Patent
1976-08-05
1977-09-06
Robinson, Thomas A.
179 16F, H04Q 128
Patent
active
040469697
ABSTRACT:
A telephone line transformer having less iron in its core can be achieved by providing the transformer with a third winding to balance out the DC magnetization produced by the tip and ring windings. This winding must be bypassed by a capacitor of fairly large size to provide sufficiently low impedance at voice frequencies to prevent voice currents from flowing through the balancing winding. A capacitor of large enough size, however, may prevent the development of sufficient voltage change during dial pulsing for reliable dial pulse detection. This problem is solved by providing a dynamic capacitor bypass arrangement which includes a negative feedback operational amplifier together with a balancing winding bypass capacitor of reasonable size. The dynamic circuit magnifies the bypass capacitance with respect to voice frequency signals but saturates during dial pulsing and thus leaves the capacitance temporarily unamplified. The arrangement thus yields an effective bypass capacitance of approximately 35,000.mu.f at voice frequencies from a 15.mu.f capacitor which is small enough to permit adequate voltage changes to occur during dial pulsing. Background of the Invention
Bell Telephone Laboratories Incorporated
Popper Howard R.
Robinson Thomas A.
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