Note: Descriptions are shown in the official language in which they were submitted.
114Z281
01 This application is a division of Canadian Application
02 Serial No. 297,883 filed February 28th, 1978.
03 Thi~ invention relates to a bidirectional amplifier, and
04 more particularly to a bidirectional trunk circuit for coupling a
~05 first pair of leads ~uch as a trunk and a second pair such as a
06 junctor.
07 Trunk circuits are generally used for handling incoming
~08 or outgoing calls to or from a central office or PBX. Due to the
09 existence of a wide variety of Xinds of central offices and
PBXs, certain operational characteristics of trunks have become
ll standardized. Yet because of the requirement of trunks to inter-
~12 face with various different internal circuits of the central
~13 offices or private exchanges which circuitry often does not match
14 the trunk characteristics, trunk circuits are in addition requir-
ed to efect the various matching conversions.
16 Trunk impedances have become standardized at typically
` 17 either 900 or 600 ohms, and are comprised of balanced tip and
18 ring leads. Trunk circuits also carry signalling information and
' l9 typically carry current of either polarity at 48 volts DC. They
normally carry signals at a level of about 0 dbm, although the
21 ~ signal levels often vary widely and sometimes are as high as ~6
~22 dbm, which is between 5 and 6 volts peak to peak at the approxi-
23 mate impedance levels noted.
24 Yet due to the large number of trunks exposed to the
weather and due to other environmental factors such as high volt-
, .
26 age power lines, poor or changing ground resistance, temperature
- 27 variations mechanically affecting terminations, etc, sometimes
28 large common mode signals result on the trunks, sometimes as high
29 as +200 volts AC.
Trunk circuits further must terminate the tip and ring
31 leads with a matching impedance for AC signals, yet must have
~32 internal resistance for direct current w~ich has been standardiz-
~33 ed at under 250 ohms.
34
,
~1~22~31
01 The trunks must further be able to carry either signal-
02 ling alone (tone duplex or battery reversal) or signalling in-
03 cluding ringing signals. They must also provide separate control
04 of the gain of both incoming and outgoing voice or other message
05 signals in order that they can be set at the proper working
06 levels for the central office or PBX.
07 While trunks themselves are normally electrically balanc-
08 ed, their signals are inherently differential in nature, due to
09 the aforenoted common-mode signal problem. Thus a PBX or central
office which operates on simple AC audio signals must have means
11 for coupling the audio signals to the trunks in a differential,
12 isolated manner. The unbalanced pair feeding a trunk circuit
13 (i.e one terminal and ground) can be of whatever impedance iis
14 most convenient to the switching office or PBX designer, but is
often 600 ohms.
16 The trunk circuit must therefore translate the unbalanced
17 input aircuit to a balanced output circuit and vice versa, must
18 match the respective trunk or unbalanced impedances, must adjust
19 the signal levels to the standard signal level required both on
the~trunk and unbalanced lead pair, must remove very high common
21 mode signals often carried by the trunk, must present a different
~i ~
22 AC impedance to the trunk than DC impedance, etc.
23 Trunk circuits have previously used circuits involving
24 hybrid transformers to provide the matching and common mode re-
duction required. However with the advent of electronic switch-
26 ing offices and PBXs of considerably reduced size and weight, the
27 use of such transformers is an impediment to miniaturization.
28 For example where the circuitry of a PBX is built into a small
29 console, cabinet or desk, within which the circuitry is disposed
on printed circuit boards, such transformers add bulk, weight and
31 increa~ingly unacceptable size to the unit, and as well are to
32 2
il~2281
01 some extent incongruous with printed circuit boards carrying
~02 microelectronic components. The bulk of the transformers are of
03 course due to the requirement for a significantly sized core
04 which will not saturate in the presence of high AC and DC trunk
05 currents.
06 The present invention is a trunk circuit which provides
~07 the requirements of a trunk noted above, but which does not re-
i 08 quire the use of a hybrid transformer. Physically the circuitry
09 is light and small and can be fabricated on a printed circuit
^10 board plug-in card for a PBX or central office.
11 It should be noted that while the description of this in-
12 vention is directed to its use as a trunk circuit, the invention
13 is not restricted thereto and can be used for other 1/2/4 wire bi
14 directional amplifier applications. Accordingly the disclosure
and claims should be read with this in mind, to which it is in-
16 tended that this invention i8 directed.
17 In general, the inventive trunk circuit is comprised of
18 tip and ring terminals for connection to tip and ring leads, for
, ~
19 carrying signals and a direct current, an ~C terminating impe-
~20 dance connècted between the tip and ring terminals, and a high
21 impedance signal source connected between the tip and ring term-
22 inals, having a direct current conduction path. Means i9 includ-
~23 ed for controlling the signal source, including a first light
~24 sensitive solid state device for carring a signal which is a
function of a signal to be transmitted on the tip and ring leads,
;26 and for controlling the passage of current through the first
'27 signal source as a function of the current passing through the
.`',7 28 light sensitive solid sate device so as to apply a desired signal
29 to the tip and ring terminals. Means is included for connecting
a second lead pair, including a terminal for carrying trunk
31 signals. An operational amplifier circuit having a signal
;~32 3
,,
.~ .
, ,
1~4ZZ~3~
01 responsive light emitting means is connected in a load circuit
-02 therewith and is optically coupled to the first light sensitive
03 solid state devices. The operational amplifier has its input
04 connected in a signal path to the terminal for carrying trunk
05 signals. A second light sensitive solid state device optically
06 coupled to the light emitting means has a transfer function with
: 07 the light emitting means which is similar to the transfer func-
08 tion of the first light sensitive solid state device with the
09 light emitting means, the second light sensitive solid state de-
~10 vice being adapted to apply a differential signal between the in-
11 put terminals of the operational amplifier means. The tip and
12 ring terminals are thus caused to carry signal currents corres-
-13 ponding to a signal current appearing at the terminal for carry-
14 ing trunks signals.
,15 More generally the invention is a trunk circuit compris-
16 ing a first lèad pair, a second lead pair, and means terminating
17 the first lead pair with a first matching a.c. impedance and a
18 second resistance means. A first circuit applies an incoming
~19 signal from the second lead pair to the first lead pair, and a
.
second transformerless circuit applies an incoming signal from
21 the first lead pair to the second lead pair. A third
~22 transformerless circuit cancels signal from said first circuit
23 applied to the first lead pair from being reapplied to the second
24 lead pair. A fourth transformerless circuit cancels signals from
. ~,
;`25 the second circuit applied to the second lead pair from being
~26 reapplied to the first lead pair.
27 In the description to follow, a signal on "the terminal"
28 will be referred to: it is intended that this be construed as
~`29 either signal current carried by a lead which may be connected
to that terminal, or a signal voltage which can be measured be-
31 tween that terminal and common or circuit ground.
32 4
:
:
9 ~4Z2~31
01 A better understanding the invention will be obtained by
02 reference to the detailed description below of the preferred em-
03 bodiment, and to the following figures, in which:
04 Figure 1 is a block schematic of the invention in its
05 broadest form,
06 Figure 2 is a schematic of the invention in its simpliest
07 form, and
08 Figure 3 is a schematic of the preferred form of the in-
09 vention in its most detailed form.
Turning first to Figure 1, tip and ring terminals T and R
11 are provided for connection to the first tip and ring leads of a
12 trunk. The trunk is terminated with an impedance means 1, which
13 matches the trunk impedance for AC, and also provides required DC
14 resistance. While the AC impedance is selected to match the
~15 trunk impedance (i.e. 900 ohms or 600 ohms), the DC resistance
16 typically is under 250 ohms. The AC impedance can of course be
17 provided by a resistor, for instance of 900 or ~00 ohms, ln
18 series with a large valued capacitor.
~19 Means is also provided for connecting a second lead pair
to terminal U and system ground.
21 A first transformerless circuit means 2 is connected be-
-`22 tween the unbalanced terminal U in a circuit path to t~e tip and
~ 23 ring terminals T and R for applying an incoming signal from the
.,
24 second lead pair to the first lead pair. A second transformer-
less circuit means 3 is connected in a circuit path between the
:.~
26 tip and ring terminals T and R to the terminal U, for applying an
27 incoming signal from the first lead pair to the second lead pair.
28 Third transformerless circuit means 4a is provided for
~29 cancelling signal from the first circuit means 2 which is applied
to the first lead pair from being reapplied to the unbalanced
31 lead pair and fourth transformerless circuit means 4b is provided
32 5
., .
.~
i~4228~
Ol for cancelling signal from the second circuit means 3 which is
02 applied to the second lead pair from being reapplied to the first
03 lead pair.
04 Accordingly a transformerless translation is provided
05 which individually controls the paths of the signals from the
06 second to the first lead pair, and from the first to the sec-
07 ond lead pair, and yet prevents sidetone or positive feedback of
08 the signal from being reapplied back to its source.
09 In the following description, it will be assumed that the
tip and ring leads are balanced and the U terminal to ground is
ll unbalanced, for example purposes only. However the invention is
12 not restricted thereto.
13 Figure 2 is a schematic of the invention in its simplest
14 form. Tip and ring terminals T and R are provided for connection
r` 15 to the balanced tip and ring leads, and normally carry both trunk
16 signals and a direct current. Terminating the tip and ring term-
~,
17 inals is a matching resistor 5 in series with a large valued cap-
,18 acitor 6, which series circuit is connected between the tip and
-~19 ring terminals.
-~20 Terminating the tip and ring terminals for direct current
,. " ~ ,
21 is the collector - emitter circuit of transistor 7, which has
22 its collector connected to the tip or ring terminal and its emit-
. .,
23 ter in series with a small valued resistor 8 to the other of the
~`24 tip or ring terminal.
Also connected to either the tip or ring lead is one
26 terminal of a light sensitive solid state device, such as light
. ~
27 sensitive diode 9. ~he diode is also connected for AC, e.g. via
;28 a capacitor lO9 between the inverting and noninverting inputs of
'~29 operational amplifier lO. The output of operational amplifier lO
; 30 is connected to the base of transistor 7.
j.,~
i 31 Also connected from the junction of light sensitive diode
32 6
''
,................................ .
1~4ZZ81
01 9 and the input of operational amplifier 10, to the junction of
02 resistor 8 and the emitter of transistor 7 is a resistor 11.
~03 The terminal U is also connected to the input of the
04 operational amplifier 12 through a resistor 16. A second light
05 sensitive solid state device such as light sensitive diode 13 is
06 connected between the inverting and noninverting input of opera-
07 tional amplifier 12. The output of operational amplifier 12 is
;08 terminated by a load which includes the series of two light emit-
09 ting diodes 14 and 15.
;~10 Light emitting diode 14 is optically coupled to light
11 sensitive diode 9 and light emitting diode 15 is optically coupl-
12 ed to light sensitive diode 13.
13 Of course the circuitry shown will require other circuit
14 components for biasing, appropriate loading, etc, but these will
be obvious to a person skilled in the art. For instance, the
16 output of operational amplifier 10 preferably has a low valued
17 reslstance in series therewith; the noninverting input of the
18 same amplifier will require biasing, etc.
19 The circuitry involvlng the light sensitive solid state
device and light emitting devices can alternatively be of other
~21 form, as long as the result is linear translation of the input
.;
'22 signal. Various kinds of line translation circuitry which can be
~,
2~3 ~ used are described in Electronics magazine, January 19, 1978 pag-
~24 es 121-125, in the article Differential Optical Coupler Hits New
.
125 High In Linearity, Stability, by Bill Olschewski. For example,
'26; light emitting diodes 14 and 15 can be a single signal responsive
~27 light emitting device.
28 Amplifier 10, while described as an operational ampli-
~29 fier, can be a transl~tor for example of similar polarity as
transistor 7. Its collector can be connected to the collector of
31 transistor 7, its emitter to the base of transistor 7, and
32 7
., .
.. ; . ,~
:;
~i~2Z8:1
01 diode 9 can be connected between its base and collector. Resis-
02 tor 11 would then be connected between the junction of diode 9
03 and its base, and the ring lead.
04 In operation of the circuit shown in Figure 2, it will be
05 seen that signals on the U lead are applied to the input and
06 are amplified by operational amplifier 12. These signals cause
07 current to be drawn through light emitting diodes 14 and 15.
08 Diodes 14 and 15 are chosen to have similar transfer characteris-
09 tics with diodes 3 and 13. Since light emitting diode 15 is op-
tically coupled to light sensitive diode 13, a feedback loop is
11 provided whereby the gain of the amplifier and the current drawn
`12 through light sensitive diode 13 are controlled.
;13 Since the transfer characteristics of diodes 14 and 9,
14 and 15 and 13 are similar, and since diode 9 is connected for AC
directly between the input terminals of operational amplifier 10,
~16 the signal current which appears thereacross is virtually an
~17 exact duplicate of the signal current through diode 13. Since
18 the signal through diode 13 i8 controlled due to it appearing
~19 within the aforenoted feedback loop, as a result the signal
`20 applied to operational amplifier 10 i8 that which would exist
21 were it connected in the same feedback loop, as i operational
22 amplifier 12 were translating the same signal. Accordingly the
23 input signal from terminal U i8 applied to operational amplifier
24 10 in a controlled, yet isolated manner.
The output signal of operational amplifier 10 is applied
~;~26 to the base of transi6tor 7, and the DC current conducted in its
,
27 collector - emitter circuit is thereby modulated, and is thus
28 applied to the tip and ring terminals T and R.
i29 Therefore, the collector current is dependent on the sig-
nal current passing through light sensitive diode 9 rather than
'31 signal voltage applied to the base.
;`32 8
.. ..
42281
01 Due to the nature of the input characteristics of an
02 operational amplifier, diode 9 connected directly across input
03 terminals of operational amplifier 10 results in virtually zero
0~ volts of signal appearing across light sensitive diode 9.
05 Accordingly considering the circuit between node A and the ring
06 terminal R, the signal voltage between them is equal to the cur-
07 rent in diode 9 multipled by the value of the resistance of re-
: 08 si~tor 11. The current passing through resistor 8 is equal to
~09 the voltage at node A divided by the resistance of resistor 8,
~10 which current, it will be noted, is equal to the current through
11 diode 9 multiplied by the ratio of resistor 11 to resistor 8.
12 The emitter current of transistor 7 is equal to the
13 aforenoted ratio plus the current through diode 9 which passes
14 through resi6tor 11. Accordingly the collector current is equal
to the common base current gain of the transistor multiplied by
~16 the aforenoted emitter current. If the current gain of the tran-
17 si~tor is virtually equal to unity, and if the resistance of re-
18 sistor 11 is much greater than the resistance of resistor 8 (as
19 is preferred), then the collector current of the transistor is
equal to the aforenoted ratio of the resistors 11 and 8 multi-
21 plied by the current carried by diode 9. Since the AC signal
22 applied to the tip and ring terminals T and R i~ derived from the
.~23 collector current of transistor 7, it is clear that it is direct-
s~24 ly related to the signal current in diode 9 multiplied by a line-
:-25 ar relationship, that is, the relationship of the aforenoted
:26 ratio of the resistances of resistors 11 ana 8.
;27 It is further clear that the AC signal current in diode
28 13 is closely controlled by its location in the earlier - des-
29 cribed feedback loop, and is similar current as appears with
diode 9, which itself i8 multiplied by an easily controllable
31 value, the aforenoted ratio of resistances of resistors 11 and 8.
:32 9
''
~,'
ll~Z281
01 Turning now to figure 3, a detailed schematic of the in-
02 vention is shown. Ignoring for a moment the bridge rectifier
03 which will be re~erred to later, a signal from the tip and ring
04 leads i8 applied to tip and ring terminals T and R. This signal
05 is applied to operational amplifier 20 through input resistors 21
06 and 22 and DC blocking capacitors 23 and 24. It is preferred
07 that the value of each of resistors 21 and 22 should be at least
08 100 times the AC impedance of the tip and ring balanced lead
09 pair. It is also preferred that the gain of this stage be
~10 approximately 0.1 or 0.11. A feedback resistor 25 connects the
11 output of operational amplifier 20 to its inverting input term-
12 inal in the conventional manner. Amplifier 20 is thus connected
13 as a standard operational amplifier, to remove common mode sig-
14 nals which may appear on the tip and ring leads. The gain of
ampliier 30 is low to prevent saturation thereof in the presence
16 of large common mode signals.
17 The output of operational amplifier 20 is connected
18 through resistor 26 to the noninverting input of an operational
19 amplifier 27. This amplifier has a conventional feedback resis-
tor 28 between its output and inverting input (bypassed by a
21 small capacitor), and a resistor 29 to common or ground. It ls
22 preferred that with the present circuit the gain of this stage
23 should be approximately 8.
,~24 The output of operational amplifier 27 iæ connected
through resistor 30 to the unbalanced terminal U. Resistor 30
26 should be of value to match the unbalanced external impedance of
~27 the unbalanced pair at terminal U. For example, where this trunk
!` 28 circuit is used in a PBX, the impedance at terminal U would be
`~29 typically 600 ohms while the tip and ring balanced impedance
would be typically 900 ohms.
31 A gain of 0.1 in operational amplifier 20, followed by a
32 10
~ . .
:,
1~42281
01 gain of 9 in operational amplifier 27 provides a total loss of
02 1.9d6. This is an example of a circuit for coupling of 900 ohms
03 trunk to a 600 ohms termination at terminal U, with optional
04 matching. A zero dbm power level in 600 ohms is of course at a
05 lower voltage than zero dbm in 900 ohms.
06 The signal level at terminal U is reduced to half at the
'07 output of operational amplifier 27 if terminated by a resistor
~08 equal in value to resistor 30 (i.e 600 ohms~.
09 Other impedance conversions can be made by adjusting the
gains of the amplifier and circuit impedance.
"11 The tip and ring signal level as applied to the circuit
,12 of operational amplifier 20 is reduced by a factor of 10, then
, 13 raised by a factor of 8 at the output of operational amplifier
,',14 27. A 5 volt input signal i8 therefore transformed to approxi-
;15 mately 4 volts unbalanced. The signal level at terminal U would
16 then be 2 volts assuming it has a matching termination. These
'17 signal level proportions have been chosen as they are the intern-
,~18 al central office or PBX and external trunk signal levels stand-
jl9 ardized by,telephone companies.
It is a feature of the present invention that the level
,21 of the signal as applied to terminal U can be selectively in-
22 creased, to accommodate reduced levels which would oacur during
23~ internal conferencing within the central office on PBX which in-
~ 24 cludes a call on the instant trunk. Resistors 31 and 32 indivi-
'"25 dually bypass resistor 29 through externally actuated individual
;26 switch contacts 33 and 34. The gain of the stage including oper-
27 ational amplifier 27 can thus be increased by switching resistors
28 31 or 32 in parallel with resistor 29. The three gains thus pro-
29 vided by this amplifier are preferred to be about 8, 11, and 15.
For signals passing in the return direction, operational
'31 amplifier 35 has its noninverting input connected to terminal U.
~2 11
1142281
01 The output of this amplifier is connected through input resistor
02 36 to the inverting input of operational amplifier 38. As was
03 described with reference to figure 2 this operational amplifier
04 has a light sensitive diode 39 (corresponding to diode 13)
05 connected for AC across its input terminals. One terminal of
06 light sensitive diode 39 is connected for DC to a source of nega-
~07 tive potential, and the noninverting input of operational ampli-
;08 fier 38 is connected to a source of bias preferably the same as
09 the source of negative potential; if not, the source of bias and
source of negative potential should be connected together for
11 AC. Resistor 37 is connected between resistor 36 and ground.
12 The output of operational amplifier 38 is connected to
13 the base of transistor 40 to increase its drive capability.
14 The collector - emitter circuit of transistor 40 is connected to
"'15 a source of potential -V and +V, and its emitter is connected
y 16 through a current limiting resistor 41 to a pair of series con-
,`17 nected light emitting diodes 42 and 43 which are connected to a
~18 source of potential +V (assuming an NPN transistor). The trans-
19 fer and light emitting characteristics of light emitting diodes
42 and 43 should be very closely matched. Light emitting diode
21 42 and light sensitive diode 39 are optically coupled.
22 An operational amplifier 44 is also provided which has
, 23 light ~ensitive diode 45 (corresponding to diode 9 in figure 2)
~`~24 connected for AC across its input terminals. This is realized by
connecting the cathode of diode 45 to the inverting input of
26 operational amplifier 44 and its anode to the tip or ring lead
~27 (shown as the ring lead for explanation purposes herein). Diode
28 45 is optically coupled to light emitting diode 43. The nonin-
~29 verting input of operational amplifier 44 is connected through
resistor 46 to the ring lead; resistor 46 is bypassed by capaci-
31 tor 47.
32 12
' ''
il~2Z81
01 The output of operational amplifier 44 is connected
02 through resistor 48 to the base of a relatively heavy current
03 carrying transistor 49. Transistor 49 has its collector - emit-
04 ter circuit connected between the tip and ring leads, through
05 small valued resistor 50. The junction of the emitter of tran-
06 sistor 49 and resistor S0 is connected through resistor 51 to the
07 junction of diode 45 and the inverting input of operational amp-
08 lifier 44.
09 A terminating resistor 52 having a value matching the
balanced tip and ring lead impedance (i.e typically 900 ohms) is
11 connected in series with a large valued capacitor 53 (for provid-
12 ing an AC short circuit), which series circuit is connected be-
13 tween the tip and ring leads.
14 In operation o the above described circuit, signals from
,15 the unbalanced pair at terminal U, which are at 1/2 the tip and
16 ring amplitude level, are applied to operational amplifier 35 and
,17 as a result to operational amplifier 38. Light sensitive diode
18 39' typically can be biased to about 12.5 microamperes. The gain
19 in operational amplifier 35 is preferably set such that full sig-
nal level at the tip and ring amplitude level is obtained at its
`21 output, and is appli.ed to the circuit comprising operational amp-
22 li~ier 38.
'23 The output signal current of operational amplifier 38 i8
'24 applied to the base of transistor 40, which conducts current in
its collector - emitter circuit resulting in current passing
26 through light emitting diodes 42 and 43. As described earlier
27 light emitting diode 42 and light sensitive diode 39 are optical-
28 ly coupled and form a feedback loop around operational amplifier
29 38.
It was noted earlier that the transfer characteristics of
.31 light emitting diode 43 with light sensitive diode 45, and light
32 13
~1422~:~
01 emitting diode 42 with light sensitive diode 39 should be closely
~2 matched. The signal current at the output of operational ampli-
03 fier 35 passes through resistor 36 to the negative input of oper-
04 ational amplifier 38. Due to the high gain of the operational
05 amplifier, the current through the light emitting diodes is such
06 that light sensitive diode 39 is caused to draw current suffici-
';07 ent to cause the potential at the inverting input of operational
08 amplifier 38 to be at -V (bias), which is at ground potential for
;09 AC signals. The current in light sensitive diode 39 is thus the
j~10 signal voltage at the output of operational amplifier 35 divided
; 11 by the resistance of resistor 36. The DC current is the bias
,12 voltage at the output of operational amplifier 35 minus the
,~13 voltage -V divided by the resistance of resistor 36.
'~14 Due to the matched transfer characteristics, the current
`lj
~15 passing through light sensitive diode 45 is the same as the
,I6 current through diode 39.
~17 As was also described with reference to figure 2, the
~18 collector signal current in transistor 49 is equal to the ratio
. "
~19 of the resistances of resistors 51 and 50 multiplied by the sig-
~20 nal current in diode 45, plus the noninverting voltage at the in-
21 put of operational amplifier 44 divided by the resîstance of re-
~22 sistor 50. Accordingly the level of the outgoing signal of the
~23 tip and ring leads can be controlled by the ratio of resistors 51
~24 to 50. The preferred ratio is 1,000; the current in diode 45
,1l25 need only be l,OOOth of the signal current applied to the tip and
26 ring and there is accordingly a 30 DB power gain.
27 The DC current is controlled by the bias current in diode
28 45 and the noninverting voltage at the input of operational
29 amplifier 44, the latter being fixed by the ratio of the resis-
tances of resistors 60 and 46, and is proportional to the DC
31 voltage across the tip and ring leads.
32 14
ll~Z281
01 As an example, it will be noted that the signal current
02 in diode 39 i8 equal to the voltage at the output of operational
03 amplifier 35 divided by the resistance of resistor 36. This is
04 the same signal current as in light sensitive diode 45. The sig-
05 nal current applied to the tip and ring leads is the collector
06 current of transistor 49, which is equal to the voltage at its
07 emitter divided by the resistance of resistor 50. This is equal
08 to the current in light sensitive diode 44 times the ratio of the
09 resistances of resistors 51 and 50. Accordingly the current
applied to the tip and ring leads is equal to the voltage at the
11 output of operational amplifier 35 di~ided by the resistance of
12 resistor 36 times the ratio of the resistance of resistors 51 and
13 50- `
14 The signal voltage applied to the tip and ring leads is
~,lS the product of the impedance across the tip and ring leads and
16 the current applied thereto. The impedance across the tip and
17 ring leads is the termination impedance in parallel with the
18 impedance applied to the tip and ring lead. Assuming, for
~19 example, that the tip and ring line impedance is 900 ohms and the
~20 termlnation impedance (resistor 52) is 900 ohms, and as~uming the
21 resistance of resistor 51 is 20,000 ohms, resistor 50 i8 20 ohms
22 and resistor 36 is 374,000 ohms, the voltage applied to the line
23 is
~24 450 x 20,000 = 1.203. This value, it will be recognized, is the
374,000 x 20
26 voltage ratio required to transfer a signal of the same power
27 from a 600 ohm line to a 900 ohm line.
28 The circuit also provides means for safeguarding against
29 sidetone, or positive feedback of the ~ignal arriving from either
the tip and ring balanced pair or the U terminal and being fed-
31 back to the source.
32 15
~142281
01 It will be recalled that signal is applied from opera
,02 tional amplifier 27 through a 600 ohm resistor 30 to the U term-
03 inal. This signal also appears on the noninverting input of
04 operational amplifier 35. The same signal, but with a different
05 amplitude is applied through resistor 54 to the noninverting in-
06 put of the same and resistor 55 is connected between the output
07 of operational amplifier 35 and the noninverting input. Resistor
-08 54 is chosen of such value sufficient that gain of amplifer 35
09 multiplies the signal from the output of operational amplifier 27
~10 applied to the noninverting input of operational to a different
11 extent than the same signal applied to its inverting inputs such
12 that the signal is cancelled.
"13 It should be noted that signals at the output o opera-
,~14 tional amplifier 27 should be at full balanced line level and are
i15 reduced by half in resistor 30 for application to the U terminal.
;16 Signals originating from the U terminal are applied to the nonin-
17 verting input of operational amplifier 35. Accordingly a differ-
18 ential signal is produced at the input of operational amplifier
~,19 35 from signals originating at the U terminal, but signals orig-
~20~ inating at the balanced tip and ring terminals and which appear
, '
21 the output of operational amplifier 27 are canaelled within the
22 circuit of operational amplifier 35. The latter signals are
23 accordingly not applied to operational amplifier 38 and do not
24 appear as part of the signal coupled from the light emitting
r~ 2S diodes to the circuit which provides an output to the tip and
26 ring leads.
27 It was noted that signals originating on the U terminal
28 appear at the output of operational amplifier 35. Connected
29 thereto is an AC coupling circuit including capacitor 55 and
series resistor 56 which are coupled to the inverting input of
31 operational amplifier 57. A conventional feedback resistor 58
32 16
.~
114Z2~1
,~01 connects the output of operational amplifier 57 to the inverting
02 input. Its noninverting input is connected to common or ground.
03 The output of operational amplifier 57 is connected through
~,04 resistor 59 to the noninverting input of differential amplifier
05 20.
j06 Signals arriving from the U terminal at the output of
07 operational amplifier 35 are coupled via capacitor 55 and resis-
;'08 tor 56 to the input of operational amplifier 57. From there they
;09 are coupled to the noninverting input of operational amplifier
,10 20. Preferably the gain of the stage which includes operational
11 amplifier 57 is 1/8, for the present case of 900 ohm to 600 ohm
12 conversion. The gain of the stage including operational ampli-
13 fier 20 to ~ignals from operational amplifier 57 should be
14 approximately 1/10 (with the value of resistor 59 about 1/10 the
'15 value of resistors 21 and 22), and the gain of operational amp-
16 lifier 20 should be appro~imately -1/10 for outgoing signals on
17 the tip and ring lead applied by transistor 49. If the signals
18 are identical, they will cancel within operational amplifier 20.
9 Signals translated to the tip and ring lead originating
. ~,
-1~20 ~rom the U terminal are applied to operational amplifier 20 via
;~ 21 resistors 21 and 22, and are also applied thereto at the same
,~ ,
~22 level via operational amplifier 57. These signals therefore can-
23 cel, and do not pass back through amplifier 27 back to the U
24 lead.
- i
Accordingly the circuitry so far described has translat-
26 ed signals from the balanced tip and ring leads to the unbalanced
27 U lead, at half original signal level. It has also translated
28 signals originating on the unbalanced U lead at said half signal
29 level and has applied them to the balanced tip and ring leads at
full level. Yet the circuit has safeguarded against signals
31 originating on the tip and ring leads being fed back through the
,
32 17
28~
01 signal path of the U lead to the tip and ring leads, and has
02 safeguarded against signals originating on the U lead and applied
03 to the tip and ring leads from being fed back from the tip and
04 ring leads to the U lead. A transformerless bidirectional amp-
~05 lifier, including a four to two wire network (as it is commonly
06 called) is thus obtained. Bulky hybrid transformers are accord-
07 ingly eliminated and the entire circuit can be provided on a
08 printed circuit card.
~09 The circuit has also provided an AC terminating impedance
which matches the tip and ring impedance, and as well provides a
11 DC impedance (through transistor 49) which is lower than the AC
12 impedance.
~13 The noninverting input of operational amplifier 44 has
14 been noted as being connected to resistor 46 which is connected
~15 to the ring lead, to which a direct current is normally applied
16 from the trun~. The noninverting input is also connected to the
17 tip lead terminal of capacitor 53 via resistor 60. Resistor 60
18 can typically be 10 times the value of resistor 46. The ratio of
19 the two, connected across the tip and ring leads provides a bias
~20 level to operational amplifier 44 preferably to cause it to have
~21 its operation point near the minimum expected trunk power supply
-`22 potential level, ~or example 2 volts. Its DC current draw is
23 determined by the resulting voltage across resistor 50. If de-
`24 sired, the noninverting input could be connected directly to the
ring lead, which causes the operational amplifier 44 to act as a
26 current source.
-27 A power supply circuit for operational amplifier 44 is
~28 also provided, comprising transistor 61 which has its collector -
29 emitter circuit connected between a power input terminal of
operational amplifier 44 and the tip lead terminal of capacitor
;31 53. The other power input of operational amplifier 44 is con-
32 1~
114ZZ81
i
01 nected to the ring lead. The base of transistor 61 is connected
02 to the junction of a bias voltage di~ider comprising resistor 62
-03 connected to ground and in series with a light sensitive solid
04 state device such as a phototransistor 63, the latter being con-
05 nected between the base and the tip lead end of capacitor 53.
06 In operation, current is supplied through transistor 61
07 to the power input terminals of operational amplifier 44. How-
08 ever when phototransistor 63 is illuminated (as will be described
09 below), the base voltage of transistor 61 is brought near its
emitter voltage, effectively cutting off conduction and thus cut-
11 ting off operation current from operational amplifier 44.
12 Transistor 63 can be illuminated by its inclusion in a
13 photoisolator, which operates upon reception of outpulsing sig-
1~14 nals at the U terminal. Operation power is thus applied and cut-
;l~15 off from operational amplifier 44 in synchronization with the
16 pulse.
7 In addition a relay can be operated in synchronization
~18 with the outpulse on terminal U. The relay break contract 69 i8
19~ connected in the tip lead between resistor 52 and the junction of
transistor 61 and capacitor 53, to ensure that capacitor 53,
~21 which acts as a power supply for operational amplifier 44, does
~22 not discharge through transistor 49.
~23 In the event that the outpulsing ability of the current
24 i8 not desired, transistors 61 and 63 can be deleted as well as
break contact 69 and resistor 62. The power input terminals of
,26 operational amplifier 44 can be connected directly across capaci-
27 tor 53.
28 Preferably the trunk circuit also includes zener diode 75
r~, 29 which is connected acros~ capacitor 53, to which the operational
amplifier 44 power circuit is connected. Zener diode 75 act~ as
l31 a voltage protector for the power circuit.
`32 19
i~4~281
01 The tip and ring terminals al 90 preferably are connected
02 to the tip and ring leads through a bridge rectifier comprising
03 diodes 76a, 76b, 76c and 76d, connected in conventional form. As
04 is well known, the bridge rectifier will cause the correct pola-
05 rity for operation of this circuit to the T and R terminals, no
06 matter what polarity is the DC voltage across the tip and ring
-07 leads. The circuit is therefore insensitive to polarity revers-
08 ing signalling on the balanced trunk comprising the tip and ring
` 09 leads.
~10 The circuit described above is insensitive to common mode
11 AC signals on the balanced pair, since they are cancelled in dif-
12 ferential amplifer 20. It provides a balanced to unbalanced in-
;13 put, and an unbalanced to balanced input bidirectional ampliier,
14 with safeguarding against sidetone or positive feedback of sig-
nals incoming from either trunk or the switching office. Com-
16 plete control of the translated signal levels and of the AC and
17 DC impedances is provided, as well as means for increasing the
Ci
18 level of the incoming signal level in the event this is required
19 due to conferencing on the unbalanced U terminal. The signal
~20 applied to the tip and ring is also controllable by ~eans of the
` 21 variation of the ratio of a pair of resistors.
22 It will become clear that this circuit is highly useful
23 as a universal trunk for a PBX or other telephone switching
; 24 office. However itæ bidirectional attributes, impedance control
~25 and signal level control makes it equally applicable for use in
26 other environments.
27 It may also become clear to a person skilled in the art
28 understanding this invention that numerous other embodiments or
29 variations may be made. All are considered within the sphere or
- 30 scope of this invention, as defined in the appended claims.
`31 20
.,
'-,
ll~Z2~3i
01 SUPPLEMENTAPcY Dl SCLOSURE
02 It has been found that the signal cancellation effects
03 of t:he present invention can also be obtained with the use of a
04 somewhat simpler structure by which the photocoupled circuitry
05 i8 replaced by an inexpensive transformer coupled stage. No
06 hybrid transformer signal coupling need be utilized with
07 attendant impedance balancing requirements, and while an
08 inexpensive transformer is utilized to couple the signal passing
09 from one direction to the second, a transformerless circuit is
~10 used to couple the signal passing in the opposite direction.
11 Furthermore, transformerless means are used to cancel signals
12 originating from either direction from being fed back thereto.
~13 The present embodiment is thus a form of the invention described
~14 with reference to Figure 1.
In general the embodiment of the invention to be
16 described below is a trunk circuit comprising a first lead pair
17 for carrying two-way signals, a second lead pair for carrying
18 two-way signals, first circuit means for applying an incoming
19 signal from the second lead pair to the first lead pair, second
`20 transformerless circuit means for applying an incoming signal
21 from the first lead pair to the second lead pair, and third
j
22 transformerless circuit means for preventing signals from the
23 first circuit means applied to the first lead pair from being
24 reapplied to the second lead pair and for preventing signals
from the second circuit means applied to the second lead pair
26 from being reapplied to the first lead pair.
~27 Preferably the first circuit means is comprised of a
28 circuit including a transformer having a primary winding
29 connected in a low impedance voltage source circuit to the
second lead pair, and having a secondary winding connected in a
- SD21 -
~ -Q
1142281
01 very high impedance to direct current circuit means for
`~02 applying the incoming signal from the second lead pair to the
03 first lead pair. Due to the resulting very low current levels
04 in the transformer, the effective bandwidth of the transformer
05 iB substantially increased, allowing use of an inexpensive
06 transformer which can be readily incorporated on a plug-in
07 printed circuit board.
08 The details of this embodiment will be understood by
09 reference to the description below, and to Figures 4 and 5, in
~ 10 which:
; 11 Figure 4 is a schematic of the present embodiment in
12 its simplest form, and
13 Figure 5 is a schematic diagram of the embodiment of
14 Figure 4.
Turning now to Figures 2 and 4, it will be noted that
- 16 Figure 4 is similar to Figure 2 except for the deletion of the
'17 photosensitive diode 9 and photoemissive diodes 14 and 15. The
~18 operational amplifier 12 i8 not shown in Figure 4 but a circuit
19 of similar form may be utilized.
In place of photosensitive diode 9 (Figure 2), an
21 inexpensive transformer 80 is used which has a high impedance to~;22 direct current circuit connected to its secondary winding. The
23 high impedance circuit should be connected between the inverting24 input of operational amplifier 10 and the junction between
resistor 8 and ring lead R.
,.
26 The circuit which has high resistance to D.C. is
27 comprised of resistor 81 in series with capacitor 82, in series
2~3 circuit with secondary winding 83 of transformer 80.
29 Capacitor 82 allows modification of the frequency
response of the circuit besides being of infinite resistance to
31 - SD22 -
'
:
114Z2~1
01 D.C., and its presence is optional. The value of resistor 81 in
02 one model of the present invention was 10,000 ohms.
03 The primary winding 84 of transformer 80 is connected
04 in a circuit to a low impedance voltage source 85. This voltage
05 source preferably is formed by the output circuit of operational
06 amplifier 12 (Figure 2).
07 In operation, the transformer 80 is driven by signals
08 originating at terminal U, translated through operational
09 amplifier 12. The signal is applied to resistor 81. Since
virtually no (or very little) direct current flows through the
11 secondary of the transformer, its frequency and amplitude
12 characteristics are optimized. Since there is virtually no
13 loading of the transformer, a size reduction of 2 can be
14 realized, and an additional size reduction of 4-6 can be
realized due to there being virtually no direct current flow
~16 through the secondary winding.
17 The signal current flow can be calculated by
~ 18 multiplying the ratio of the signal voltage at the low impedance
- 19 source and the value of resistor 81 by the ratio of resistors 11
to 8 times the normalized filter transmission characteristics
21 formed by capacitor 82 and resistor 81.
~22 It is preferred that the primary and secondary
23 windings of the transformer are wound so that stray capacitances
24 to ground are not amplified by operational amplifier 10.
Accordingly it is preferred that the ends of the primary and
26 secondary windings which are not connected to, or adjacent the
27 ring lead (or low impedance node) should be separated as widely
~ 28 as possible in order to provide lowest capacitance therebetween.
;~29 Figure 5 depicts a schematic diagram of this
.~ .
embodiment of the invention. Comparing it to Figure 3, it will
31 - SD23 -
:
. .
~,
~14Z281
01 be noted that transistor 40, resistor 41, light emitting
02 diodes 42 and 43 and a light sensitive diodes 39 and 45 have
03 been eliminated. In their place is disposed the transformer
04 described with reference to Figure 4, having its primary winding
05 84 connected to the output of operational amplifier 35 and
06 ground. Secondary winding 83 has one terminal connected to the
07 ring lead terminal R, and its other lead connected through the
08 series circuit of capacitor 82 and resistor 81 to the inverting
09 input of operational amplifier 44.
With the application of an input signal to terminal U,
11 which signal is translated in operational amplifier 35, it
12 passes through the low impedance input circuit to the primary
13 winding 84 and the high impedance circuit connected to the
~14 secondary winding 83 of the transformer, and is applied between
terminal R and the inverting input of operational amplifier 44.
16 The presence of capacitor 82 effectively bars the passage of
17 direct current through the secondary winding, and also can be
18 used to limit the low frequency response of the circuit path,
lg with resistor 81. However, even were capacitor 82 omitted, the
~20 high resistance of resistor 81 would limit the flow of direct
21 current to a very low level.
22 It may be seen that the present circuit substitutes
23 adequately for the photo-coupled circuit of the
~24 earlier-described embodiment. The remainder of the circuit
operates as described previously, including the transformerless
26 circuit coupling signals from the tip and ring leads to the U
27 lead, and the signal cancellation circuits.
28 It should be noted that the secondary winding 83 of
29 the transformer can be connected in a circuit to the input of a
`30 coder-decoder or other 4 wire apparatus, rather than to
31 - SD24 -
114Z281
01 operational amplifier 44. The output of the coder-decoder can
02 be connected to the T and R terminals (Figure 5), removing the
03 requirement of diodes 76A, 76B, 76C and 76D, and the input of
04 the coder-decoder can be connected in a circuit to said
05 secondary winding 83. In this structure the need for one of the
06 circuit means to cancel signals from the unbalanced pair from
07 feeding back thereto can be deleted. Hence operational
08 amplifier 57, resistors 56, 58 and 59 and capacitor 55 can be
09 removed. The impedance of the T and R terminals and the output
circuit connected to the secondary winding 83 can be adjusted in
11 a manner known to persons skilled in the art to match the output
12 and input circuits respectively of a coder-decoder or other 4
.,
13 wire apparatus.
14
~16
17
18
19
21
j;
22
~24
. 26
27
. 28
:29
- SD25 -
