Active transformers for off-hook sensing in telephone applications

A novel transformer and circuit for off-hook sensing and isolation functions required in telephone applications are described. The transformer features a thin-film InSb Hall sensor integrated into the gap of a modified -core transformer, connected in a feedback circuit. A prototype transformer with 1000 turn 800 mH 100 Ω windings was demonstrated to pass 10 mA voice signals in the presence of 50 mA dc off-hook signals nulled to less than 1 mA by feedback. The sputtering process used to deposit 1-μm thick InSb films on polished ferrite, fabrication of sensors, and the design and implementation of the transformer are also described.