This is the basic configuration of an operational amplifier sample & hold circuit. The FET and transistor are commonly used. The circuit is from an 80's documentation.
Found in Electronics Today International in the 90s, this circuit turns an operational on and off...
At dusk this circuit triggers a relay switching lamps. The circuit can be powered from 6 to 12 V...
This second-order low-pass Salen-Key filter was obtained from an American 1980s documentation. Formulas...