This 1970 Electronic Design circuit uses a varactor or varicap. The circuit divides the frequency of a 150 MHz signal by 2. Similar versions appear on this site.
In the figure we have a wireless wiretapping circuit for the frequency range determined by L1. For FM...
This voltage follower or amplifier with unit gain uses an unusual operational amplifier nowadays. The...
Found in a 90’s Electronics Now, this circuit has its output connected to an audio amplifier. The...