This traditional dimmer configuration for incandescent lamps and heating elements was found in an RCA transistor manual. SCR can be a more modern equivalent like TIC106, depending on the power grid.
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,...