Electronic bridge circuit for supplying dc power and tone...

H - Electricity – 04 – M

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379/63

H04M 19/00 (2006.01)

Patent

CA 1116767

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.

296101

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