This peak detector has a memory to prolong the response. The circuit is from American documentation from the 1980s. The supply does not have to be symmetrical.
This digital instrument input circuit was found in the September 1989 issue of Radio Electronics...
In this circuit of a 1970s publication, data from the coil to oscillate at the desired frequency...
This circuit is from an old Elektor using components that are not currently common. The circuit is...