The heart of any voltage regulator is the voltage reference. The output voltage stability and accuracy are no better than the reference. Since this series of articles concentrates on vacuum tube circuits, references for high-voltage supplies will be covered here.
Some references can also serve as regulators by themselves, i.e. they can supply significant power and still maintain a constant voltage. Others, usually the more precise ones, only maintain their specifications under very light loading, and so must be connected to a regulator circuit to control power.
References fall into two main categories: low voltage and high voltage. Many good low voltage references have been designed for the solid-state market. In order to use them with high voltage regulators, their reference voltage needs to be greatly scaled up, sometimes as much as 100 or 200, which can introduce drift and errors, particularly in the simpler regulator circuits. High voltage references are ones that are within a factor of two to five of the output voltage. There are fewer high voltage references available than low voltage ones, but their use can simplify the regulator design.
The important specs for references are: reference voltage, absolute accuracy, output impedance (in two-terminal devices, this is called dynamic impedance) and the temperature coefficient. The first three affect the reference's integration into the regulator design. Variations in absolute accuracy in both the reference and associated components are usually handled by a trim pot. Unless carefully compensated, the temperature coefficient can