Browse Prior Art Database

# FEEDBACK LOOP PROVIDING DC CHARACTERISTIC IN MALC SYSTEM

IP.com Disclosure Number: IPCOM000005690D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-26
Document File: 3 page(s) / 107K

Motorola

## Related People

Patrick Clement: AUTHOR

## Abstract

Within the MALC system the subscriber's telephone line is current fed. However the DC characteristic for feeding the line requires more than a simple current source. This characteristic should have two polarities and in each case three parts are distinguished as shown in FIG. 1: - VOLTAGE FEED: the system provides a constant line voltage VL = VF;

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MO7DROLA Technical Developments Volume 7 October 1987

FEEDBACK LOOP PROVIDING DC CHARACTERISTIC IN MALC SYSTEM

by Patrick Clement

Within the MALC system the subscriber's telephone line is current fed. However the DC characteristic for feeding the line requires more than a simple current source. This characteristic should have two polarities and in each case three parts are distinguished as shown in FIG. 1:

- VOLTAGE FEED: the system provides a constant line voltage VL = VF;

- CURRENT FEED: the system provides a constant line current IL = IF;

- RESISTIVE FEED: the system works like an ideal source with an internal resistance RF.

Because the MALC system should be able to work as a voltage source, a feedback loop is needed. FIG. 2 shows the way the DC characteristic is achieved. The behaviour of this loop may be easily understood if the VF part is explained first. In this case the two voltage limiters and the resistive voltage limiter are inactive. The differential DC voltage on the line VL is measured across the input dividing network providing between DC+ and DC- a voltage equal to rDc VL. This voltage is then compared to a reference voltage V, and the error, con- verted into a current by means of 4-input OTA (OTA = Operational Transconductance Amplifier), is integrated on the capacitor Cot. This integration produces a high gain for the DC signal in the loop and also has a low pass effect which filters out the AC signals. Furthermore the voltage VL on capacitor Cot is buffered and transformed into a differential current in the output stage. This current is then amplified by a factor Akv in the HV-MALC and sent onto the line. A stable point is reach when

When the voltage limiters become active the loop provides the IF part of the characteristic. One of the voltage limiters is used for normal polarity, the othe...