levels noted to be obtained. Nevertheless it is exemplary of a circuit which can equalize different signal 21 levels, or can amplify, in one stage, two different signals to 22 different degrees. Indeed, should various ones of the resistors ~2~3 be made variable, i~e., by means of a signal variable resistor 24 such as a field effect transistor, an optical isolator, or the ~25 like, one or both of the input signals can be subjected to ~26 various gain modifications or can be modulated at will ~rom an ~;27 outside source. Interaction or various degrees of addition of 28~ the two input signals can thus be readily obtained.
`29 Returning now to figure 2, it had been noted earlier that 30~ the circuit contains means for applying ringing voltage to the 31 ~subscriber's line. A ringing relay 50 has the usual back EMF
32~ 17 ~, : ; ~ ;' : ~ :: ~ ~ . - ..
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01 protection diode 51 connected thereacross. It is connected to a 02 source of voltage -~8v and to the collector oE a PNP transistor 03 52, which itself has its emitter connected to ground.
04 A source of ring actuate signa:L is applied from a ring 05 actuate lead throu~h resistors 53 and 54 to the base of transis-06 tor 52, while a source of potential +V is connected through re-07 sistor 55 to the ring actuate lead as bias for transistor 52.
08 A signal is applied to the OFH~ lead Eor the provision of 09 remote ringing shut off. 'rhe OFHK lead is connected via diode 56 10 through resistor 5~ to the base of transistor 52.
11 In operation, a source of low potential is applied to the 12 ring actuate lead. This drops the potential level at the base, 13 and causes operation of transistor 52, which conducts and opera-t-14 es ringing relay 50.
In the meantime the OFHK lead has a source of low potent--16 ial amplified, and diode 56 is back biased. In the event of an 17 indication of the local subscriber going off-hook, from a remote ~18 circuit (not shown), the level on OFHK lead changes and carries a ~19 high potential, causing diode 56 to c~onduct, raising the level at ~20 the junction of resistors 53 and 54 and thus the base of transis-~21 tor 52, to high potential. Transistor 52 accordingly ceased con-22 duction, open circuiting the current path through ringing relay , , .
23 50, which is shut off.
~2~ It is preferred to connect a light emitting diode 57 through resistor 58 to a negative source of potential and to the ~26 OFHK lead. Accordingly when high level potential appears on the 27 OFHK lead causing shutoff of ringing relay 50, light emitting ~28 diode S7 operates, providing a visual indication to an operator ,, .
~29 or serviceman that the circuit has functioned.
~30 Ringing relay 50 is comprised of a single pole double ~31 throw set of contacts, having a moving pole contact 59, a brea~
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01 contact 60 and a ma]ce contact 61. r~ake contact 61 is connected 02 through resistor 62 to a terminal for connect:ion as a source of 03 ringing current 63.
04 When ringing relay 50 is engergized, break contact 60 05 opens and moving pole contact 59 contacts make contact 61, which 06 applies ringing current to the tip and ring terminal 64 for 07 application to the ringer of the loca:L subscribers telephone 08 set connected to terminals 64 and 65. Once ringing has been 09 tripped by the subscriber going off-hook, ringing relay 50 is caused to open, causing moving contact 59 to contact break con-11 tact 60, and thus stopping ringing current from being applied to 12 terminal 64 and the subscriber's telephone set, and re-establish-13 ing the circuit from the telephone set to the line circuit.
14 It will be noted that when the ringing relay has operated, ;15 the subscriber's line becomes unbalancedr and ringing signals as ~16 well as an altered DC level passes through and appears at the out-;17 put of differential amplifier 10. Both the ringing and DC signal 18 are applied through resistor 66 to a low pass active filter 67 of 19 well known contruction. Active filter 67 is fabricated so as to ~20 block the 20 Hz ringing signals, but to pass signals of lower 21 frequency, including the DC off-hook signal level from the output 22 of operational amplifier 10. It is also preferred that active 23 filter 67 should pass 10 Hz signalsr the frequency of rotary dial 24 pulsesr although this is not essential for the present circuit.
The output of low pass active filter 67 is connected to 26 the noninverting input of differential amplifier 68r the output :27 of which is connected through resistor 69 and diode 70 to the 28 noninverting input of differential amplifier 71~ The output of 29 diode 70 is connected to ground through an integrating filter ,, .
;30 circuit comprising copacitor 72 in parallel with resistor 73 con-~31 19 `' ~ ''`
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01 nec~ed to a negative source o~ potential. The inverting input 02 oE diffe~enti~l am~liEier 71 is connected to grount~, and the in-03 verting ;nput of differential amplifier 68 is connected to a 0~ source off-'nook bias via an OFHlC BIAS lead. The DC level o~ this 05 lea~ sets the sensitivi~y oE the circuit for sensing the DC
06 oEf-hoolc level from the output oE diEEerential ampli~ier 10.
07 In operation, the DC potential level output o~ dif~eren-03 tial operational ampli~ier 10 is applied through resistor 66 to 03 low pass ac-tive filter 67. Here all signals below ringing fre-ln quency are passed throug'n the Eilter, and ringing signals are re-11 -jected or greatly attenuated. The resulting DC level is applied 12 to diEEerential amplifier 63, which provides an output signal 13 once the DC level is higher than the potent'ial level on the 14 o~E-hook bias lead. This signal is rec-ti-~ied and Eiltered to DC
in ~iode 70~ capacitor 72 and resistor 73, the resulting DC
lG signal being applied to the inpu-t of diEferential ampliEier 71.
17 When an off-hook condition exists, the output o~ di~erent-1~ ial amoliEier 71 provides a high potential level, causing diode 19 5~ to become forward biased, and cutting off operation oE trans-istor 52. As described earlier wit'n respect to a high level 21 signal on the OF~IK lead, ringing relay 50 is caused to open, re-22 moving ringing signal.s which had been applied to the subscribers 23 line.
24 In the meantime a signal on the RING ACTUATE lead had been removed, and ~he +V potential, applied through resistors 53 and ~ . , 26 54 are operative to cause the emitter-base junction oE transistor 27 52 to be reverse biased, thus holding ringing relay 50 inopera-28 tive.
2~ Switch lS, whlch preEerably is a CMOS switch also is oper- -atec~ by a~signal on the RING ACTUATE lead. Accordingly when 31 ringing is applied to~the subscriber's line upon the actuation of 32 ~ ~ 20 -'~
-~ `' ':.''" '' ' ' , , 01 ringing rela~ 50, switch 15 is caused to open. Ringing signals 02 thus appearing at the output of differential amplifier 10 are 03 stopped from being applied to the output line via output line 04 terminal 19, as well as from returning through amplifier circuit 05 34.
06 It should be noted that with the conduction of diode 56, 07 light emitting diode 57 also conducts and illuminates, providing 08 an indication that the subscriber has gone off-hook. A
Og serviceman at the PBX examining the line circuit printed circuit boards thus can obtain an indica-tion of which subscriber's lines ~11 are busy and which have remained on hook.
12 As noted earlier, the present line circuit can be used as 13 a standardized circuit for interfacing with standard telephone 14 sets. Two circuits connected with adjoining output terminals provide a four wire to two wire two way amplifier conversion.
`16 Each circuit converts a balanced tip and ring pair to an unbal-17 anced pair, whereby single wire switching can be used. The cir-18 cuit also contains provision for externally controlled gain in-19 crease for conferencing or the like.
~20 As one of the important elements of the invention, a novel ~21 amplifier has been used which amplifies two different signals -22 with different amounts of gain.
23 The circuit also provides means for applying ringing sig-24 nal to subscribers telephone set, and for detecting a subsequent off-hook condition when the subscriber has answered the tele-~26 phone, and for ring tripplng. Facility is also provided for ex-~27 ternally tripping the ringing, as might be provided, for in-~28 stance, by an external timing circuit.
`29 It will now become clear to a person skilled in the art understanding this invention that varlous variations and modifi--~31 cations could be made. All are considered within the scope of 32~ the present invention as defined in the appended claims.
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01 ~U P EM NTARY DI C OStJRE
02 Figure 4 shows a schematic of an alternative form of 03 the invention.
04 It should be noted that the subject line circuit can 05 be connected to a tip and ring lead at one port, to an incoming 06 signal circuit at another port/ and to an outgoing signal 07 circuit, at a third port, the second and third ports being 08 adapted for connection to such apparatus as a Codec. The input 09 to differential amplifier 16, rather than being connected to terminal 19, can be connected to a circuit ~or receiving an 11 input signal, e.g., the output circuit of the Codec. In 12 addition, the output of operational amplifier 17 can be 13 connected to an input circuit, e.g., the input circuit o~ the 14 Codec. The unbalanced output circuit of Figure 2 thus can be converted to two ports, an input port and an output port, from a 16 single bidirectional port.
17 The circuit of Figure 4, which shows various ~: 18 modifications to the circuit of Figure 2, depicts how the 19 circuit can be converted to have two ports on one side, and a bidirectional port on the other.
21 ~ The two inputs to differential amplifier 16 are Z2 connec-ted via respective resistors 75 and 76, one to ~;23 bidirectional unbalanced line input/output terminal 19, and the ~` 2~4~ other to;the output of differential amplifier 17. The inverting 25 ~; input of operational amplifier 16 is also connected through ;~26 reSlstQr ~0 to ground.
~27 ~lowever, for connection to a Coder-decoder (Codec) or ~ - -~28 other such 4 wlre apparatus, the conductive path to terminal 19 2~9 ~ is broken at polnt 77 ~and resis:tor 75 is connected instead to ;30 input terminal~ 78 instead. The conductive path to the output of :
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01 differential amplifier 17 is broken at point 79 and resistor 7 02 is connected instead to ground; both of the alternative 03 connections are shown in dashed line. In this case amplifier 16 04 need not be of differential type.
05 To simplify the circuit of Figure 2, ra-ther than 06 utilizing transistors 26 and 27 in the output circuit of 07 differential amplifier 16, a sing:Le transistor 81 i5 used in -the 08 circuit of Figure 4. This allows connection of the collector 09 of transistor 81 to a simplified power supply, e.g. to a -~5 vo]t source, rather than to a split vo:Ltage power supply. The 11 emitter is connected through resistor 30 to the tip lead, in 12 this embodiment. Resistor 82 is connected between the base and ~13 the emitter of transistor 81.
14 Similar to the emitters of transistors 26 and 27 in Figure 2, the emitter of transistor 81 is connected through 16 resistor 30 to the tip lead, and through resistor 32 to the 17 noninverting input of differential amplifier 10.
18 The circuit of Figure 4 has been modified for 19 operation with PBXs utilizing a lower voltage (e.g. -24 volts) than is normally used by a central office, which has the 21 availability of a -48 volt supply. The present circuit is ;22 adapted to use power supplies having filtering which is somewhat 23 poorer than might otherwise be desired, as would be economically 24 advantageous in a low-cost PBX.
~25 Since a low cost power supply would often contain ;-~ 26 significant amounts of A.C. ripple, the power supply leads are 27 applied to the tip and ring leads in a manner similar to the ~8 signals received ~rom ~ifferential amplifier 16. The -24 volt ~;~29 supply is connected (optionally through switch 83, which switch ~ - , is also connected to a source of ringing current~ to the ring ;: :
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. : .- - - ~ .: - -01 lead through r~sistor 84 and to the inverting input of 02 diEferential amplifier 10 through resistor 85. The resistance 03 of resistor 8~ should be the same as that of resistor 30, e.g., 04 one half the line resistance (or about 300 ohms) and resistor 85 05 should be of similar resistance as resistor 32 (e.g., about 06 200,000 ohms). Resistor 86 is connected from the noninverting 07 input oE differential amplifier 10 to ground, and can be, e.g., 08 about 10,000 ohms.
09 It may be seen that with the above noted di~erence in resistance between resistors 84 and 85, the major portion of 11 D.C. and A.C. ripple current from the -24 volt supply passes 12 through resistor 84 to the ring lead. The current is applied 13 with substantially reduced amplitude to the inverting input of 14 differential amplifier 10. The effect of the reduced current being applied to the inverting input, and the high curren-t being 16 applied (with reduced amplitude due to resistor 2) to the 17 noninverting input causes cancellation of the A.C. ripple within 18 the differential amplifier, and no significallt ripple appears at 19 the output of differential amplifier 10 which could be passed to output terminal 19.
21 A.C. ripple which appears between the tip and ring 22 terminals 64 and 65 can be cancelled by the application of the 23 -24 volt supply through capacitor 87 in series with resistor 88 24 to the noninverting input of differential amplifier 16, which blocks the -24 volt D.C. but applies the A.C. ripple to the 26 differential amplifier. me values of capacltor 87 and resistor 27 88 should be adjusted so that there is unity gain to the tip 28 lead, or more precisely so that the applied A.C. ripple cancels 29 whatever ripple appears bet~een the tip and ring leads.
In a simplified line circuit in ~hich no conferencing .
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01 function is required, the output of dlfferential amplifier 10 0~ can be connected directly through capacitor 14 to the input of 03 differential amplifier 17. However, to facilitate remote muting 04 of the outgoing signal, a pair of resistors 89 and 90 are 05 connected in series with the output circuit o~ differential 06 amplifier 10. The junction of resistors 89 and 90 is connected 07 to the collector of a transistor 91, which has its emitter 08 connected to the ~5 volt supply. A lead from a muting logic 09 circuit, not part of this invention/ is connected to the base of transistor 91.
11 Upon the muting logic circuit applying a low level 12 signal to the base of transistor 91, the ~5 volt supply is 13 connected through the emitter-collector circuit of transistor 91 14 to the junction of resistors 89 and 90. This effectively shunts the output of differential amplifier 10 to the supply, cutting 16 off the transmission of signal to the input of differential 17 amplifier 17.
18 In addition, an indication of an off-hook condition on ~19 the tip and ring leads can be obtained by connecting one input ~`20 of an operational amplifier 92 to the output of differential 21 amplifier 10. The other input is connected to a voltage divider ~22 comprising resistors 93 and 94 connected between the -24 volt 23 power supply lead and ground. With a change in the D.C. voltage ~24 across the tip and ring leads, the output voltage level of ~25 differential amplifier 10 changes, causing operational ~26 amplifier 92 to conduct once the threshold set by the voltage 27 divider has been exceeded. The output lead OFHR of operational 28 ~ amplifier 92 provides an output signal which is indlcative of 29 the on or off-hook ccndition of the tip and ring leads.
~30 ~25 :
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Claims (18)
1. A telephone line circuit comprising (a) balanced tip and ring leads for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals also connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the offhook impedance of the telephone set, (c) a buffer amplifier having its input connected in a first circuit path to the output of the first differential amplifier means and its output connected in a second circuit path to the line output lead, (d) second differential amplifier means having its input termin-als connected respectively in circuit paths to the first and second circuit paths, (e) the output of the second differential amplifier means being connected to one of the tip or ring leads, (f) the output of the second differential amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second differential.
amplifier means to the input of the first differential amplifier means which is connected to said other of the tip or ring leads such as to substantially cancel a signal within the first differential amplifier means which is applied to its input terminals via said one of the tip or ring leads from the output of the second differential amplifier means.
amplifier means to the input of the first differential amplifier means which is connected to said other of the tip or ring leads such as to substantially cancel a signal within the first differential amplifier means which is applied to its input terminals via said one of the tip or ring leads from the output of the second differential amplifier means.
2. A telephone line circuit as defined in claim 1 in which the first differential amplifier means has a gain of less than unity, the buffer amplifier has gain of about the reciprocal of the gain of the first differential amplifier means, and the second differ-ential amplifier means is comprised of amplifying means having gain which is different for different signals applied to each of its inputs from the first circuit path and from the second circuit path respectively, the gains being selected to translate the individual signals applied from the output of the first differential amplifier means to each of the inputs of the second differential amplifier means to amplitude levels which are substantially equal whereby they substantially cancel within the second differential amplifier means.
3. A telephone line circuit as defined in claim 2 further in-cluding means for selectively increasing the gain of the buffer amplifier so as to increase the level of an output signal applied to the line output lead.
4. A telephone line circuit as defined in claim 3 further in-cluding a resistor means connected in series with the output of the buffer amplifier to the second circuit path, having a value similar to the line output impedance.
5. A telephone line circuit as defined in claim 1, 3 or 4, in which the output of the second differential amplifier means is connected to one pole of a source of current and to said one of the tip or ring leads through a first matching resistor means having value of about 1/2 the off hook impedance of the telephone set, the opposite pole of the current supply being connected through a second matching resistor means of similar value as the first matching resistor means to the other of the tip or ring leads.
6. A telephone line circuit as defined in claim 1, in which the output of the second differential amplifier means is connected to one pole of a source of current, and to said one of the tip or ring leads through first matching resistor means having value of about 1/2 the off hook impedance of the telephone set, the opposite pole of the current supply being connected through a second matching resistor means of similar value as the first matching resistor means to the other of the tip or ring leads;
further including a tip terminal, a ringing relay comprising an operate signal input lead, and a set of single pole double throw contacts, the pole of the contacts being connected to the tip terminal, a break contact being connected to the tip lead and a make contact being connected to means for connection to a source of ringing current; a switch means connected in series between the output of the first differential amplifier means and the first circuit path, having an enable input for causing operation of the switch means and opening of the first circuit path, the enable input being connected to the operate signal input lead of the ringing relay, whereby upon reception of a ring operate signal on said operate signal input lead, the ringing relay is caused to operate, causing the pole of its contacts to break the current path to the tip lead and to connect it to the means for connection to the source of ringing current, and said switch is caused to open said first circuit path.
further including a tip terminal, a ringing relay comprising an operate signal input lead, and a set of single pole double throw contacts, the pole of the contacts being connected to the tip terminal, a break contact being connected to the tip lead and a make contact being connected to means for connection to a source of ringing current; a switch means connected in series between the output of the first differential amplifier means and the first circuit path, having an enable input for causing operation of the switch means and opening of the first circuit path, the enable input being connected to the operate signal input lead of the ringing relay, whereby upon reception of a ring operate signal on said operate signal input lead, the ringing relay is caused to operate, causing the pole of its contacts to break the current path to the tip lead and to connect it to the means for connection to the source of ringing current, and said switch is caused to open said first circuit path.
7. A telephone line circuit as defined in claim 6, further including means for sensing the off hook condition of a telephone station set connected to the ring lead and to the tip terminal and for applying an inhibiting signal to the operate signal input lead of the ringing relay for causing release of said relay and of said switch means upon sensing of the off hook condition.
8. A telephone line circuit as defined in claim 7, in which the means for sensing is connected to the output of the first differ-ential amplifier means, and includes a low pass filter adapted to block ringing frequency signals but to pass lower frequency signals.
9. A telephone line circuit as defined in claim 1, in which the output of the second differential amplifier means is connected to one pole of a source of current supply and to said one of the tip or ring lead through a first matching resistor means having value of about 1/2 the off hook impedance of the telephone set, the opposite pole of the current supply being connected through a second matching resistor means of the same value as the first matching resistor means to the other of the tip or ring lead, and further including a gate having an output connected to means for operating a ringing relay, an enable input to the gate for causing operation of the ringing relay, and a D.C. potential level sensing means having an input connected to the output of the first differential amplifier means and an output connected to an inhibit input of the gate for causing release of the ringing relay upon sensing of a predeterminal level of said D.C. potential at the output of said first differential amplifier means caused by the conduction of current from the current supply through an external telephone set from the ring to the tip lead.
10. A telephone line circuit as defined in claim 9 further in-cluding means for selectively varying the threshold of sensing of said D.C. potential level.
11. A telephone line circuit as defined in claim 10 in which the potential level sensing means is comprised of a low pass filter having a passband lower than ringing current frequency.
12. A telephone line circuit comprising:
(a) balanced tip and ring leads for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input term-inals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being large relative to the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to the line output lead for receiving incoming signals, (e) the output of the second amplifier means being connected to one of the tip or ring leads to which one input of the first differential amplifier is connected, (f) the output of the second amplifier means also being connected by a further resistor means to the other input of the first differential amplifier means, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means such as to substantially cancel a signal within the first differential amplifier means which is applied to said input terminal via said one of the tip or ring leads from the output of the second amplifier means.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
(a) balanced tip and ring leads for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input term-inals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being large relative to the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to the line output lead for receiving incoming signals, (e) the output of the second amplifier means being connected to one of the tip or ring leads to which one input of the first differential amplifier is connected, (f) the output of the second amplifier means also being connected by a further resistor means to the other input of the first differential amplifier means, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means such as to substantially cancel a signal within the first differential amplifier means which is applied to said input terminal via said one of the tip or ring leads from the output of the second amplifier means.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
13. An amplifier circuit as defined in claim 12 further including means for applying current from a D.C.
power source to said other of the tip or ring leads; and to theinput of the first differential amplifier means which is connected to said one of the tip or ring leads through a resistor of similar value as said further resistor means, whereby A.C. ripple which may be carried by the power source current is applied to said other of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means.
power source to said other of the tip or ring leads; and to theinput of the first differential amplifier means which is connected to said one of the tip or ring leads through a resistor of similar value as said further resistor means, whereby A.C. ripple which may be carried by the power source current is applied to said other of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means.
14. An amplifier circuit as defined in claim 12 further including means for applying current from a D.C. power source via a capacitor in series with a resistor to the input of the second amplifier means, whereby A.C. ripple generated by the power source is applied to said tip and ring leads so as to cancel hum of similar frequency as said ripple carried by said leads.
15. An amplifier circuit as defined in claim 12 further including means for applying current from a D.C. power source to said other of the tip or ring leads, and through a resistor of similar value as said further resistor means to the input of the first differential amplifier means which is connected to said one of the tip or ring leads, whereby any A.C. ripple current which may be carried by the power source current is applied to said one of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means, and means for applying current from said D.C. power source via a capacitor in series with a resistor to the input of the second amplifier means, whereby A.C. ripple current generated by the power source is applied to said tip and ring leads so as to cancel hum of similar frequency as said ripple carried by said leads.
16. A telephone line circuit comprising:
(a) a balanced tip and ring lead for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to a line input lead for receiving incoming signals, (e) the output of the second amplifier means being connected in a circuit path to one of the tip or ring leads, (f) the output of the second amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means so as to substantially cancel signals within the first amplifier means appearing at its input terminals which were applied from the output of the second amplifier means to the tip and ring leads, and (h) means for applying current from a D.C. power source to said other of the tip or ring leads, and to the input of the first differential amplifier means which is connected to said one of the tip or ring leads through a resistor of similar value as said further resistor means, whereby A.C. ripple which may be carried by the power source current is applied to said other of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means.
(a) a balanced tip and ring lead for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to a line input lead for receiving incoming signals, (e) the output of the second amplifier means being connected in a circuit path to one of the tip or ring leads, (f) the output of the second amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means so as to substantially cancel signals within the first amplifier means appearing at its input terminals which were applied from the output of the second amplifier means to the tip and ring leads, and (h) means for applying current from a D.C. power source to said other of the tip or ring leads, and to the input of the first differential amplifier means which is connected to said one of the tip or ring leads through a resistor of similar value as said further resistor means, whereby A.C. ripple which may be carried by the power source current is applied to said other of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means.
17. A telephone line circuit comprising:
(a) a balanced tip and ring lead for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to a line input lead for receiving incoming signals, (e) the output of the second amplifier means being connected in a circuit path to one of the tip or ring leads, (f) the output of the second amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means so as to substantially cancel signals within the first amplifier means appearing at its input terminals which were applied from the output of the second amplifier means to the tip and ring leads, and (h) means for applying current from a D.C. power source via a capacitor in series with a resistor to the input of the second amplifier means, whereby A.C. ripple generated by the power source is applied to said tip and ring leads so as to cancel hum of similar frequency as said ripple carried by said leads.
(a) a balanced tip and ring lead for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to a line input lead for receiving incoming signals, (e) the output of the second amplifier means being connected in a circuit path to one of the tip or ring leads, (f) the output of the second amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means so as to substantially cancel signals within the first amplifier means appearing at its input terminals which were applied from the output of the second amplifier means to the tip and ring leads, and (h) means for applying current from a D.C. power source via a capacitor in series with a resistor to the input of the second amplifier means, whereby A.C. ripple generated by the power source is applied to said tip and ring leads so as to cancel hum of similar frequency as said ripple carried by said leads.
18. A telephone line circuit comprising:
(a) a balanced tip and ring lead for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to a line input lead for receiving incoming signals, (e) the output of the second amplifier means being connected in a circuit path to one of the tip or ring Leads, (f) the output of the second amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means so as to substantially cancel signals within the first amplifier means appearing at its input terminals which were applied from the output of the second amplifier means to the tip and ring leads, and (h) means for applying current from a D.C. power source to said other of the tip or ring leads, and through a resistor of similar value as said further resistor means to the input of the first differential amplifier means which is connected to said one of the tip or ring leads, whereby any A.C. ripple current which may be carried by the power source current is applied to said one of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means, and means for applying current from said D.C. power source via a capacitor in series with a resistor to the input of the second amplifier means, whereby A.C. ripple current generated by the power source is applied to said tip and ring leads so as to cancel hum of similar frequency as said ripple carried by said leads.
(a) a balanced tip and ring lead for connection to a telephone set, and an unbalanced line output lead, (b) first differential amplifier means having its input terminals connected through individual equal valued resistor means to the tip and ring leads, the values of each of the resistor means being at least 10 times the off hook impedance of the telephone set, (c) the output of the first differential amplifier means being connected in a circuit path to said line output lead, for applying outgoing signals thereto, (d) second amplifier means having its input connected in a circuit path to a line input lead for receiving incoming signals, (e) the output of the second amplifier means being connected in a circuit path to one of the tip or ring Leads, (f) the output of the second amplifier means also being connected by a further resistor means to the input of the first differential amplifier means which is connected to the other of the tip or ring leads, (g) the value of the further resistor means being selected to apply sufficient output signal from the second amplifier means to said other input of the first differential amplifier means so as to substantially cancel signals within the first amplifier means appearing at its input terminals which were applied from the output of the second amplifier means to the tip and ring leads, and (h) means for applying current from a D.C. power source to said other of the tip or ring leads, and through a resistor of similar value as said further resistor means to the input of the first differential amplifier means which is connected to said one of the tip or ring leads, whereby any A.C. ripple current which may be carried by the power source current is applied to said one of the tip or ring leads and to the input of the differential amplifier means so as to substantially cancel within the first differential amplifier means, and means for applying current from said D.C. power source via a capacitor in series with a resistor to the input of the second amplifier means, whereby A.C. ripple current generated by the power source is applied to said tip and ring leads so as to cancel hum of similar frequency as said ripple carried by said leads.
Priority Applications (10)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP53055443A JPS5921237B2 (en) | 1978-02-01 | 1978-05-09 | telephone line circuit |
| US05/906,772 US4178485A (en) | 1978-02-01 | 1978-05-17 | Transformerless telephone line circuit |
| DE2824468A DE2824468C2 (en) | 1978-02-01 | 1978-06-03 | Telephone booster circuit |
| FR7822336A FR2416600B1 (en) | 1978-02-01 | 1978-07-24 | TELEPHONE LINE CIRCUIT |
| GB7848666A GB2014398B (en) | 1978-02-01 | 1978-12-15 | Telephone line circuit |
| FR7917277A FR2460076B2 (en) | 1978-02-01 | 1979-06-27 | TELEPHONE LINE CIRCUIT |
| US06/062,989 US4281219A (en) | 1978-02-01 | 1979-08-02 | Telephone line circuit |
| CA000371449A CA1117232A (en) | 1978-02-01 | 1981-02-20 | Telephone line circuit |
| GB08200788A GB2106345B (en) | 1978-02-01 | 1982-01-12 | Telephone line circuit |
| JP57124279A JPS5853556B2 (en) | 1978-02-01 | 1982-07-15 | Amplifier circuit for telephone line circuit |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP2925444.2 | 1979-06-23 | ||
| DE19792925444 DE2925444C2 (en) | 1979-06-23 | 1979-06-23 | Telephone booster circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1105162A true CA1105162A (en) | 1981-07-14 |
Family
ID=6074008
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA296,137A Expired CA1105162A (en) | 1978-02-01 | 1978-02-01 | Telephone line circuit |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1105162A (en) |
| DE (1) | DE2925444C2 (en) |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3889072A (en) * | 1973-10-19 | 1975-06-10 | Itt | Bi-directional amplification apparatus |
-
1978
- 1978-02-01 CA CA296,137A patent/CA1105162A/en not_active Expired
-
1979
- 1979-06-23 DE DE19792925444 patent/DE2925444C2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE2925444C2 (en) | 1985-07-04 |
| DE2925444A1 (en) | 1981-01-15 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |