Constant current circuit for supplying telephone terminals - has high symmetry AC attenuation without complex symmetry restoring controls

摘要

The constant current circuit, for supplying telephone terminals from a central exchange, achieves high symmetry AC attenuation without using complex controls to maintain symmetry. A transistorised constant DC current source (T1) is connected into one wire (a) of the loop circuit and acts also as a constant AC current source. The second wire (b) contains a transistorised constant DC voltage source (T2) that also acts as a constant AC current source. The transistors are IGFETs. The advantage lies in the stability achieved by using only one constant DC current source - and thereby preventing asymmetry arising from any opposite regulation that might occur with two sources in series.