| 摘要 |
PROBLEM TO BE SOLVED: To measure subscriber loop impedance in the inside by generating first and second currents proportional to sensed loop voltage and loop currents and generating a third current by adding second current to the first current. SOLUTION: A loop voltage sensing circuit 20 measures the voltage between subscriber lines. This voltage is scaled by a scaling circuit 14 and passed through a resistor 18 to yield the first current. A loop current sensing circuit 10 measures the voltage applied to a resistor. This voltage is passed through a resistor 16, lead to a first current mirror 24 and scaled to yield the second current. A second current mirror 26 gives the first current to a third current mirror 28. The third current mirror 28 generates the reproductions of the first current on two lines. One of the reproductions of the first current is added to the second current to yield the third current. A fourth current mirror 30 generates reproduction of the third current.
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