This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit uses a diac and the transistor can be general-purpose.
This operational amplifier circuit with variable gain was obtained in an 80's Electronic...
The circuit shown was obtained in American documentation from 1968. MOSFETs can be replaced by...
We found this circuit in a Popular Electronics from the 80s. This circuit eventually doesn't work with...