In the figure we have one of the many options found in this section for an intercom using ordinary phones not in use. This was obtained in a Popular Electronics from the 90s.
This anti-ringing circuit was found in an English documentation from 1996. The integrated circuit is...
The figure shows how to increase the gain of the amplifier stage of the previous circuit. The...
This circuit uses unusual operational amplifiers, but equivalents can be tried. FETs can also be...