Found in a 1924 book by Gernsback, this circuit uses a double variable capacitor, a component not very common at the time. Listening was done on a loudspeaker at the time.
In the figure we have the way to connect two high power operational amplifiers LM12 to double the...
This circuit is from a publication about thyristors from the 70s. It consists of a pulse generator...
This is the receiver circuit for the transmitter of the previous circuit using an integrated...