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 stabilized zener voltage as the supply voltage! This is obviously only possible if the stabilized voltage is higher than the zener voltage. This is the figure of the circuit of stable zener reference;
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 stabilized 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 op amp 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.
