Self biased Zener reference using Wilson current mirrors - Page 1
Forum section: electronics / projects

author: Dabbot
author_rank: Regular Contributor
timestamp: January 12, 2025, 02:41:19 am
content: I saw a video by @w2aew where he demonstrates the Wilson current mirror operating with little to no change across different burden voltages:I decided to try a self biased Zener voltage reference using two Wilson current mirrors to see how it performs, and thought I'd share.The circuit here is biasing a 5.1V Zener diode close to where its temperature coefficients cancel out (set by R3). Q1/Q2, and Q5/Q6 are matched by hand. I built the circuit up on a breadboard and supplied either 9V or 18V while measuring the output, and found it differed by 1-2mv at most.I noticed that when I used my DMM to measure the voltage across R2, the upper mirror went into oscillation, hence the presence of C1. This is quite possibly because it's on a breadboard. C1 could be reduced, or even taken out entirely if the circuit is soldered.

author: moffy
author_rank: Super Contributor
timestamp: January 12, 2025, 07:23:12 am
content: That's not a bad idea, using the reference/zener to create it's own fixed bias current with power supply rejection built in.

author: tggzzz
author_rank: Super Contributor
timestamp: January 12, 2025, 11:04:08 am
content: Good to see someone showing imagination and inventiveness, and realising solderless breadboards have problemsYour schematic doesn't show any PSU decoupling capacitor, which would be necessary on a virtually any construction technique.Another way of self-biassing a zener is inside an opamp feedback loop (TAoE3, p676)

author: Dabbot
author_rank: Regular Contributor
timestamp: January 12, 2025, 11:38:59 am
content: Quote from: tggzzz on January 12, 2025, 11:04:08 amNot pictured.It was the first thing I checked when I noticed something funny about the top current source.Quote from: tggzzz on January 12, 2025, 11:04:08 amI have the remnants of a zener and op-amp circuit beside this one. What is the case where you would need a startup circuit? I assume if you're using an op-amp with PNP input stage and negative rail referenced zener, it's not necessary? I was using a LM358 and didn't have any issues with startup.

author: mawyatt
author_rank: Super Contributor
timestamp: January 12, 2025, 01:57:00 pm
content: Clever use of the Wilson Current Sources, well doneOne could also investigate using a ~6.2V Zener with TC ~+2mv/C and take the output from the base pair connection of Q5 and Q6, thus the output voltage would be Vz + Vbe which should have ~ 0 TC due to Vbe ~ -2mv/C. Use R3 to adjust Zener current for best overall TC.Best

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 12, 2025, 11:09:23 pm
content: Quote from: mawyatt on January 12, 2025, 01:57:00 pmYes, tho usually if chasing temperature compensation, you get two diodes in the same package.The Nexperia PLVA26xx is an example of dual diodes designed for this use.Attached data sheet plots show how TC varies slightly with Zener current, and another from TI showing the average TC effects of Zener + diode(s) in series.

author: mawyatt
author_rank: Super Contributor
timestamp: January 13, 2025, 03:37:39 pm
content: Quote from: mawyatt on January 12, 2025, 01:57:00 pmTaking the Output Voltage from bases of Q5 and Q6 leaves a temperature gradient which can't be corrected alone by R3 in changes in simulation.Here's an adaptation utilizing a 6.2V Zener and adding a Vbe junction in series for TC compensation. With this configuration the Vout Positive TC can be adjusted by R3 and the Negative TC by R4 and as shown can be balanced to producing the classic bowed Vout curve.There's all sorts of reasons why this isn't practical for use as a TC Compensated Reference, however an interesting circuit concept to play around with.Anyway, thought some folks might be interested.Best

author: iMo
author_rank: Super Contributor
timestamp: January 13, 2025, 04:13:27 pm
content: Quote from: mawyatt on January 13, 2025, 03:37:39 pmThe zeners in the LTspice (ie in your above sim) usually do not have the specific params defined needed for their TC..PS: Here is a model I experimented with in past (in some posts I did re Vrefs sims). Example only.Quote

author: RoGeorge
author_rank: Super Contributor
timestamp: January 13, 2025, 04:18:58 pm
content: Nice trick with Q7 to help adjusting both the +/- temp coeficients, thanks.I'm tempted to think about the initial circuit (upper and lower half) as being two current sources in series. Thumb rule is to avoid putting current sources in series (or voltage sources in parallel) or else they'll fight with each other. I wonder if there will be a bigger benefit in using only half of the schematic. I'm tempted to believe that will be more stable, but that's only a guess, I didn't try.I wonder which one will be more beneficial, two mirrors like it is now, or a single mirror, and with the extra transistors used to make Darlingtons or Sziclay transistor combination instead of Q3 (so to have bigger Beta and less Ib in Q3). Oh, and a red LED instead of Zener, for low noise.

author: mawyatt
author_rank: Super Contributor
timestamp: January 13, 2025, 04:37:48 pm
content: Quote from: iMo on January 13, 2025, 04:13:27 pmThis is just a limitation of the model not necessarily LTspice, the same would be for any simulator including Cadence.Creating a specific model for a specific diode as you've done would be in order if one was considering actually building this circuit, then you would still be up against not having all the circuit elements on a common thermal platform such as a silicon die, so basically likely a waste of time, why we indicating"playing around"If we were still designing chips, this might be an interesting circuit to throw onto a test chip, but those chip designing days have long since passedBest

author: iMo
author_rank: Super Contributor
timestamp: January 13, 2025, 04:44:53 pm
content: Quote from: mawyatt on January 13, 2025, 04:37:48 pmMeaning - in "LTspice's libraries".. So you have to find the params and add them to the LTspice's zener models. Otherwise your zener will not show typical zener's TC in your sim..

author: mawyatt
author_rank: Super Contributor
timestamp: January 13, 2025, 04:56:18 pm
content: Quote from: RoGeorge on January 13, 2025, 04:18:58 pmThe current sources are really mirrors and actually not having the high Z outputs driving each other. The top Current Mirror has a low Z input impedance on the right side as one "sees" a low Z due to the 2 Vbe junctions. The top Mirrors drives the lower mirror with it's high Z output driving the low Z input of the bottom mirror as "seen" on the left side thru 2 Vbes and the Zener.So this isn't like having 2 high Z current sources in series but more like having the high Z Current Mirror drive the lower Z of the opposite mirror which in turn has its output driving the 1st Mirror low Z.One potential point is that this circuit may not start up because of the coupled mirrors driving each other and may need some sort of start-up circuit (bleed resistor).Note as iMo mentioned the Zener model used was just grabbed from the library, and any "serious" work should 1st start off with a fully developed Zener model, same is true for the transistors.Anyway, still a fun circuit to play around withBest

author: David Hess
author_rank: Super Contributor
timestamp: January 13, 2025, 05:21:23 pm
content: Last time I checked, a couple years ago, temperature compensated reference zener diodes were about the same price as equivalent LT1236 or LT1021 buried zener series references.$3.47 1N823 5.9-6.5V 50 ppm/C$3.98 1N825 5.9-6.5V 20 ppm/C$5.68 1N827 5.9-6.5V 10 ppm/C$8.00 1N829 5.9-6.5V 5 ppm/C$6.66 LT1236BC 5.0V 5 ppm/C$7.29 LT1236BC 10V 5 ppm/C$8.93 LT1021DC 5.0V 3 ppm/C$8.93 LT1021DC 10V 5 ppm/C

author: RoGeorge
author_rank: Super Contributor
timestamp: January 14, 2025, 07:13:08 pm
content: Tried out of curiosity a minimalistic version, where the second mirror was replaced with an emitter voltage follower.Click to enlarge.- at low frequency (100Hz Vcc ripple) it has about the same performance as the schematic with 2 current mirrors (~112dB rejection of Vcc).- at higher frequency (e.g. 1MHz Vcc ripple) it behaves much better because of the lack of C1. If the C1 is added here, too, it is only marginally better than the circuit with 2 mirrors.- this mirror + follower version uses 4 transistors instead of 6, so if it were to make use of 6 BJTs, we could try a 3rd schematic with composed transistors for a higher gain (and thus an even bigger rejection), and indeed the 3rd version has a bigger Vcc rejection.Click to enlarge.The Vcc ripple was added as another AC source in series with the Vcc. AC response is to estimate the performance of all 3 versions (in terms of Vcc ripple rejection).At least in simulation, the transistors from the second mirror doesn't seem to contribute with any improvement, but I didn't try in practice, and didn't investigate the stability. Didn't try any noise simulation either (for the idea of using a LED for its low noise, instead of the Zener).Another interesting test was to look at the thermal drift.Click to enlarge.While the Vcc ripple rejection is stellar in all 3 versions, the thermal drift is expected to produce much large deviations. Will worth the effort of tuning the resistor for minimal thermal coefficient, or to put the circuit in an thermostat oven.

author: mawyatt
author_rank: Super Contributor
timestamp: January 14, 2025, 07:39:36 pm
content: Like your 4 transistor versionBest

author: magic
author_rank: Super Contributor
timestamp: January 14, 2025, 10:53:02 pm
content: I don't like its thermal driftBut it could be corrected with a diode on top of the zener, or using a lower voltage zener maybe.Note that PSRR may be unrealistic in simulation due to perfect matching.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 14, 2025, 11:08:08 pm
content: Quote from: David Hess on January 13, 2025, 05:21:23 pmYes, and note the other issue with Zener based circuits is the relatively poor tolerance.Two diodes in series is a common thermal fix for Zeners, eg the PLVA26xx series do this, but they only spec 3% as I guess they expect post-trim in usage.You can get Dual Zeners down to 2% in single packages - eg BZB84-B6V2,215, but 1% Zeners only come as singlesThe Nexperia BZT52H-A6V2-QX is 1%, and comes in SOD123F, whichmaybecan thermally couple in pairs, but the price climbs for that 1% bin.The data does suggest the cathode has best thermal couplingDevice mounted on an FR4 PCB, single-sided copper, tin-plated, mounting pad for cathode 1 cm2

author: Dabbot
author_rank: Regular Contributor
timestamp: January 15, 2025, 09:50:18 am
content: Quote from: RoGeorge on January 14, 2025, 07:13:08 pmI built this up from components of my original circuit and it at least matched the original. ~1mv change between 9V and 18V. I like it.Quote from: RoGeorge on January 14, 2025, 07:13:08 pmFor a laugh I put the thing in the freezer (sans DMM) and it changed ~5mV (5.041V to 5.036V). It's 27°C at the moment, and the freezer is -18°C, so ~110uV per °C. I don't think I'll go through the effort of putting it in the oven right now.

author: tggzzz
author_rank: Super Contributor
timestamp: January 15, 2025, 09:59:03 am
content: Quote from: Dabbot on January 15, 2025, 09:50:18 amIs it constructed on a solderless breadboard? If so, how do you take account of the poorly-defined contacts between components and internal spring contacts?Is there any condensation on the circuit? That could unbalance it.Anyway, a fun exercise. it would be even more of a laugh to put a solderless breadboard in an oven

author: Dabbot
author_rank: Regular Contributor
timestamp: January 15, 2025, 10:38:51 am
content: Quote from: tggzzz on January 15, 2025, 09:59:03 amI was hoping any combination of dicky contacts and condensation would balance out across the circuit somehow.Quote from: tggzzz on January 15, 2025, 09:59:03 amI'm sure I'll stop laughing once I notice the funny taste in my next casserole.

author: magic
author_rank: Super Contributor
timestamp: January 15, 2025, 01:02:30 pm
content: I wonder if this would work?(If unstable, try a capacitor in the indicated position.)

author: RoGeorge
author_rank: Super Contributor
timestamp: January 16, 2025, 07:53:03 am
content: Quote from: magic on January 15, 2025, 01:02:30 pmWow!Indeed, the 3rd BJT in the Wilson current is to avoid the imbalance caused by Early voltage when the Vcb is different in the two arms of the mirror, except in this particular usecase (with an emitter follower between the mirror arms), the Vc will vary the same (at 0.6V apart), so the Early voltage is not a problem any more, and the 3rd transistor in the Wilson mirror becomes redundant. I failed to see that.Quote from: Dabbot on January 15, 2025, 09:50:18 amImpressive result, you might be very lucky with that Zener.Never tried to measure the TC of a Zener, but now I'm curious to test various Zener diodes I might have through the scrap boxes. I would like to log V vs T, but the DMM can only log one of the two.I didn't need a Vref, but now I'm tempted to build one.

author: Dabbot
author_rank: Regular Contributor
timestamp: January 16, 2025, 07:55:54 am
content: Quote from: magic on January 15, 2025, 01:02:30 pmIt works pretty well. Doesn't noticeably budge when I flip it from 9V to 18V. No capacitor required. Very nice!

author: Dabbot
author_rank: Regular Contributor
timestamp: January 16, 2025, 08:12:09 am
content: Quote from: RoGeorge on January 16, 2025, 07:53:03 amI doubt it. I got some of these because their datasheet specified a min/max TC which crossed zero, which means easily tuned by current.Quote from: RoGeorge on January 16, 2025, 07:53:03 amYou could bias a thermistor on the same board and multiplex the thermistor voltage with your test Vref.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 16, 2025, 09:11:01 am
content: Quote from: Dabbot on January 16, 2025, 08:12:09 amThe min-max is the production spread, not the current spread.A problem with the simple current adjust to find nominal min TC, is the knee of those lower voltage zeners is quite soft.See the attached plot, of both Zener TC and KneeThat's why it is more common to see a Diode + Zener or VBE multiplier + Zener pairing used.The higher voltage Zener knee is sharper, which means lower Z which also means lower noise.The other issue here is that in 2025, you can buy a 0.05% sub 6ppm precision reference for about $1. Lower current, and no trimming and checking needed.You can get corrected Zener simulations to under 6ppm, but as 2mV/K is about 330 ppm/K, you are asking for a temperature tracking of about 20 milli Kelvin.That's why you see these on IC's wafers, with heaters.See this thread for a modern example of a (multiple) Zener with heating control referencehttps://www.eevblog.com/forum/metrology/ref80-a-competition-to-adr1001/

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 10:50:05 am
content: 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.

author: magic
author_rank: Super Contributor
timestamp: January 17, 2025, 12:15:00 am
content: Quote from: Dabbot on January 16, 2025, 07:55:54 amThe basic idea is stolen fromLinear 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 offThe 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.

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2025, 11:12:58 pm
content: I've done a similar thing before, but it was far more crude: just a couple of transistors.

author: magic
author_rank: Super Contributor
timestamp: January 20, 2025, 11:16:47 pm
content: I think you'd get better AC PSRR if the compensation cap is referenced to the input rail rather than ground.

author: LordXaos
author_rank: Contributor
timestamp: January 21, 2025, 10:48:12 am
content: Ah, so I am not the only one who tried thisWasn'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...

author: tggzzz
author_rank: Super Contributor
timestamp: January 21, 2025, 11:14:15 am
content: Quote from: LordXaos on January 21, 2025, 10:48:12 amWith 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.

author: LordXaos
author_rank: Contributor
timestamp: January 21, 2025, 02:48:21 pm
content: 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