Electronic hybrid

An improved electronic hybrid for coupling signals between two wire and four wire transmission circuits. The hybrid includes a pair of amplifiers associated with a pair of terminating impedances which are serially connected between the amplifier outputs and the two wire port. The amplifiers driving the two wire port through the terminating impedances are biased to couple a dc potential to the two wire line whereby the terminating impedances serve as battery feed resistors. Each amplifier-impedance arrangement includes a feedback circuit connected from the junction between the impedance and the two wire port and an input of the amplifier for providing positive feedback for the amplifier so that the output impedance of the hybrid is maintained at a desired level using battery feed resistors of smaller value than heretofore required. The result is the ability to drive longer loops through the smaller series impedances while at the same time exhibiting the desired terminating impedance.

DESCRIPTION OF THE INVENTION 
This invention relates to hybrid circuitry for coupling signals between two 
wire and four wire transmission circuits and more particularly concerns an 
electronic hybrid circuit of the type which provides dc current for the 
two wire line. 
Hybrid circuits are used in communications networks where it is necessary 
to couple a bidirectional two wire line to individual unidirectional 
sections of a four wire line. In telephony, for example, bidirectional 
signals may be carried over a two wire line, such as in a subscriber loop, 
but must be split into separate transmit and receive unidirectional 
signals, such as for a central office. The most recent general form of 
such hybrid circuits has been a transformerless or electronic hybrid. A 
particularly useful electronic hybrid has been described in U.S. Pat. No. 
4,064,377. The hybrid circuit described therein provides an electronic 
hybrid capable of meeting the operating requirements of practical 
telecommunications systems. Among the features of that hybrid are 
precisely matched terminating impedances interposed in series between the 
circuity driving the two wire line and the two wire line itself, said 
impedances being arranged to transform all voltages in the two wire loop, 
whether normal or abnormal, to currents of manageable proportions. 
The above-mentioned hybrid has excellent longitudinal balance and also uses 
its terminating impedances to serve as battery feed resistors. That is, 
all of the two wire line loop current flows through the terminating 
impedances. 
Causing loop current to pass through the same terminating impedances as are 
provided for matching the line impedance imposes one limitation on the use 
of the hybrid circuit. In a typical subscriber loop, the nominal line 
impedance to be matched by the terminating impedances of the hybrid totals 
600 ohms. A basic telephone requirement is to provide a minimum of 23 
milliamps loop current to subscriber loops. Considering a 48 volt dc 
supply with about 10 volts allowed for signal swing, there is a maximum 
possible dc loop resistance of about 1,650 ohms. By using 600 ohms for the 
hybrid terminating resistors in the path of all dc line current, the 
maximum external subscriber loop resistance cannot be much larger than 
about 1,000 ohms. It is desirable for long loop operation to be able to 
handle about 1500 ohms. 
It is therefore an object of the present invention to provide a hybrid 
circuit with series connected battery feed resistors in which a desired 
terminating impedance is achieved by multiplying the effect of the battery 
feed resistors so that the actual value thereof can be reduced to provide 
increased loop driving capability. 
It is a further object of the invention to provide such a hybrid circuit 
which has a wider dynamic range of operation due to a reduced amplifier 
series impedance. 
It is a subsidiary object of the invention to provide such a hybrid circuit 
which is less sensitive to low frequency longitudinals.

While the invention is susceptible to various modifications and alternative 
forms, a specific embodiment thereof has been shown by way of example in 
the drawings and will herein be described in detail. It should be 
understood, however, that it is not intended to limit the invention to the 
particular form disclosed, but, on the contrary, the intention is to cover 
all modifications, equivalents, and alternatives falling within the spirit 
and scope of the invention as defined by the appended claims. 
A simplified functional diagram illustrating the major components of a 
hybrid exemplifying the present invention is shown in FIG. 1. The hybrid 
10 is adapted to couple signals between a two wire bidirectional line and 
a pair of unidirectional lines in a four wire circuit, and includes a two 
wire port 11 having terminals 12, 13 for connection to a two wire line. 
The hybrid 10 further includes a transmit port 14 having terminals 16, 17 
for connection to the transmit circuitry of a four wire line and a receive 
port 18 having terminals 19, 21 for connection to the receive circuitry of 
the four wire line. The transmit port 14 and the receive port 18 together 
comprise a four wire port 15. The illustrated two wire terminals 12, 13 
are connected directly to the two wire line without the need for 
transformers or other magnetic elements. 
A pair of impedances 22, 23 are serially coupled between the terminals 12, 
13 of the two wire port and the circuitry of the hybrid in such a way that 
the impedances 22, 23 function as battery feed resistors for the hybrid. 
Ignoring for the moment the feedback circuits 24, 26 illustrated 
diagrammatically, the impedances 22, 23 serve as terminating impedances 
for the two wire line with the circuitry of the hybrid connected to the 
terminating impedances providing ac ground points for each impedance at 
these connections. A pair of operational amplifiers 27, 28 are connected 
to the respective terminating impedances for driving the two wire line 
through the impedances. The amplifiers are arranged such that their 
outputs are an effective ac ground as described, for example, in U.S. Pat. 
No. 4,064,377, by provision of feedback resistors 20, 25 with the point of 
feedback establishing the ac ground. 
The magnitudes of the impedances viewed from each of the terminals 12, 13 
looking into the hybrid 10 must be precisely matched in order to provide 
the required longitudinal balance and in combination must match the 
nominal impedance of the equipment coupled thereto at the two wire port 
11. A discussion of longitudinal balance and impedance matching for hybrid 
circuits such as is functionally shown in FIG. 1 may be found in the 
above-mentioned U.S. patent. 
In the hybrid of FIG. 1 it is further desired to provide battery feed to 
the two wire line from the hybrid 10. The amplifiers 27, 28 which produce 
the ac ground are biased to establish their quiescent operating points at 
separated dc levels so that the amplifiers themselves produce the dc loop 
currents. The impedances 22, 23 serve as battery feed resistors. 
In a typical subscriber loop application, the terminal 12 is connected to 
the tip conductor and the terminal 13 to the ring conductor of the loop. 
Typically, subscriber loops are driven by a battery potential of 
approximately -48 volts with respect to ground. To power such a line from 
the illustrated hybrid and allowing for signal swing, the amplifier 27 may 
be biased at -5 volts and the amplifier 28 at -43 volts. Although in the 
absence of the feedback circuits 24, 26, and as discussed in the above 
mentioned 4,064,377 patent, the amplifier 27 may be replaced with a ground 
with all of the bias being provided through the amplifier 28, in the 
present instance it is preferred to use both of the amplifiers 27, 28. 
As schematically illustrated in FIG. 1 the receive port 18 is coupled to 
the amplifiers 27, 28 for driving the two wire line in response to signals 
imposed upon the receive port. It will be appreciated that the amplifiers 
27 and 28 must be configured to respond 180.degree. out of phase with 
respect to each other so as to provide differential ac drive to the two 
wire line. 
For coupling signals received at the two wire port 11 to the transmit port 
14, means are provided for recovering information signals imposed on the 
two wire port while rejecting noninformation-bearing longitudinal signals. 
To that end, an amplifier 29 has its inputs differentially coupled to 
thThe values of the resistors 31, 32 are precisely matched and are several 
orders of magnitude larger than the values of the resistors 22, 23. As a 
result, the impedance of the reverse signal recovery circuitry has a 
negligible effect upon the longitudinal balance of the hybrid. Because the 
amplifier 29 is differentially coupled to the two wire line, it will 
ignore in-phase signals and thereby provide excellent longitudinal 
rejection. The output of the amplifier 29 is coupled to one of the inputs 
of an amplifier 33 having its output in turn driving the transmit port 14 
of the four wire line. As a result, signal currents detected by the 
amplifier 29 are coupled to the amplifier 33 and thereby to the transmit 
port. 
Cancellation means are provided for preventing signals imposed on the 
receive port, coupled to the two wire line, and thereafter detected by the 
amplifier 29 from being returned to the transmit port. To that end, the 
terminal 19 of the receive port is coupled to a second input of the 
amplifier 33, the amplification being adjusted so that a signal imposed on 
the receive port will completely cancel the response of the amplifier 29 
to the effect of that signal coupled to the two wire line. 
Further discussion of the signal recovery function of the amplifier 29 and 
its related circuitry, as well as the use of the amplifier in a 
supervisory capacity, may be found in the above-identified U.S. patent. 
In accordance with the invention, feedback circuits 24, 26 are provided for 
maintaining the desired output impedance of the hybrid 10 as presented to 
the two wire line while reducing the contribution of the hybrid circuitry 
to the dc battery feed loop resistance. To accomplish this, positive 
feedback is provided around each of the amplifier and battery feed 
impedance arrangements, so that the output impedance presented at the two 
wire port 11 is maintained, such as at 600 ohms for a subscriber loop, 
while the magnitude of the battery feed impedances 22 and 23 can be 
reduced. The positive feedback increases the ac impedance looking into the 
hybrid at frequencies of interest to maintain the required output 
impedance match with the two wire line, while allowing a reduction in the 
value of the impedance through which the dc loop current passes. 
Accordingly, it is possible to drive a subscriber loop wherein virtually 
all of the dc resistance is contained in the subscriber loop with very 
little resistance contributed by the hybrid. At the same time, with the 
amplifier series impedances smaller, the amplifier dynamic operating 
ranges are wider because a smaller voltage swing at each amplifier output 
is required to drive the two wire line. As an example, it is possible to 
reduce the magnitude of the battery feed resistors to 100 ohms each, 
substantially increasing the long loop capability of the hybrid while at 
the same time configuring feedback circuits 24, 26 to produce feedback 
over the frequencies of interest to effectively multiply the value of the 
battery feed resistors to achieve in the example a 600 ohm terminating 
impedance. 
Turning now to FIG. 2, there is illustrated the circuitry of a hybrid 
constructed in accordance with the invention. The hybrid 40 is coupled to 
a two wire port 41 including a terminal 42 for a tip line and a terminal 
43 for a ring line. Matched battery feed impedances, resistors 44, 46 are 
interposed in series between the terminals 42, 43 and the circuitry 
driving the two wire line. The resistor 44 is coupled in series between 
the terminal 42 and a junction 47 which is maintained at ac ground. 
Similarly, the resistor 46 is interposed between the terminal 43 and a 
junction 48 which is also maintained at ac ground. 
The ac ground 47 is produced by amplification circuitry including a 
feedback amplifier 49 having a transistor 51 coupled in emitter follower 
configuration between ground and the point 47, with negative feedback from 
the ac ground point 47 to the inverting input of the amplifier 49 being 
provided by a resistor 52. The ac ground at the junction 48 is provided by 
amplification circuitry including a feedback amplifier 53 having a 
transistor 54 coupled in the output circuit thereof in emitter follower 
configuration between a -48 volt bus and the point 48. Feedback from the 
point 48 to the inverting input of the amplifier 53 is provided by a 
resistor 56. It is apparent from the circuit as thus far described that 
the junctions 47, 48 serving as the feedback junctions for the amplifier 
circuitry provide ac ground points and thereby establish the battery feed 
impedances 44, 46 as the sole terminating impedances looking into the 
hybrid from the port 41. 
In accordance with the invention, the values of the battery feed resistors 
are reduced to levels considerably smaller than heretofore used and 
feedback means are provided for multiplying the effect of those values to 
maintain the output impedance of the hybrid at the desired level. In order 
to accomplish that, a positive feedback circuit is connected from a point 
50 on the side of the resistor 44 which is coupled to the tip terminal 42, 
to the noninverting input of the amplifier 49; and a positive feedback 
circuit is connected from a point 55 on the side of the resistor 46 
coupled to the ring terminal 43 to the noninverting input of the amplifier 
53. The structure of those feedback circuits will be further described 
below. 
At the outset, it should be noted that the dc operating point of the tip 
terminal 42 is determined by the resistance ratio of resistors 57a and 
57b. The junction of this dc voltage divider is connected to the 
amplifier's 49 noninverting input to establish a bias thereon. The 
parallel combination of 57a and 57b form a composite resistance 57 whose 
significance will be discussed later. A similar reasoning may be applied 
to resistors 59a and 59b in assessing their function in connection with 
amplifier 53 and the associated ring terminal 43. 
The level of positive feedback for the amplifier 49 is determined by a 
resistative divider connected from the point 50 comprising resistor 
combination 57 and a resistor 58. The voltage across the resistor 57b is 
fed back to the non-inverting input of the amplifier 49 to provide the 
desired positive feedback. The positive feedback for the amplifier 53 is 
provided by a voltage divider comprising resistor combination 59 and a 
resistor 61 connected from the point 55. The voltage across the resistor 
59b is connected to the non-inverting input of the amplifier 53 to provide 
the desired positive feedback. Each feedback path also includes a 
capacitor 62, 63 to provide roll off for the output impedance of the 
hybrid so that at dc the battery feed voltage is not fed back to produce a 
large amplifier voltage drop. 
With the positive feedback, the terminating impedance looking into the two 
wire port 41 is the sum of the impedances 44, 46 to each of their 
respective ac grounds, as increased by the feedback taken from the points 
50, 55. The value of the dc battery feed resistance remains the sum of the 
impedances 44, 46. For example, a nominal 600 ohm subscriber loop, calling 
for a hybrid output impedance of a matching 600 ohms, the dc loop 
impedance contributed by the hybrid may be reduced to 200 ohms, the 
impedances 44 and 46 being 100 ohms each, by setting the gain of the 
amplifiers 49, 53, due to the positive feedback, at three over the 
frequency range of interest. In one such instance, the resistor 
combinations 57, 59 are each 10,000 ohms and the resistors 58, 61 are each 
20,000 ohms. Since the gain of each amplifier due the positive feedback is 
one plus the quotient of the resistor 58 or 61 divided by the value of the 
resistor combinations 57 or 59, respectively, the gain is 3. Therefore, 
the impedance looking into each terminal 42, 43 toward ac ground 47, 48 is 
three times the 100 ohms of the battery feed resistance 44, 46 giving an 
effective impedance at each terminal of 300 ohms, and an effective 
terminating impedance of the hybrid 40 of 600 ohms, matching the impedance 
of the subscriber loop. Meanwhile, with the battery feed impedances 44, 46 
reduced to 100 ohms each, the contribution of the hybrid to the maximum 
allowable 1,650 ohms driven by the battery feed voltage is now 200 ohms, 
rather than 600 ohms, so that the dc resistance of the loop may be 
increased from about 1,000 ohms to about 1400 ohms. The increased dc 
resistance allowable in the loop enables the use of a longer subscriber 
loop for a given hybrid circuit 40. Clearly other, smaller, values for the 
impedances 44, 46 may be selected so that there is even less loss in the 
battery feed from the hybrid 40. For example, if the resistors 44, 46 are 
50 ohms each, the positive feedback for each amplifier 49, 53 is increased 
to a gain of 6, maintaining the 600 ohm output impedance of the hybrid, 
while the dc resistance for the battery feed is reduced to 100 ohms. 
The capacitors 62, 63 in the positive feedback circuits are selected so 
that the frequency roll off of the positive feedback is established at 
about 180 hz, below which frequency the terminating impedance of the 
hybrid as seen from the two wire port 41 decreases below the 
characteristic 600 ohms. Introduction of the capacitors 62, 63 eliminates 
the large voltage drops which would otherwise occur across the amplifiers 
49, 53 at dc. This reduction is also advantageous because a lower 
terminating impedance is therefore presented to the majority of 
longitudinals which typically are at about 60 hertz. The normal frequency 
range for operation of a hybrid is generally between about 300 and about 
3,000 hertz. 
The protection circuits associated with the hybrid 40 are similar to those 
disclosed in the above mentioned U.S. Pat. No. 4,064,377 and will be noted 
only briefly. The ac ground points 47, 48 are each provided with 
appropriately poled clamping diodes for protecting the circuitry from 
excessive voltages, such diodes being indicated generally at 64, 66. 
Diodes 67, 68 are coupled in the output circuitry of the amplifiers 49, 
53, respectively, so as to become reverse biased during short circuit 
conditions on the line, maintaining the associated amplifiers within their 
dynamic operating ranges. Current sensing resistors 69, 71 are connected 
in series with the emitter follower transistors 51, 54 and in the base 
circuit of clamping transistors 72, 73 so that when current flow through 
the sensing resistors is sufficient to forward bias the base-emitter 
junction of the associated transistor, the transistor will conduct, 
clamping the output of the associated driving amplifier to limit current 
in the loop. The transistors 74, 76 and their associated circuitry are 
provided to maintain current flow through the terminating impedances, to 
maintain longitudinal balance, when longitudinal currents attempt to 
override the quiescent current provided by the hybrid. A phase inverter 
circuit 60 is interposed between the receive port of the hybrid and the 
amplifier 53 so that the amplifier 49, 53 are driven out of phase to 
differentially drive the two wire port 41. 
Reverse signal recovery is provided by an amplifier circuit designated 
generally as 77 which also supervises the loop coupled to the port 41. The 
signals recovered from the two wire line are coupled from the output of 
the amplifier circuit 77 through an amplifier circuit 78 to the transmit 
terminals of the four wire port. The other input to the amplifier 79 is 
provided from the receive terminal of the four wire port in order to 
cancel the transmission of signals on the two wire line from the receive 
terminals of the four wire port. A precision balance network 81, which can 
be a gyrator, is also coupled to the noninverting input of the amplifier 
79 to present a frequency dependent impedance to signals from the receive 
port which approximates the impedance characteristic of the line presented 
to the two wire port so that cancellation will occur across the frequency 
band. Further, more detailed, discussion of the recovery circuit 77, the 
amplifier and cancellation circuit 78 and the balance circuit 81 may be 
found in the above-mentioned U.S. patent. 
The two amplifier dc loop current drive shown in FIG. 2 is preferred in the 
practice of the present invention in order to most easily eliminate 
longitundinal problems and provide a balanced terminating impedance at all 
frequencies. In an unbalanced system, such as where all of the dc drive is 
provided by the amplifier 53, with the amplifier 49 circuitry being 
substantially replaced by a ground connection, it would be necessary to 
connect a complex impedance on the tip line terminal 42, that would track 
the amplifier characteristics on the ring line. It is not overly difficult 
using thick film technnology to precisely match the characteristics of the 
amplifiers so that even though the terminating impedance decreases below 
the roll off frequency, the impedance in the tip and ring line track 
precisely so that longitudinals will not prove troublesome. 
It can be seen that a hybrid has been described herein having series 
connected battery feed resistors in which a desired terminating impedance 
is achieved by multiplying the effect of the battery feed resistors so 
that the actual values thereof can be reduced, providing increased loop 
driving capability. It can be further seen that such a hybrid circuit has 
been described which has a wider dynamic range of operation due to a 
reduced amplifier series impedance. Further, such a hybrid has been 
described which is less sensitive to low frequency longitudinals.