This circuit is from a 60's documentation. The capacitance C1 can be between 20 pF and 0.01 uF depending on the generated frequency. Equivalent transistors may be employed.
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...