This circuit is for traditional old-style telephone lines found in an American 1980s documentation. Components are common and the optical coupler can be replaced with an equivalent.
The time constant of this circuit is 100 x R1 x C1, which leads to the possibility of obtaining times...
This circuit with a double-gate MOS transistor was found in a Popular Electronics from the 90's....
We found this circuit in the 1994 Linear Applications Handbook from National Semiconductor. The...