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.
Found in an Electronic Design from the 90's, this circuit uses common components. The circuit is...
With this circuit it is possible to modulate a small signal in amplitude using a common germanium...
This circuit is from the 80's integrated circuit manual itself. The component is not as common today...