This simple circuit with two transistors was found in a 1985 publication. The power can come from batteries or battery, as consumption is very low.
This circuit can be applied to other 3-terminal regulators and equivalent transistors can be used....
This circuit is from a 1990 publication, but the idea can be used in a current project to supply...
This simple circuit for headphones or small speakers was found in documentation from the 90s. The...