This input circuit for meters, which can also be used as a shield, is intended for operation with very low frequency signals. The power supply must be symmetrical.
We found this protection circuit for the power supply in a 1992 documentation. This circuit has...
I found this circuit in a documentation for radio amateurs from 1975. The FET could be the BF245....
Found in an American 80's documentation, this circuit can use common FETs like the BF245 and cover...