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Subscriber Telephone Line Circuit

IP.com Disclosure Number: IPCOM000095254D
Original Publication Date: 1965-Oct-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Croisier, A: AUTHOR [+2]

Abstract

This subscriber telephone line circuit allows most of the DC to be eliminated in the subscriber transformer. The circuit differs from the usual circuits by the addition of transistors K1 and K2 and zener diodes Z1 and Z2. The function of each transistor-zener diode combination is to derive the DC which would otherwise flow through each half A and B of the transformer T primary.

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Subscriber Telephone Line Circuit

This subscriber telephone line circuit allows most of the DC to be eliminated in the subscriber transformer. The circuit differs from the usual circuits by the addition of transistors K1 and K2 and zener diodes Z1 and Z2. The function of each transistor-zener diode combination is to derive the DC which would otherwise flow through each half A and B of the transformer T primary.

The DC part which flows through winding A also flows through the base of K1. This causes a current beta times greater to flow through the K1 collector. Thus, only 1/(beta+1) of the current from the -48 v source actually flows through winding A. The same applies to transistor K2 and winding B. Z1 and Z2 regulate the voltage drop across each transistor to about 5 v. Thus, voice signals of 10 v peak-to-peak can be accommodated without reverse-biasing K1 and K2.

The circuit provides an automatic limitation since T is not designed to operate with a DC bias. When the voice frequency amplitude becomes too high, the DC actually flows into A and B, saturating T and blocking transmission. The voice frequency current in A and B is bypassed by capacitor C. Thus, K1 and K2 introduce no voice frequency feedback. C is a component normally provided in the line circuit.

Other circuit arrangements based on the fundamental circuit can be provided. The zener diode circuits can be replaced by the following circuits. Two resistors are series-connected between point a' and the K...