We found this circuit in the April 1993 Electronics Now magazine. Almost any general-purpose JFET can be used. Power is supplied by a battery (2 x 9V).
In the figure we have the way to implement a 5 to 45 V zener diode with 50 mA of maximum current...
This circuit is from a Radio Electronics magazine from December 1958. The indicator instrument is analog...
These integrated ones consist of class B amplifiers with gain fixed 24 internally for better...