I'm hope that you are not refering to TL431 @ 1mA bias with 5V supply? Due to divider, the TL are internally disabled without any gain, and probably will show 2V (1mA times 2000R) between cathode/anode (a little less than that due to internal biasings). So the 3µ3V is residual from the biasings and thermal noise etc.
The 20µV @ 2V5 relays accordingly when the device are operational at 5V (5mA up; 40µV noise).
Versus the 6V8 specimen, the TL will have probably 55µV of noise at 6V8 and the zener 21µV, then.
I believe that you know that but costs nothing to mention this in case another fellow like me are reading all these pages (sometimes I read entire topic when is not soo long).
Most zeners are variable in this regards, and some brands have LESS noise at gray "avalanche-zener" zone (5V6-6V8). The TL at least have the 20µV/2V5 more consistently from maker to maker or batch to batch, although for me is more fun to play with discrete components (fun for others, nightmare for some)...
PS.:
Old thread, but with good information like these measurements from another pages. I arrived here first.
BTW about the topic. Remember that open loop series regulator will show output impedance that varies with load, being higher for low currents. Theorically 1/gm of the device at operating current. For constant or low varying current loads like most preamps, no big deal.
So when one asks me "about this O/L series reg out Z" I say, "at what load current"?
So when one asks me "about this O/L series reg out Z" I say, "at what load current"?
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