This circuit is from a 1968 Motorola power circuit manual. The circuit uses transistors of the time. The passing band is 45 Hz to 100 kHz.
The circuit with a common operational amplifier uses a zener as a reference. The circuit is from...
This circuit was found in a 90’s Popular Electronics. The circuit is used to analyze noise in...
This 1989 circuit activates a protection relay if one or two phases are missing from a three-phase...