Found in a 1967 documentation this circuit can be applied to any operational amplifier. Components without values determine the frequency to which the circuit is tuned.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...