CA1116767A

CA1116767A - Electronic bridge circuit for supplying dc power and tone signals to subscriber line

Info

Publication number: CA1116767A
Application number: CA000296101A
Authority: CA
Inventors: Richard N. Brooksbank
Original assignee: Plessey Overseas Ltd
Current assignee: Plessey Overseas Ltd
Priority date: 1977-02-01
Filing date: 1978-01-31
Publication date: 1982-01-19
Also published as: AU3267378A; AU514376B2; IT7819856A0; GB1560922A; NZ186342A; FR2379210A1; IT1092555B; ZA78513B

Abstract

A B S T R A C T

In a telephone system having a two-conductor power supply to a subscriber's telephone, there is provided a circuit for supplying the conductors with d-c power using the principle of transforming a capacitance into an inductance between the conductors and a d-c power source. The circuit comprises an electronic feed bridge consisting of a PNP transistor having its emitter connected to the positive side of the d-c power source via a variable resistor, and its collector connected to the first conductor, and an NPN transistor having its collector connected to the second conductor and its emitter connected via first and second series connected resistors to earth. The bases of the two transistors are connected via bias resistors to their collector circuits and also to opposite terminals of a capacitor, the common point of the first and second resistors being connected to a tone generator, whereby various tones can be introduced to the subscriber's telephone. The invention eliminates the need for large inductances and provides a means of compensating for the large variations in earth potential in different geographical areas to maintain interference from noise sources at a minimum.

Description

~6~ 7 The present invention relates to telephone systems and more particularly to electronic feed bridges for feeding D.C.
power and/or tones to a subscribers line.
It is necessary to supply a subscribers telephone with D.C. and also in many instances it is required to inject tones for example for use by the subscriber in identifying the line condition. It is desirable to be able to balance the impedance to earth presented by the electronic feed bridge.
It is, therefore, an object of the present invention to ) provide an electronic feed bridge, for a telephone system, with the facility of balancing the impedance to earth and/or the facility of introducing tones onto the subscribers line.
According to the present invention there is provided an electronic feed bridge for a subscrlbers telephone line including first transistor means for connection to one of the subscri~ers lines, ~econd transistor means for connection to a ~urther one of the subscribers lines, and includ;ing variable resistance means connected in a main current pa~h of the first transistor means for variation of the impedance of ~he first transistor means.
O In a preferred embodiment there is provided tone genera-tion means connected to a main current path of the second tran-sistor means for the supply of tones to the further one of the subscribers lines.
Preferably, the tone generation means comprises a tone generator which is connected to the common point of two series connected resistors which resistors are connected in the main current path of a transistor in the second transistor means.
Embodiments of the present invention will now be

- 2 -described by wa~ of example with reference to the accompanying drawing in which:-FIGURE 1 shows an electronic feed bridge incorporatingboth the tone injection and the earth balancing facility and, FIGURE 2 shows an equivalent circuit of the electronic feed bridge of F~GURE 1.
The subscribers lines Ll, L2 are shown connected to a subscribers telephone instrument T. The electronic feed hridge is connected to the lines Ll, L2 at terminals Tl, T2 and comprises a first transistor 1 and a second transistor 2. The first tran-sistor 1 is provided with a bias resistor 3 and the second tran-sistor 2 with a bias resistor 4. The bases of transistors 1 and 2 are connected to respective opposite terminals of a capacitor 5.
Txansistors 1 and 2 are of opposite conductive types and in their emitter circuits respectlvely have resistors 6 and 7, 8. Resistor 6 i5 variable and the common point of resistors 7 and 8 is con-nected to a ton~ generator Gl.
Resistor 6 is adjusted, a~ter installation of the feed bridge, until the impedances Zl, Z2 (see FIGU~E 2) are equal, ~0 Zi and Z2 being the total resistances of each side of the feed bridge presented to earth. Thls balancing is essential in tele-phone lines if interference from noise sources is to be kept at a minimal level.
Generator Gl can be connected to the common point of resistors 7, 8 and when so connected can provide tones for the

- 3 -' . : , .

operation of the telephone exchange by modulating the current supplied to the subscribexs telephone.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a telephone system having a two-conductor power supply to a subscriber's telephone, a circuit for supplying said conductors with d-c power using the principle of trans-forming a capacitance into an inductance between said con-ductors and a d-c power source, said circuit comprising an electronic feed bridge consisting of a PNP transistor having its emitter connected to the positive side of the d-c power source via a variable resistor, and its collector connected to a first of said conductors, and an NPN transistor having its collector connected to the second of said conductors and its emitter connected via first and second series connected resistors to earth, the bases of the two transistors being connected via bias resistors to their collector circuits and also to opposite terminals of a capacitor, the common point of said first and second resistors being connected to a tone generator, whereby various tones can be introduced to the subscriber's telephone.