CA1117232A - Telephone line circuit - Google Patents
Telephone line circuitInfo
- Publication number
- CA1117232A CA1117232A CA000371449A CA371449A CA1117232A CA 1117232 A CA1117232 A CA 1117232A CA 000371449 A CA000371449 A CA 000371449A CA 371449 A CA371449 A CA 371449A CA 1117232 A CA1117232 A CA 1117232A
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Abstract
ABSTRACT OF THE DISCLOSURE
The present invention is an amplifier circuit having gain which is different for different signals applied to each of its inputs. The circuit is comprised of a differential operational amplifier having an inverting and a noninverting input, and an output, a feedback resistor (R1) connected between the output and inverting input, a first signal source having its output connected to the noninverting input, a buffer amplifier having its input connected to the first signal source, a second signal source having its output connected to the output of the buffer amplifier and through an input resistor (R2) to the inverting input of the differential operational amplifier, and a bypass resistor (R3) connected from said inverting input to ground, whereby the gain provided by said amplifier circuit to a signal from the first signal source is given by the expression and the gain provided by said amplifier circuit to a signal from the second signal source is given by the expression
The present invention is an amplifier circuit having gain which is different for different signals applied to each of its inputs. The circuit is comprised of a differential operational amplifier having an inverting and a noninverting input, and an output, a feedback resistor (R1) connected between the output and inverting input, a first signal source having its output connected to the noninverting input, a buffer amplifier having its input connected to the first signal source, a second signal source having its output connected to the output of the buffer amplifier and through an input resistor (R2) to the inverting input of the differential operational amplifier, and a bypass resistor (R3) connected from said inverting input to ground, whereby the gain provided by said amplifier circuit to a signal from the first signal source is given by the expression and the gain provided by said amplifier circuit to a signal from the second signal source is given by the expression
Description
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01 This invention relates to a transformerless bidirectional 02 4 wire/2 wire line circuit which is usefully employed in a PBX.
03 Line circuits which are used, for example in a P~X provide 04 a Eacility for feeding direct current via a subscriber's line -to 05 the local telephone sets to which they are connected, and as well 06 are required to block common ~ode signals while transmitting 07 voice frequency signals in both 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 modula-~ed 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 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 oE 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 signals do not cancel and as a result are 23 induced in the secondary winding and are applied to further
01 This invention relates to a transformerless bidirectional 02 4 wire/2 wire line circuit which is usefully employed in a PBX.
03 Line circuits which are used, for example in a P~X provide 04 a Eacility for feeding direct current via a subscriber's line -to 05 the local telephone sets to which they are connected, and as well 06 are required to block common ~ode signals while transmitting 07 voice frequency signals in both 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 modula-~ed 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 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 oE 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 signals do not cancel and as a result are 23 induced in the secondary winding and are applied to further
2~ switching or other circuitry.
However, since direct current must be fed to the local sub 26 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 do~s not saturate at normal line currents.
It is also highly desirable to apply ringing current to 31 the local subscriber's line from the line circuit, and to be able ~$
01 to sense the state of the subscriber's telephone set should the 02 telephone set go off hook.
03 For use particularly with an electronic PBX which uses two 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 for 07 switching, w~ile at the same time raising the amplitude of a 08 received signal from other telephone sets to a level required by 09 the local subscribers telephone set. It is also preferable to be able to change the output signal levels at will in order -that the `
11 switched signal levels might be equalized in the case of a 12 conference call controlled by the PBX.
13 ~he invention described in U.S. patent 4,007,335 dated 1~ February 8, 1977, assigned to Bell Telephone Laboratories, utilizes an integrated circuit which is connected between the tip 16 and ring leads and is intended to reduce common mode signals.
17 This circuit exhibits a high impedance to differential 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 21 operate satisfactorily for the application to which it is -22 intended, by its nature it cannot transform a balanced line to an 23 unbalanced line (which otherwise would be done by a transformer 24 circuit). Accordingly ît cannot be used in a two wire switching networX, unless a following transformer is added.
26 Further, there i5 no facility for sensing the direct ~27 current level in the subscriber's line for determining the on or 28 off hook status thereof. There is further no facility for 29 changing the signal levels applied to the switching matrix.
Nei-ther is there a facility for applying ringing current to the 31 subscribers line nor for cutting it off when the subscriber has 01 gone off hook~
02 The presen-t invention, on the other hana, is a circuit 03 which is comprised of a four to two wire bidirectional ampli-04 fier. The present circuit transforms a balanced tip and ring 05 circuit to an unbalanced line, and at the same time transforms 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 transformer. The bidirectional amplifier performs 09 without introducing positive feedback or undesired sidetone. At the same time the circuit substantially blocks common mode sig-11 nals which may appear on the balanced pair from being transfered 12 to the unbalanced output line.
13 The circuit also provides means for increasing the ampli-14 tude of the outgoing signal to the unbalanced line in order that the reduced impedance, which may appear from the line to common 16 ground due to the connection thereacross of a plurality of con-17 ferencing telephone sets which would otherwise result in a reduc-18 tion in signal amplitude, is compensated for.
19 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 subscribers line, and for causing ringing to be 22 cut off 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 set, and an unbalanced 26 line output lead, first differential 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 of the 29 resistor means being at least 10 times the offhook impedance of the telephone set, a buffer ampliier having its input connected 31 in a first circuit path to the output of the first differential ~.~1.7Z3;Z
01 amplifier means and i-ts output connected in a second circuit path 02 to a line output lead, and second differential amplifier means 03 having its input terminals connected respectively in circuit 04 paths to the first and second circuit paths. The output of 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 first differential amplifier 08 means which is connected to th~ other of the tip or ring 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 14 differential amplifier means which is applied to its inpu-t terminals via the one of the tip or ring leads from tl~e 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 which is different for different 23 signals applied to each of its inputs from -the first 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 ;~
means.
31 More particularly, the present invention is directed to a 32 differential amplifier circuit having gain which is difEerent for .~ .
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01 different signals applied to each of its inputs. The circuit is 02 comprised of a differen-tial operational amplifier having an 03 inverting and a noninverting input, and an output, a feedback 04 resistor (Rl) connected between the output and inverting input, a ~05 first signal source having its output connected to the non-:06 inverting input, a buffer amplifier having its input connected 07 to the first signal source, a second signal source having its 08 output connected to the output of the buffer amplifier and thrugh 09 an input resistor ~R2) to the inverting input of the differential operational amplifier, and a bypass resistor ~R3) connected from 11 said inverting input to ground, whereby the gain provided by said 12 amplifier circuit to a signal from the first signal source is 13 given by the expression Rl/ (R2) (R3) and the gain provided by 14 (R2~ + (R3) said amplifier circuit to a signal from the second signal source 16 is given by the expression Rl where Rl, R2, and R3 are resistance 18 values in ohms.
19 A better understanding of -the invention will be obtained by reference to the detailed description below, and to the ~21 `24 27 i ;28 33 - 4a -`:
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01 following drawings, in which:
02 Figure 1 is a block schematic of the most general form of 03 the invention, 04 Figure 2 is a detailed schematic of the invention, and 05 Figure 3, which appears on the same sheet as Figure 1, is a 06 detailed schematic of a dual gain amplifier used in the invention.
07 Turning first to Figure 1, a general block schematic of 08 the invention is shown. A balanced line comprising tip and ring 09 leads is connected via equal valued resistors 1 and 2 to differ-ential ampliEier means 3. The output of differential amplifier 11 means 3 is connected to the input of a buffer amplifier 4 and to 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 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 oE differential amp-lifier means 5.
21 In operation, a subscriber's telephone set will be con-22 nected between the tip and ring leads. Signals are generated 23 thereby, 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 26 output signal between its output and ground. This signal is 27 applied to buffer amplifier 4 which applies it to the unbalanced 28 line output 7.
29 Common mode interference signals appearing on the tip and ring leads, however, are in phase on both leads with respect to 31 ground, and are applied with equal amplitude and phase to the in-32 puts of differential amplifier means 3. Accordingly since there 33 is no difference between the signals across the input of the amp 7~3Z
01 lifier means, the signals are not amplified and do 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 of differential amplifier means 3 13 and the input of buffer amplifier 4. The inverting input of 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 ampli~ier 3 and buffer amplifier ~. 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 signals from the respective inputs, the signals from differential 26 amplifier 3 are caused to cancel, and are not reapplied from the : 27 output of differential ampliier 5 back to the tip and ring 28 leads. Accordingly positive feedback, ~xcessive 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.
32 It should be noted that buffer amplifier 4 should there- :
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01 fore be of the non-inverting type, for -the polarity of the tip 02 and ring signal as applied to differential amplifier means 5 to 03 be correct. Furthermore, buffer amplifier 4 Eunctions as an 04 isolator, restricting signals arrivi.ng 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 amplifier means 5 and are 08 thus amplified and applied to the tip leadO
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 of differential amplifier means 3 should be of high 14 resistance, each preferably at least 100 times the offhook 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 lB signal arriving from the line output) by at least 200 times.
19 This lower amplitude signal is cancelled by the applica-tion of signal from the output of differential ampliEier 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 bs of value such as to 24 apply the exact amount of signal to the differential amplifier 3 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 currents, it is pre .~
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01 ferred that resistors l and 2 each should have tolerance within 02 0.1%.
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 ll connected 08 between its output and its inverting inpu-t in the convantional 09 manner. As noted with respect to Figure l, resistors l and 2 should each be at least 100 times the impedance of the off-hook ll telephone set which is to be connected 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 inver-ting and the 14 noninverting input terminals of operational amplifier 10 to ground.
16 The output of 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-~-erting 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 difer-23 ential amplifier 17 is connected through resistor 18 to the line 24 output terminal l9.
Line output terminal 19 is connected through resistor 20 26 to the inverting input of second dif~erential amplifie~ 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 OUtpll-t terminal l9, for 31 matching purposes, since the output impedance of differential 7~Z
01 amplifier 17 would otherwise be very low.
02 The inverting input of th.ird differential amplifier 17 is 03 connected to ground through resis-tor 21, and is also connected 04 via a feedback resistor 22 to the output of differential ampli-05 fier 17.
06 The inverting input of -third differential amplifier 17 is 07 also connected through individual resis-tors 23 and 24 in series 08 with make contacts 25 and 26 to ground. 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 of amplifier 11 17 for signals applied to the non-inverting input is determined 12 by ~he ratio of 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 are closed by an externally operated switching matrix, not 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
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, 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 inver-ting input of differential ampli-28 fier 16, which resistor is bypassed by a small high frequency 29 rolloff capacitor 29.
The operation of differential amplifier 16 with resistor 31 125 and transistors 126 and 127 as well as feedback resistor 28 32 is similar to a differential amplifier with a normally connected ~:
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01 feedback loop, but having a high current capacity output stage.
02 The output stage is required to feed line current to the tip and 03 ring leads as will be described below.
04 The emitters of transistors 126 and 127 are connec-ted 05 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 -48 07 volts.
08 As was described with respect to resistor 6 in Figure 1, 09 the emitters of transistors 126 and 127 are connected through re-sistor 32 to the non-inverting input of opera-tional 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 10 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 the tip and ring leads are not subjected to imbalance.
23 In operation, a subscriber's telephone set is assumed to 24 go off-hook. Ba-ttery voltage is supplied through transistors 126 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 the battery supply, -48 volts. It is important 28 that the bat-tery feed circuit appear as a low impedance voltage 29 source.
The ratio of the sum of -the resistances of resi~tors 1 and 31 2 to the line impedance provides a vol-tage division of any common 01 mode signals which rnay appear on the tip and ring leads, as 02 applied to amplifier 10. While equal valued common mode signals 03 being applied to the input terminals of differential amplifier 10 04 would cancel in any event, it is preferred to reduce their amp-05 litude in case there is a differential interEerence component 06 modulated on the common mode signals, and in the case of differ-07 ences in the values of resistors 1 and 2, which would give rise 08 to a differential componentO
09 It is preferred that the amplification of the circuit which includes differential amplifier 10 should be about 1/10.
11 Accordingly if resistor 1 is 100,000 ohms for instance, resistor 12 11 should be about 10,000 ohms.
13 Differential voice frequency signals are also subjected to 14 a reduction in amplitude of 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 inver-ting input of 18 differential amplifier 16. The ratio of resistors 22 to 21 pre-19 ferably is 10 to 1, which provides an amplification of 10 of the voice signals applied to buffer amplifer 17. This compensates 21 completely of the signal loss in the circuit of amplifier 10, and 22 the original differential 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-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 28 crosspoint matrix 33, and is thus connected through -the P~X to a 29 similar terminal 19 of another line circuit of similar struc-ture. The line level of a signal at the line terminal 19 arriv 1.7Z32 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 amplifier 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 2~ are considered as a single ampli-11 fier circuit 34.
12 Amplifier 34 has been designed to amplify 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 the 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
t9.43K + 40.2K) 26 which equals approximately 26 (non-inverting~.
27 For signals arrived from the line terminal 19, the gain of 2s3 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 ~117~3;~
01 127 are connected through resistor 30 to the tip lead, the ring 02 lead being connected through resistor 31 to effective AC ground 03 (but which resistor in reality forms the DC line feed). The 04 impedance between -the tip and ring lead with the subscriber'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 Accordingly for a voice frequency signal on the tip and 08 ring leads applied to differential amplifier 10, there will be a 09 10 times reduction at its output, and the output signal will be 1/10 the input signal. This is the signal which is applied to 11 the non-inverting input of amplifier 16.
12 The signal i5 further amplified 10 times by the circuit of 13 differential amplifier 17, re-establishing the original level at 14 the output of the latter differential amplifier. However due to matching resistor 18 to the line impedance at output terminal 19, 16 the output signal level at output terminal 19 is 1/2 the original 17 signal 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 34 to the signals applied to its non--21 inverting and inverting inputs, the 1/10 original signal 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 -5, for an in-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 2~ negligible, but which may be desireable in some instances to 29 avoid the appearance of a "dead" line.
Clearly a selection of the values of resistors 20, 21 and 31 28, as well as resistors 22 and 21 (establishing the gain of dif-.
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01 ferential amplifier 17), or by providing one or more trimmer re-02 sistors, an exact cancellation of any signal from the tip and 03 ring leads from feeding back through differential amplifier 16 04 can be effected.
05 Considering now a signal arriving at line terminal 19 from 06 the switching matrix 33, this is applied at half the original 07 source level to amplifier 34. However due to the undirectional 08 nature of buffer amplifier 17, it is restricted from being 09 applied to the non-inverting input of aifferential amplifier 16.
Amplifier circuit 34 increases the amplitude of the signal by the 11 gain established by the ratio of resistors 28/20, noted earlier 12 as -5. The output signal of amplifier circuit 34 is thus -5 13 times the half 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 16 is a gain reduction, for the resistor values given, of 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, which is slightly but negligibly greater than the original. Clearly an increase in the 21 value of resistors 30 and 31 (i.e., to 350 ohms, for 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 24 level obtained as a result of use of 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 29 amplitude by resistors 1 and 2, which signal would be translated by differential amplifier circuit 10 as a normal differential 31 signal. Accordingly resistor 32 of at least twice the resistance 32 of the total of ~7;~:3;~
01 resistors 1 and 2 carries the output signal of amplifier circuit 02 34 to the non-inverting input of differential amplifier 10. This 03 signal provides an equal amplitude signal to the non-inverting 04 input as appears on the inverting inpu~, in order that exact can-OS cellation of the signal output of amplifier 34 should be ob-tained 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 vol-tage level will then be less than for a two party 14 call. For a conference call, one or both of switches 25 or 26 are closed 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 increased increasing the output signal level 18 to terminal 19 to the two-party call level. Compensation can 19 thus be made for a conference call with a Eew or a great many participants.
21 Accordingly, a bidirectional amplifier is provided, with a 22 battsry line feed and a translation of a balanced line to an 23 unbalanced line. The signal leve].s are completely controlled and -24 the facility provides selectable signal level compensation for conference calls.
26 Before describing the remainder of the circuit in detail, 27 reference is made to figure 3, which shows more generally the 28 amplifier circuit which provides two levels of gain for two dif-29 ferent input signals. A first signal source 40 is connected to the non-inverting input of a differential operational amplifier 31 41. The same signal source is also connected through a buffer 32 amplifier 42, and through an input resistor 43 to the inverting 01 input of differential amplifier 41. The same inverting inpu-t 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 o~ 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 expression R47/(R44 x R43) , where each of the 22(R44 ~ 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 01 added to resistors 43 as an internal part thereof for the gain 02 calculation.
03 As an example, assume resistor 46 is of low impedance, for 04 instance 600 ohms, resistor ~3 is 50K ohms, resistor 44 is lOK
05 ohms and resistor 47 is 250K ohms. Assume also that two differ-06 ent signals, each of 1 volt amplitude appear at sources 40 and 07 45.
08 The gain of this stage for the signal at source 45 will be 09 250K/~OK 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, 11 the 5.0 volt output signal has its phase inverted 1~30 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 /(50K xlOK) 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 voltage 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 23 be made variable, i.e., by means of a signal variable resistor 24 such as a field e~fect transistor, an optical isolator, or the like, one or both of the input signals can be subjected to 26 various gain modifications or can be modulated at will from an 27 outside source. Interaction or various degrees of addition o~
28 the two input signals can thus be readily obtained.
29 Returning now to figure 2, it had been noted earlier that the circuit contains means for applying ringing voltage to the 31 subscriber's line. A ringing relay 50 has the usual back EMF
!
~;7.2~
01 protection diode 51 connected thereacross. It is connec-ted to a 02 source of voltage -48V and to the collector of a PNP transistor 03 52, which itself has its emitter connected to ground.
04 ~ source o:E ring actuate signaL is applied from a ring 05 actuate lead through resistors 53 and 54 to the base o~ -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 OFHK lead for the provision of 09 remote ringing shut off. The OFHK lead is connected via diode 56 through resistor 54 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 operat-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 conduct, raising the level at the junction of resistors 53 and 54 and t~us 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.
24 It is preferred to connect a light emitting diode 57 through resistor 58 to a negative source of potential and to the 26 OF~K lead. Accordingly when high level potential appears on the 27 OFHK lead causing shutoff of ringing relay 50, light emitting 28 diode 57 operates, providing a visual indication to an operator 29 or serviceman that ~he circuit has functioned.
Ringing relay 50 is comprised o a single pole double 31 throw set of contacts, having a moving pole contact 59, a break `2 18 .7;Z3Z
01 contact 60 and a make contact 61. Make contact 61 is connected -02 through resistor 62 to a terminal for connection 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 local 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, the subscriber's line becomes unbalanced, 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 fil-ter 67 of 19 well known contruction. Active filter 67 is fabricated so as to block the 20 Hz ringing signals, but to pass signals of lower ~21 frequency, including the DC off-hooX signal level from the outpu-t 22 of operational amplifier 10. It is also preferred that active 23 filter 67 should pass 10 Hz signals, the frequency of rotary dial 24 pulses, 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 68, the output 27 of which is connected through resistor 69 and dioae 70 to -the 28 noninverting input of differential amplifier 71. The output of 29 diode 70 is connected to ground through an integrating :Eilter circuit comprising copacitor 72 in parallel with resistor 73 con-23;~
01 nected to a negative source of potential. The inverting input 02 of differential amplifier 71 is connected to ground, and the in-03 verting input of diferential a~plifier 68 is connected to a 04 source off-hook bias via an OFHK BIAS lead. The DC level of this 05 lead sets the sensitivity o the circuit Eor sensing the DC
06 off-hook level ~rom the output of differen-tial amplifier 10.
07 In operation, the DC potential level output of di~feren-08 tial operational amplifier 10 is applied through resistor 66 to 09 low pass active filter 67. Here all signals below ringing fre-quency are passed through the filter, and ringing signals are re-11 jected or greatly at-tenuated. The resulting DC level is applied 12 to differential ampliEier 68, which provides an output signal 13 once the DC leve~ is higher than the potential level on the 14 off-hook bias lead. This signal is rectified and filtered to DC
in diode 70, capacitor 72 and resistor 73, the resulting DC
16 signal being applied to the input of differential ampliEier 71.
17 When an o~-hook condition exists, the output of different 18 ial amplifier 71 provides a high potential level, causing diode 19 56 to become forward biased, and cutting of~ operation of trans-istor 52. As describea earlier with respect to a high level 21 signal on the OFHK lead, ringing relay 50 is caused to open, re-~22 moving ringing signals which had been applied to the subscribers 23 line.
24 In the meantime a signal on the RING ACTUATE lead had been removed, and the +V potential, applied through resistors 53 and 26 54 are operative to cause the emitter-base junction o~ transistor 27 52 to be reverse biased, thus holding ringing relay 50 inopera-28 tive.
29 Switch 15, which pre~erably is a CMOS switch also is oper-ated by a signal on the RING ACTUATE lead. Accordingly when 31 ringing is applied to the subscriber's line upon the actuation of .
01 ringing relay 50, switch 15 is caused to open. Ringing signals 02 thus appearing at the output of differen-tial 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 su~scriber has gone off-hook. A
09 serviceman at the PBX examining the line circuit printed circuit boards thus can obtain an indication 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 ~our 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 con-trolled gain in-19 crease for conferencing or the like.
As one of the important elements of the invention, a novel 21 amplifier has been used w~ich 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 of~-hook condition when the subscriber has answered the tele-26 phone, and ~or ring tripping. 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 various 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.
~ ., i . ~
However, since direct current must be fed to the local sub 26 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 do~s not saturate at normal line currents.
It is also highly desirable to apply ringing current to 31 the local subscriber's line from the line circuit, and to be able ~$
01 to sense the state of the subscriber's telephone set should the 02 telephone set go off hook.
03 For use particularly with an electronic PBX which uses two 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 for 07 switching, w~ile at the same time raising the amplitude of a 08 received signal from other telephone sets to a level required by 09 the local subscribers telephone set. It is also preferable to be able to change the output signal levels at will in order -that the `
11 switched signal levels might be equalized in the case of a 12 conference call controlled by the PBX.
13 ~he invention described in U.S. patent 4,007,335 dated 1~ February 8, 1977, assigned to Bell Telephone Laboratories, utilizes an integrated circuit which is connected between the tip 16 and ring leads and is intended to reduce common mode signals.
17 This circuit exhibits a high impedance to differential 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 21 operate satisfactorily for the application to which it is -22 intended, by its nature it cannot transform a balanced line to an 23 unbalanced line (which otherwise would be done by a transformer 24 circuit). Accordingly ît cannot be used in a two wire switching networX, unless a following transformer is added.
26 Further, there i5 no facility for sensing the direct ~27 current level in the subscriber's line for determining the on or 28 off hook status thereof. There is further no facility for 29 changing the signal levels applied to the switching matrix.
Nei-ther is there a facility for applying ringing current to the 31 subscribers line nor for cutting it off when the subscriber has 01 gone off hook~
02 The presen-t invention, on the other hana, is a circuit 03 which is comprised of a four to two wire bidirectional ampli-04 fier. The present circuit transforms a balanced tip and ring 05 circuit to an unbalanced line, and at the same time transforms 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 transformer. The bidirectional amplifier performs 09 without introducing positive feedback or undesired sidetone. At the same time the circuit substantially blocks common mode sig-11 nals which may appear on the balanced pair from being transfered 12 to the unbalanced output line.
13 The circuit also provides means for increasing the ampli-14 tude of the outgoing signal to the unbalanced line in order that the reduced impedance, which may appear from the line to common 16 ground due to the connection thereacross of a plurality of con-17 ferencing telephone sets which would otherwise result in a reduc-18 tion in signal amplitude, is compensated for.
19 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 subscribers line, and for causing ringing to be 22 cut off 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 set, and an unbalanced 26 line output lead, first differential 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 of the 29 resistor means being at least 10 times the offhook impedance of the telephone set, a buffer ampliier having its input connected 31 in a first circuit path to the output of the first differential ~.~1.7Z3;Z
01 amplifier means and i-ts output connected in a second circuit path 02 to a line output lead, and second differential amplifier means 03 having its input terminals connected respectively in circuit 04 paths to the first and second circuit paths. The output of 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 first differential amplifier 08 means which is connected to th~ other of the tip or ring 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 14 differential amplifier means which is applied to its inpu-t terminals via the one of the tip or ring leads from tl~e 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 which is different for different 23 signals applied to each of its inputs from -the first 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 ;~
means.
31 More particularly, the present invention is directed to a 32 differential amplifier circuit having gain which is difEerent for .~ .
7Z3;2 .
01 different signals applied to each of its inputs. The circuit is 02 comprised of a differen-tial operational amplifier having an 03 inverting and a noninverting input, and an output, a feedback 04 resistor (Rl) connected between the output and inverting input, a ~05 first signal source having its output connected to the non-:06 inverting input, a buffer amplifier having its input connected 07 to the first signal source, a second signal source having its 08 output connected to the output of the buffer amplifier and thrugh 09 an input resistor ~R2) to the inverting input of the differential operational amplifier, and a bypass resistor ~R3) connected from 11 said inverting input to ground, whereby the gain provided by said 12 amplifier circuit to a signal from the first signal source is 13 given by the expression Rl/ (R2) (R3) and the gain provided by 14 (R2~ + (R3) said amplifier circuit to a signal from the second signal source 16 is given by the expression Rl where Rl, R2, and R3 are resistance 18 values in ohms.
19 A better understanding of -the invention will be obtained by reference to the detailed description below, and to the ~21 `24 27 i ;28 33 - 4a -`:
~7~
01 following drawings, in which:
02 Figure 1 is a block schematic of the most general form of 03 the invention, 04 Figure 2 is a detailed schematic of the invention, and 05 Figure 3, which appears on the same sheet as Figure 1, is a 06 detailed schematic of a dual gain amplifier used in the invention.
07 Turning first to Figure 1, a general block schematic of 08 the invention is shown. A balanced line comprising tip and ring 09 leads is connected via equal valued resistors 1 and 2 to differ-ential ampliEier means 3. The output of differential amplifier 11 means 3 is connected to the input of a buffer amplifier 4 and to 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 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 oE differential amp-lifier means 5.
21 In operation, a subscriber's telephone set will be con-22 nected between the tip and ring leads. Signals are generated 23 thereby, 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 26 output signal between its output and ground. This signal is 27 applied to buffer amplifier 4 which applies it to the unbalanced 28 line output 7.
29 Common mode interference signals appearing on the tip and ring leads, however, are in phase on both leads with respect to 31 ground, and are applied with equal amplitude and phase to the in-32 puts of differential amplifier means 3. Accordingly since there 33 is no difference between the signals across the input of the amp 7~3Z
01 lifier means, the signals are not amplified and do 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 of differential amplifier means 3 13 and the input of buffer amplifier 4. The inverting input of 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 ampli~ier 3 and buffer amplifier ~. 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 signals from the respective inputs, the signals from differential 26 amplifier 3 are caused to cancel, and are not reapplied from the : 27 output of differential ampliier 5 back to the tip and ring 28 leads. Accordingly positive feedback, ~xcessive 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.
32 It should be noted that buffer amplifier 4 should there- :
L7;~
:
01 fore be of the non-inverting type, for -the polarity of the tip 02 and ring signal as applied to differential amplifier means 5 to 03 be correct. Furthermore, buffer amplifier 4 Eunctions as an 04 isolator, restricting signals arrivi.ng 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 amplifier means 5 and are 08 thus amplified and applied to the tip leadO
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 of differential amplifier means 3 should be of high 14 resistance, each preferably at least 100 times the offhook 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 lB signal arriving from the line output) by at least 200 times.
19 This lower amplitude signal is cancelled by the applica-tion of signal from the output of differential ampliEier 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 bs of value such as to 24 apply the exact amount of signal to the differential amplifier 3 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 currents, it is pre .~
7;23Z
01 ferred that resistors l and 2 each should have tolerance within 02 0.1%.
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 ll connected 08 between its output and its inverting inpu-t in the convantional 09 manner. As noted with respect to Figure l, resistors l and 2 should each be at least 100 times the impedance of the off-hook ll telephone set which is to be connected 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 inver-ting and the 14 noninverting input terminals of operational amplifier 10 to ground.
16 The output of 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-~-erting 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 difer-23 ential amplifier 17 is connected through resistor 18 to the line 24 output terminal l9.
Line output terminal 19 is connected through resistor 20 26 to the inverting input of second dif~erential amplifie~ 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 OUtpll-t terminal l9, for 31 matching purposes, since the output impedance of differential 7~Z
01 amplifier 17 would otherwise be very low.
02 The inverting input of th.ird differential amplifier 17 is 03 connected to ground through resis-tor 21, and is also connected 04 via a feedback resistor 22 to the output of differential ampli-05 fier 17.
06 The inverting input of -third differential amplifier 17 is 07 also connected through individual resis-tors 23 and 24 in series 08 with make contacts 25 and 26 to ground. 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 of amplifier 11 17 for signals applied to the non-inverting input is determined 12 by ~he ratio of 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 are closed by an externally operated switching matrix, not 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
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, 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 inver-ting input of differential ampli-28 fier 16, which resistor is bypassed by a small high frequency 29 rolloff capacitor 29.
The operation of differential amplifier 16 with resistor 31 125 and transistors 126 and 127 as well as feedback resistor 28 32 is similar to a differential amplifier with a normally connected ~:
~7;23Z
01 feedback loop, but having a high current capacity output stage.
02 The output stage is required to feed line current to the tip and 03 ring leads as will be described below.
04 The emitters of transistors 126 and 127 are connec-ted 05 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 -48 07 volts.
08 As was described with respect to resistor 6 in Figure 1, 09 the emitters of transistors 126 and 127 are connected through re-sistor 32 to the non-inverting input of opera-tional 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 10 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 the tip and ring leads are not subjected to imbalance.
23 In operation, a subscriber's telephone set is assumed to 24 go off-hook. Ba-ttery voltage is supplied through transistors 126 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 the battery supply, -48 volts. It is important 28 that the bat-tery feed circuit appear as a low impedance voltage 29 source.
The ratio of the sum of -the resistances of resi~tors 1 and 31 2 to the line impedance provides a vol-tage division of any common 01 mode signals which rnay appear on the tip and ring leads, as 02 applied to amplifier 10. While equal valued common mode signals 03 being applied to the input terminals of differential amplifier 10 04 would cancel in any event, it is preferred to reduce their amp-05 litude in case there is a differential interEerence component 06 modulated on the common mode signals, and in the case of differ-07 ences in the values of resistors 1 and 2, which would give rise 08 to a differential componentO
09 It is preferred that the amplification of the circuit which includes differential amplifier 10 should be about 1/10.
11 Accordingly if resistor 1 is 100,000 ohms for instance, resistor 12 11 should be about 10,000 ohms.
13 Differential voice frequency signals are also subjected to 14 a reduction in amplitude of 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 inver-ting input of 18 differential amplifier 16. The ratio of resistors 22 to 21 pre-19 ferably is 10 to 1, which provides an amplification of 10 of the voice signals applied to buffer amplifer 17. This compensates 21 completely of the signal loss in the circuit of amplifier 10, and 22 the original differential 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-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 28 crosspoint matrix 33, and is thus connected through -the P~X to a 29 similar terminal 19 of another line circuit of similar struc-ture. The line level of a signal at the line terminal 19 arriv 1.7Z32 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 amplifier 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 2~ are considered as a single ampli-11 fier circuit 34.
12 Amplifier 34 has been designed to amplify 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 the 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
t9.43K + 40.2K) 26 which equals approximately 26 (non-inverting~.
27 For signals arrived from the line terminal 19, the gain of 2s3 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 ~117~3;~
01 127 are connected through resistor 30 to the tip lead, the ring 02 lead being connected through resistor 31 to effective AC ground 03 (but which resistor in reality forms the DC line feed). The 04 impedance between -the tip and ring lead with the subscriber'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 Accordingly for a voice frequency signal on the tip and 08 ring leads applied to differential amplifier 10, there will be a 09 10 times reduction at its output, and the output signal will be 1/10 the input signal. This is the signal which is applied to 11 the non-inverting input of amplifier 16.
12 The signal i5 further amplified 10 times by the circuit of 13 differential amplifier 17, re-establishing the original level at 14 the output of the latter differential amplifier. However due to matching resistor 18 to the line impedance at output terminal 19, 16 the output signal level at output terminal 19 is 1/2 the original 17 signal 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 34 to the signals applied to its non--21 inverting and inverting inputs, the 1/10 original signal 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 -5, for an in-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 2~ negligible, but which may be desireable in some instances to 29 avoid the appearance of a "dead" line.
Clearly a selection of the values of resistors 20, 21 and 31 28, as well as resistors 22 and 21 (establishing the gain of dif-.
,~ ~ ~ ~-Jq~
01 ferential amplifier 17), or by providing one or more trimmer re-02 sistors, an exact cancellation of any signal from the tip and 03 ring leads from feeding back through differential amplifier 16 04 can be effected.
05 Considering now a signal arriving at line terminal 19 from 06 the switching matrix 33, this is applied at half the original 07 source level to amplifier 34. However due to the undirectional 08 nature of buffer amplifier 17, it is restricted from being 09 applied to the non-inverting input of aifferential amplifier 16.
Amplifier circuit 34 increases the amplitude of the signal by the 11 gain established by the ratio of resistors 28/20, noted earlier 12 as -5. The output signal of amplifier circuit 34 is thus -5 13 times the half 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 16 is a gain reduction, for the resistor values given, of 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, which is slightly but negligibly greater than the original. Clearly an increase in the 21 value of resistors 30 and 31 (i.e., to 350 ohms, for 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 24 level obtained as a result of use of 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 29 amplitude by resistors 1 and 2, which signal would be translated by differential amplifier circuit 10 as a normal differential 31 signal. Accordingly resistor 32 of at least twice the resistance 32 of the total of ~7;~:3;~
01 resistors 1 and 2 carries the output signal of amplifier circuit 02 34 to the non-inverting input of differential amplifier 10. This 03 signal provides an equal amplitude signal to the non-inverting 04 input as appears on the inverting inpu~, in order that exact can-OS cellation of the signal output of amplifier 34 should be ob-tained 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 vol-tage level will then be less than for a two party 14 call. For a conference call, one or both of switches 25 or 26 are closed 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 increased increasing the output signal level 18 to terminal 19 to the two-party call level. Compensation can 19 thus be made for a conference call with a Eew or a great many participants.
21 Accordingly, a bidirectional amplifier is provided, with a 22 battsry line feed and a translation of a balanced line to an 23 unbalanced line. The signal leve].s are completely controlled and -24 the facility provides selectable signal level compensation for conference calls.
26 Before describing the remainder of the circuit in detail, 27 reference is made to figure 3, which shows more generally the 28 amplifier circuit which provides two levels of gain for two dif-29 ferent input signals. A first signal source 40 is connected to the non-inverting input of a differential operational amplifier 31 41. The same signal source is also connected through a buffer 32 amplifier 42, and through an input resistor 43 to the inverting 01 input of differential amplifier 41. The same inverting inpu-t 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 o~ 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 expression R47/(R44 x R43) , where each of the 22(R44 ~ 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 01 added to resistors 43 as an internal part thereof for the gain 02 calculation.
03 As an example, assume resistor 46 is of low impedance, for 04 instance 600 ohms, resistor ~3 is 50K ohms, resistor 44 is lOK
05 ohms and resistor 47 is 250K ohms. Assume also that two differ-06 ent signals, each of 1 volt amplitude appear at sources 40 and 07 45.
08 The gain of this stage for the signal at source 45 will be 09 250K/~OK 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, 11 the 5.0 volt output signal has its phase inverted 1~30 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 /(50K xlOK) 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 voltage 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 23 be made variable, i.e., by means of a signal variable resistor 24 such as a field e~fect transistor, an optical isolator, or the like, one or both of the input signals can be subjected to 26 various gain modifications or can be modulated at will from an 27 outside source. Interaction or various degrees of addition o~
28 the two input signals can thus be readily obtained.
29 Returning now to figure 2, it had been noted earlier that the circuit contains means for applying ringing voltage to the 31 subscriber's line. A ringing relay 50 has the usual back EMF
!
~;7.2~
01 protection diode 51 connected thereacross. It is connec-ted to a 02 source of voltage -48V and to the collector of a PNP transistor 03 52, which itself has its emitter connected to ground.
04 ~ source o:E ring actuate signaL is applied from a ring 05 actuate lead through resistors 53 and 54 to the base o~ -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 OFHK lead for the provision of 09 remote ringing shut off. The OFHK lead is connected via diode 56 through resistor 54 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 operat-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 conduct, raising the level at the junction of resistors 53 and 54 and t~us 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.
24 It is preferred to connect a light emitting diode 57 through resistor 58 to a negative source of potential and to the 26 OF~K lead. Accordingly when high level potential appears on the 27 OFHK lead causing shutoff of ringing relay 50, light emitting 28 diode 57 operates, providing a visual indication to an operator 29 or serviceman that ~he circuit has functioned.
Ringing relay 50 is comprised o a single pole double 31 throw set of contacts, having a moving pole contact 59, a break `2 18 .7;Z3Z
01 contact 60 and a make contact 61. Make contact 61 is connected -02 through resistor 62 to a terminal for connection 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 local 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, the subscriber's line becomes unbalanced, 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 fil-ter 67 of 19 well known contruction. Active filter 67 is fabricated so as to block the 20 Hz ringing signals, but to pass signals of lower ~21 frequency, including the DC off-hooX signal level from the outpu-t 22 of operational amplifier 10. It is also preferred that active 23 filter 67 should pass 10 Hz signals, the frequency of rotary dial 24 pulses, 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 68, the output 27 of which is connected through resistor 69 and dioae 70 to -the 28 noninverting input of differential amplifier 71. The output of 29 diode 70 is connected to ground through an integrating :Eilter circuit comprising copacitor 72 in parallel with resistor 73 con-23;~
01 nected to a negative source of potential. The inverting input 02 of differential amplifier 71 is connected to ground, and the in-03 verting input of diferential a~plifier 68 is connected to a 04 source off-hook bias via an OFHK BIAS lead. The DC level of this 05 lead sets the sensitivity o the circuit Eor sensing the DC
06 off-hook level ~rom the output of differen-tial amplifier 10.
07 In operation, the DC potential level output of di~feren-08 tial operational amplifier 10 is applied through resistor 66 to 09 low pass active filter 67. Here all signals below ringing fre-quency are passed through the filter, and ringing signals are re-11 jected or greatly at-tenuated. The resulting DC level is applied 12 to differential ampliEier 68, which provides an output signal 13 once the DC leve~ is higher than the potential level on the 14 off-hook bias lead. This signal is rectified and filtered to DC
in diode 70, capacitor 72 and resistor 73, the resulting DC
16 signal being applied to the input of differential ampliEier 71.
17 When an o~-hook condition exists, the output of different 18 ial amplifier 71 provides a high potential level, causing diode 19 56 to become forward biased, and cutting of~ operation of trans-istor 52. As describea earlier with respect to a high level 21 signal on the OFHK lead, ringing relay 50 is caused to open, re-~22 moving ringing signals which had been applied to the subscribers 23 line.
24 In the meantime a signal on the RING ACTUATE lead had been removed, and the +V potential, applied through resistors 53 and 26 54 are operative to cause the emitter-base junction o~ transistor 27 52 to be reverse biased, thus holding ringing relay 50 inopera-28 tive.
29 Switch 15, which pre~erably is a CMOS switch also is oper-ated by a signal on the RING ACTUATE lead. Accordingly when 31 ringing is applied to the subscriber's line upon the actuation of .
01 ringing relay 50, switch 15 is caused to open. Ringing signals 02 thus appearing at the output of differen-tial 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 su~scriber has gone off-hook. A
09 serviceman at the PBX examining the line circuit printed circuit boards thus can obtain an indication 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 ~our 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 con-trolled gain in-19 crease for conferencing or the like.
As one of the important elements of the invention, a novel 21 amplifier has been used w~ich 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 of~-hook condition when the subscriber has answered the tele-26 phone, and ~or ring tripping. 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 various 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.
~ ., i . ~
Claims
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An amplifier circuit comprising means for amplifying two different signal with different gain comprising:
(a) A differential operational amplifier having an inverting and a noninverting input, and an output, (b) A feedback resistor (R1) connected between the output and inverting input, (c) a first signal source having its output connected to the noninverting input, (d) a buffer amplifier having its input connected to the first signal source, (e) a second signal source having its output connected to the output of the buffer amplifier and through an input resistor (R2) to the inverting input of the differential operational amplifier, and (f) a bypass resistor (R3) connected from said inverting input to ground, whereby the gain provided by said amplifier circuit to a signal from the first signal source is given by the expression and the gain provided by said amplifier circuit to a signal from the second signal source is given by the expression , where R1, R2, and R3 are resistance values in ohms.
(a) A differential operational amplifier having an inverting and a noninverting input, and an output, (b) A feedback resistor (R1) connected between the output and inverting input, (c) a first signal source having its output connected to the noninverting input, (d) a buffer amplifier having its input connected to the first signal source, (e) a second signal source having its output connected to the output of the buffer amplifier and through an input resistor (R2) to the inverting input of the differential operational amplifier, and (f) a bypass resistor (R3) connected from said inverting input to ground, whereby the gain provided by said amplifier circuit to a signal from the first signal source is given by the expression and the gain provided by said amplifier circuit to a signal from the second signal source is given by the expression , where R1, R2, and R3 are resistance values in ohms.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000371449A CA1117232A (en) | 1978-02-01 | 1981-02-20 | Telephone line circuit |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA296,137A CA1105162A (en) | 1979-06-23 | 1978-02-01 | Telephone line circuit |
| CA000371449A CA1117232A (en) | 1978-02-01 | 1981-02-20 | Telephone line circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1117232A true CA1117232A (en) | 1982-01-26 |
Family
ID=25668635
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000371449A Expired CA1117232A (en) | 1978-02-01 | 1981-02-20 | Telephone line circuit |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1117232A (en) |
-
1981
- 1981-02-20 CA CA000371449A patent/CA1117232A/en not_active Expired
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| MKEX | Expiry |