It has long been known that the simple combination of a depletion-mode MOSFET (and before these were available, a JFET) and a resistor made a simple, serviceable current source such as that seen on ...
A recent Design Idea (DI), “Silly simple precision 0/20mA to 4/20mA converter,” by prolific DI contributor Stephen Woodward uses the venerable LM337 regulator in a creative configuration along with a ...