LEDs can be, have been and are used as voltage references. Standard LEDs in various colors are cheap and hence sensible when wanting a low accuracy low voltage reference. Remember that low voltage Zeners have bad knee characteristics and poor tolerances. Zener diodes do make noise when in the breakdown region. In fact, there are noise generators based on this known principle in antique ARRL handbooks. However, is the LED less noisy than the Zener?
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