This circuit was found in an American documentation two years 80. The MOSFET is of double port, difficult to obtain now and the circuit has a tuning frequency determined by RC.
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...