With this circuit found in an American documentation from 1969 it is possible to vary the selectivity (Q factor) of a tuned circuit. The operational is from that time.
This circuit, which can act as a shield, was found in a 90’s Radio Electronics. It enables a power supply...
This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit...
This circuit was found in a 1978 American publication. It beeps if the car's lights are on and the...