We found this circuit in a 1968 US documentation. The power FET is a component of the time and the current is 4.5 mA only.
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...