I can never remember which way round the TC goes, but below about 5 and a bit
volts, it's Zener breakdown, and above that, it's avalanche breakdown. Because
of the turnover, at about 5.6 to 6 volts, the TC is essentially zero. So for
best stability, you make up higher voltages with a string of 5.6 or 6 volt
diodes. These days of course, it's all integrated, and reference voltages are
produced from a band gap, compensated if you don't want a PTAT (Proportional To
Absolute Temperature) source. Definitely, using series diodes in forward
conduction gives you -2mV/deg C TC, so as Bud says, the voltage from 9 diodes
drops 18mV/deg C. This may not be a problem in many cases, of course.
73
Peter G3RZP
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