This circuit that appears in several versions in this section and on this website was found in documentation from the 1980s. The transistors can be of any general purpose NPN type like the BC548.
We found this circuit in the Linear Applications Handbook of the 1994 National Semiconductor. The...
This circuit from a Japanese publication in 1968 uses germanium transistors from the time like the...
This circuit, found in a 1994 publication, allows the use of two common analog telephone sets on a...