This circuit was found in a 1966 EEE. The circuit uses transistors from the time but can be made with modern equivalents. The operating frequency ranges from 0.1 to 1 kHz.
In the absence of a modulation signal, the carrier of this simple short-range transmitter is cut...
This circuit was obtained from an 80's Electronic Engineering. The circuit detects ice formation...
Other operational amplifiers series operatives, such as MC34072, MC34074, MC35071, MC35072,...