This circuit is from a Motorola documentation from 1968. The integrated one is an operational of the time still in metallic casing. Modern equivalents can be used.
This circuit using an operational amplifier was found in old American documentation. It can be...
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Like the previous circuit, this is used in the analog part of the telephone lines. The same...