Author Topic: Self biased Zener reference using Wilson current mirrors  (Read 1892 times)

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Offline Kleinstein

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #25 on: January 16, 2025, 10:50:05 am »
One can tune the TC somewhat with the current, but this has only a somewhat limited range.  The current needed to get zero TC may be too low (too much noise) or too high to be practical.
For really stable voltage one may also have to care about the 2nd order TC.  The transistors VBE in the 3 BJT suggestion causes a current that goes slightly  up with temperature. This usually gives a more positive TC with increasing temperature, so a positive 2nd order contribution. For many zeners this would be at least the opposite sign of the usually negative 2nd order part of the Zener itself.
 

Offline magic

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #26 on: January 17, 2025, 12:15:00 am »
It works pretty well. Doesn't noticeably budge when I flip it from 9V to 18V. No capacitor required. Very nice!

The basic idea is stolen from Linear Technology's zener reference ICs, although their mirrors are more complex to reduce errors like Q1, Q2 base currents or the still remaining small difference between their collector voltages.

The circuit obviously relies on good matching of these two transistors, they need to behave similarly when their Vce is varied.

It also has the "interesting" property that there are two stable states: the desired one, and one when all transistors are off ::)
The latter may be avoided if there is enough leakage somewhere to turn at least one transistor somewhat on. It may be reliably avoided by adding a startup circuit, for example a diode from some low precision 3.3V rail to Vz.
« Last Edit: January 17, 2025, 12:18:00 am by magic »
 
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Online Zero999

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #27 on: January 20, 2025, 11:12:58 pm »
I've done a similar thing before, but it was far more crude: just a couple of transistors.
 
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Offline magic

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #28 on: January 20, 2025, 11:16:47 pm »
I think you'd get better AC PSRR if the compensation cap is referenced to the input rail rather than ground.
 

Online LordXaos

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #29 on: January 21, 2025, 10:48:12 am »
Ah, so I am not the only one who tried this  :) Wasn't sure if this was in any way a good idea or I just had a strike of luck...

Anyways, in May 2021 I did give this a full-blown try, using LM399, proper PCB and a little extra effort. Experimenting showed that, in addition to the two stable points magic mentioned, there could be some additional more or less stable points at very low currents, and with parasitic capacitance on the ballast side it could get stuck there (happened on breadboard, where there's more parasitic capacitance), I did put in some JFET that will kick-start the voltage, and then in essence just influences the circuit via gate leakage (I hope).

Overall I went with a pair of 10k low TC ballast resistors in parallel with the LM399, mostly because 10k were obtainable for a reasonable price, and 1.4mA is not that far off the suggested 1mA... Also used matched mirror SOT23 parts for the current mirrors, no ballast, but a pair of L7812 (one for the reference side, one for the heater) to suppress more supply noise.

I did build a total of four of these, though the first one was stillborn (messed up soldering temp...), and the last one plagued by an LM399 specimen with a lot of popcorn noise (this one featured two plateaus separated by about 7µV, very obviously). But the other two turned out fine, and as far as I can tell have not drifted any more than my measurement uncertainty since then (apart from an induced drifting event when I modified one of them as an experiment).

They usually are quite low-noise also (but I never managed to achieve more than 10 minutes of continuous recording without some externally induced spike) - over those 10 minute intervals the Keithley 7510 showed standard deviations between 0.6 and 1.2µV, although there seems to be some temperature dependence overall (as it got cooler in the evenings the voltage changed ever so slightly, somewhere in the single µVs).

Overall these seem to work quite well for my very limited skills...  ;D

 

Online tggzzz

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #30 on: January 21, 2025, 11:14:15 am »
... and with parasitic capacitance on the ballast side it could get stuck there (happened on breadboard, where there's more parasitic capacitance)...

With solderless breadboards, inductance is often more of a problem - but capacitance doesn't help.

Very simple example, inspired by Horowitz and Hill: https://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/ Includes how to miss it, how to spot it, the cause, and some ways to avoid those problems.
« Last Edit: January 21, 2025, 11:22:59 am by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online LordXaos

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Re: Self biased Zener reference using Wilson current mirrors
« Reply #31 on: January 21, 2025, 02:48:21 pm »
I think I'd generalize that as "solderless breadboards do have a lot of parasitics"  ;) in that particular case I did experiment with adding some intentional capacitance there, which made the previous sporadic problem into repeatable behavior, thus my conclusion, though I tend to be wrong ... but anyways, the proper PCB-based version works, I dare say, unreasonably fine. But at least now I know I am not the only one that came up with that idea and got good results  :)
 


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