Line feed circuit

A line feed circuit for a telephone subscriber loop is disclosed which can be used to supply constant current to the subscriber loop. An operational amplifier, operating in its direct current mode, is used to maintain the current constant. The same operational amplifier, operating in its alternating current mode, magnifies the value of a capacitance bypassing the line feed circuit. In this way the alternating current impedance of the line feed circuit is maintained at a very low level.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to telephone subscriber loop circuits and, more 
particularly, to a constant current line feed circuit for such loop 
circuits. 
2. Description of the Prior Art 
It has become increasingly common to serve telephone subscribers over 
carrier systems operating on the subscriber loops. This permits the 
transmission of a larger number of telephone conversations on a fewer 
number of metallic pairs. In some situations, such as long rural loops, 
the cost of additional metallic pairs can exceed the cost of carrier 
terminals and therefore the use of electronic carrier systems in the loop 
plant can be justified. One such carrier system is shown in the copending 
application of the present applicant Ser. No. 974,384, filed of even date 
herewith. 
In such carrier derived loops, and in many standard telephone loops, it is 
desirable to maintain the level of direct current in the metallic 
connection to the subscriber (the local drop) at a preselected value 
regardless of the length of the metallic path. In a system that is power 
limited, such as one totally powered from the central office and limited 
due to the resistance of the transmission medium, it is imperative to 
insure that the power utilized by remote circuits is kept to a minimum 
within the constraints of the system. Therefore, instead of providing 
power to the local drop in such a way as to result in variable currents to 
the telephone set, as is the case with the standard constant voltage power 
feed circuits provided by local central offices, it becomes desirable to 
provide all telephone sets with the minimum current permissible, i.e., 
constant current power feed. In this manner the current provided becomes 
essentially independent of the impedance, and hence length of the local 
drop, as well as independent of the impedance of the telephone set. 
A problem with line feed circuits is the inherent internal impedance of 
such feed circuits. Any longitudinal sixty hertz signals induced in the 
subscriber loop from adjacent power lines appear as a voltage across the 
line feed impedance, causing audible interference with voice transmission. 
Capacitor bypasses for these audible components require a very large 
capacitor to accommodate the low frequency sixty hertz signal. 
SUMMARY OF THE INVENTION 
In accordance with the illustrative embodiment of the present invention, an 
operational amplifier operating in its direct current mode is utilized to 
regulate the amount of current delivered to the local loop to provide a 
constant current power feed. The same operational amplifier, operating in 
its alternating current mode, magnifies the value of a capacitor bypassing 
the current feed circuit. This magnified capacitance reduces the source 
impedance of the feed circuit to an acceptable value. 
A major advantage of the line feed circuit of the present invention is that 
it provides adequate talking current for each subscriber and yet limits 
the current to this adequate value, thereby conserving power, reducing the 
size of the power supply and providing a standard signal level at the 
carrier terminal. At the same time, induced alternating current 
longitudinal noise signals are bypassed around the line feed circuit to 
minimize the generation of noise voltages.

DETAILED DESCRIPTION 
In the drawing, a hybrid transformer 10 couples the central office side of 
a subscriber loop to the subscriber side of the loop at tip lead 11 and 
ring lead 12. Ring lead 12 is connected through one winding of transformer 
10 and a diode 13 to the line feed circuit comprising the balance of the 
drawing. The line feed circuit comprises a controlled transistor 14 which 
delivers current to the loop from negative voltage source 15 through 
resistor 16. The base of transistor 14 is controlled by the emitter 
current of transistor 17 in such a fashion as to maintain the current 
through the collector-emitter path of transistor 14 constant. To this end 
an operational amplifier 18 is provided. 
Operational amplifier 18 comprises two input transistors 19 and 20 feeding 
a "long-tailed pair" comprising transistors 21 and 22. Transistors 19 and 
20 are biased from a positive voltage source 23 through resistors 24 and 
25, respectively. The emitters of transistors 21 and 22 are connected 
together through resistor 26 to negative supply voltage 15. A diode 27 in 
the collector circuit of transistor 22 and a transistor 28, the 
collector-emitter path of which is in the collector circuit of transistor 
21, help insure equal current flow in the transistors of the long-tailed 
pair. 
One input to amplifier 18 is the voltage across feed resistor 16, thereby 
providing a voltage proportional to the current flow in the subscriber 
loop. The other input to operational amplifier 18 is supplied across a 
voltage divider comprising resistors 29 and 30, connected between positive 
voltage source 23 and negative voltage source 15. The voltage supplied to 
the base of transistor 20 is therefore constant and the operational 
amplifier operates to maintain the current through resistor 16 at a 
related constant value. Thus the output of the operational amplifier 18, 
taken from the collector of transistor 21, is supplied to the base of 
transistor 17. The emitter of transistor 17 is connected to the base of 
transistor 14. The collector of transistor 17 is biased through resistor 
31 from positive voltage source 23. 
It can be seen that operational amplifier 18, operating in its direct 
current mode, serves to supply a constant current through the 
collector-emitter path of transistor 14. The same operational amplifier 
18, operating in its alternating current mode, serves to magnify the 
capacitance of a bypass capacitor 32. Thus, alternating current signals 
from the ring conductor 12 cause an alternating current voltage to appear 
on the collector of transistor 14. Capacitor 32, being a low impedance 
compared to resistors 29 and 30, permits transmission of that voltage 
directly to the base of transistor 20, one input to the operational 
amplifier. The action of the operational amplifier causes an equal voltage 
to appear at the emitter of transistor 14. Since the same voltage appears 
at both the collector and emitter of transistor 14, it is effectively 
bypassed. The capacitor 32 therefore must be a low impedance with respect 
to resistors 29 and 30 rather than a low impedance compared to resistor 16 
which would be the requirement if the transistor 14 were bypassed 
directly. Since the input impedance to the operational amplifier at the 
base of transistor 20 is quite high, resistors 29 and 30 can be made much 
larger than resistor 16 which is in the direct current path and must be 
kept small in order not to waste power. Hence a substantially smaller 
capacaitor can be used to perform the bypassing function, or 
alternatively, the smaller capacitor that is used has a greatly magnified 
effect by action of the operational amplifier. 
The need to obtain capacitance magnification arises not only from the fact 
that the current source must maintain a low impedance at audio frequencies 
for proper signal transmission, but from the fact that longitudinal power 
line frequency currents induced in the cable pair to the customer cause 
current flow in the same path. It is necessary to provide a low impedance 
for these sixty hertz currents since any voltage developed in the loop due 
to these currents appears as noise in the subscriber receiver and becomes 
audible to the customer. Since the frequency of these currents is well 
below audio frequencies, much larger capacitors are required for 
bypassing. The size and cost of these capacitors can become prohibitive, 
making capacitor magnification an extremely valuable tool in realizing the 
line feed circuit. 
The direct talking current is supplied from negative voltage source 15, 
through resistor 16 and transistor 14 to the ring conductor 12. The 
talking current path is completed through the subscriber telephone set, 
tip conductor 11 and back to ground potential. 
It can be seen that the line feed circuit of the present invention not only 
maintains a constant current feed to the subscriber telephone set, but 
itself has a very low source impedance. This low source impedance reduces 
the talking signal drop across the line feed circuit and thereby provides 
a maximum talking signal across transformer 10 to be delivered back to the 
central office as well as to the customer. In addition, the effects of 
longitudinal power line induction are minimized.