This 50 Hz rejector or notch filter was found in a 1975 documentation. Other frequencies can be obtained by changing components. The filter uses a Wien network and must use a symmetrical power supply.
Found in Linear Technology documentation from the 90s, this circuit uses an operational amplifier...
This simple circuit that interrupts the supply of a 12 V load in the event of an overvoltage was found in...
The circuit shown is from National Semiconductor, having been found in the 1994 Linear Application...