Stable Zener Reference

Nowadays some first-rate
voltage references are available. Take the LM385 for example: this is
available for different voltages and even comes in an adjustable
version. What is more, the current consumption may be kept very small
(10 µA). But as often happens, you may not have one to hand when you
need one for an experimental circuit. In that case, you could use an
ordinary zener diode for the reference. Unfortunately, they have a
somewhat higher internal resistance (about 5 ?), which means they won’t
be very stable when the supply voltage varies. The solution is right in
front of us: use the stabilised zener voltage as the supply voltage!
This is obviously only possible if the stabilised voltage is higher than
the zener voltage.

Circuit diagram:

Stable Zener Reference Circuit

Stable Zener Reference Circuit Diagram

It therefore has to be amplified a little. This is exactly what this
circuit does: it amplifies it by a factor of two. The current limiting
resister should be chosen such that a current of 1 to 3 mA flows through
the zener diode. Manufacturers usually state the zener voltage at a
current between 3 to 5 mA. The zener diode is fed from a stabilised
voltage and hence has a very stable operating point, which is
independent from the supply voltage. The graph speaks for itself. It is
clear that the output voltage is much more stable. The graphs have been
plotted to different scales to make the comparison easier. In reality
the opamp output is twice the zener voltage. Zener diodes also have a
temperature coefficient, which is smallest for types with a zener
voltage around 5 volts. Virtually any type of opamp should be suitable;
even our old friend the 741 works well enough.

Author: Karel Walraven
Copyright: Elektor Electronics

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