This adjustable circuit from 0 to 200 nA was found in a 90's EDN. The components can still be found relatively easily, as they are CMOS.
Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values...
This circuit was found in the August 1994 issue of Electronics Now magazine. The 4046 is uncommon...
This circuit is based on an operational amplifier that is no longer common today but can be...