[OT] Impedance Matching a Phone Line
David Huisman email (remove spam text)
The phone line impedance should be 600 or 900 Ohms.
You can match to this with an opamp.
Use 47k in series with inverting input with a capacitor (where C = 1/
2*PI*47k*F, F = lowest frequency .. say 20Hz).
Now make feedback resistor = Gain Required x 47k. Say gain is 5, then
feedback resistor is 235k (use 220k or maybe a 500k pot if you want to
adjust the level).
The output of the op-amp goes to a 600 Ohm isolation transformer (Farnell
have small PCB mounting versions for Telecom) via a resistor equal to your
line impedance, the other side of the winding goes to +ve.
This now gives you a balanced, isolated drive to the line. You will need to
do a little more on the line side depending on your application (line seize
and hold circuitry etc)
Hope this is useful to you as a starting point.
See also: www.piclist.com/techref/index.htm?key=
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