Driver transistor for variable voltage reference stage with LM399 - Page 1
Forum section: electronics / general

author: jorgemef
author_rank: Regular Contributor
timestamp: February 15, 2023, 10:43:05 am
content: Hello,I attempted to produce a variable voltage reference similar to what is on the LM399 datasheet. Used a BC547 driver transistor as didnt had the LM195k in my part box. But it has a lot variation with the temperature.Any other suggestion to use instead with better tolerance to temperature? LM195K is not easy to find.I thought that the OP07C would compensate for the temperature fluctuations of the transistor but it seems it does not do that very well.I thought I might be getting affected by the DC-DC EMI noise but should not be it as the LM399 module uses the same source for its driver OPAMP and is stable. Negative rail of -5V pump is not under a voltage regulator. Positive rail has a 7824 regulator. LM399 board has a second 7815 regulator from the 24V rail.The potentiometer will be updated later for multi turn but is not introducing non-linearities to the circuit, at least not in the order that matters currently.The voltage reference output seems stable.PS1:Measurements with a BM869s.PS2: in case not bether discret solution then BC547 could LM317 be used as kind of transistor with 1.25V "BEv" instead of 0.7v to replace BC547 instead of LM195K? At least has some temperature compensation.Cheers,Jorge

author: Andreas
author_rank: Super Contributor
timestamp: February 15, 2023, 11:00:51 am
content: Hello,do you really need the 1A output for your reference?The BC547 is good only for some mA in this cirquit before it blows up.The LM195 is a very special part and has much higher amplification.BTW. I would look for a LM395 (commercial temperature range version) as replacement of the LM195.RS components has some in stock.with best regardsAndreas

author: jorgemef
author_rank: Regular Contributor
timestamp: February 15, 2023, 11:15:15 am
content: Hello,I don't need 1A. For now I will use it as source for a wheatstone bridge to sort some resistances for other projects, later I may use for other projects, but I believe I will not need anything above 100mA.Cheers,Jorge

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2023, 11:31:33 am
content: The OP-amp should compensate for the temperature dependence of the transistors. There are other parts that are the bigger problem:The pot to set the voltage is a poor choice. One should use a trimmer only for a small correction to the amplifier gain.The resistors R4,R5 to set the gain are also critical parts for the stability - more critical than the ref. chip.Another point to whatch it the ground connections - with the ref. and amplifier on different PCBs this can go wrong.I a somewhat higher current is wanted, a BD135 transistor may be an option to allow for a little more power loss.One may consider a current limit though.The excitation to a weatstone bridge is usually not critcal.

author: jorgemef
author_rank: Regular Contributor
timestamp: February 15, 2023, 11:58:13 am
content: Hello,The external potentiometer is there to divide the voltage from the reference. I believe is not critical as both upper and lower part of the variable resistor will suffer same effects from temperature. Will replace in the future with a multi-turn for better precision setting.The blue pot on the board is not for gain compensation. Is not needed for precise gain as any fluctuation will be compensated by the setting voltage potentiometer. This is only for nulling input offset voltage of the opamp.R4 and R5 are metallic 1%. Need to search for bether parts, and see how to handle the different grounds part.Thanks,Jorge

author: Andreas
author_rank: Super Contributor
timestamp: February 15, 2023, 12:30:08 pm
content: Quote from: Kleinstein on February 15, 2023, 11:31:33 amDefinitely not.The LM195 amplifies a base current of 3-10 uA to a output current of 1A. So more than 100 000 fold current gain.Even for 100 mA at 24V supply (and short cirquit) you will need a small heat sink for the LM195/LM395.The -5V has to be capable to sink the leakage current of the LM195. (This is why the 1K resistor is there).with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2023, 12:54:00 pm
content: The zero trim at the op-amp is rarely needed and should not be a problem, though still not ideal. In the circuit it would be more like a point to trim the offset drift. For BJT based OP-amps the offset added by the trim is about proportional to the absolute temperaure.Even if the pot for setting the voltage has perfect matching, many of the pots are still pretty noisy. So one would excess 1/f noise from the resistor part alone. The simple carbon type pot could explain the observed noise and drift. The wiper contact is also not good and can give additional drifting. Even a good multiturn pot is likely still a mediocre solution.For adjusting the voltage the coarse part should be more done with switches. It at all the pot is something for a fine part (e.g. 1-5%).The 1% thick film resistors are often not that great. Though usually better than the pots, they can still show quite some exess noise - that is extra 1/f noise, that can be comparible to the reference noise.Ideally one would avoid the gain when not absolutely needed (e.g. make it switchable).In the combination of LM399 and resistors for the gain or divider chances are the resistors are the more critical parts than the reference itself.A big plus of the LM395 is the thermal protection. A cheap solution with thermal protection would be a LM317 - though this would not allow to go all the way to zero.

author: iMo
author_rank: Super Contributor
timestamp: February 15, 2023, 09:39:12 pm
content: What about a darlington - see below the idea, the 0.4ohm is the shunt for max current and the x axis is the load resistance..

author: jorgemef
author_rank: Regular Contributor
timestamp: February 15, 2023, 10:58:27 pm
content: The darlington may work. Indeed the LM317 also worked and goes down to 0. Since there is a negative rail, the opamp pulls the adj pin to -1.25V when set to 0.Anyway I will attempt the Darlington idea or go back to bc547.The problem with the LM317 is that it requires minimum regulation current and for having 5mA close to zero voltages it needs 1Kohm on R7 which then when at 20V takes 0,625W from the system. This would drain the batteries much faster.With the transistors or Darlington I believe biasing current and thus R7 can be more relaxed. In fact I was using 4.7K before with the BC547.The main source of noise seems indeed the main potentiometer. Attached the traces for the system with LM317 for half range (10V) and full range (20v).With full range the opamp gets the signal directly from the LM399 and much less noise is seen. I had similar results with the BC547.

author: iMo
author_rank: Super Contributor
timestamp: February 15, 2023, 11:33:20 pm
content: As Kleinstein wrote above the potentiometers or trimmers of that kind are "nogo" when talking "voltage reference". It could be a 10turn wire-wound potentiometer may work a bit better (but it has somehow discrete steps).

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2023, 03:14:52 am
content: An LM317 can replace the LM395 if necessary as a protected power pass element. Add a 120 ohm resistor between the output and adjustment terminal to provide the constant current sink on the output, and then buffer the output of the precision operational amplifier with a PNP bipolar, p-channel JFET, or PMOS transistor.Or design a discrete output stage.The 308 operational amplifier was used as an early precision part with low drift, so there are lots of modern alternatives starting with the OP-07.Quote from: imo on February 15, 2023, 11:33:20 pmIn combination with the wirewound 10 turn potentiometer for high stability, a good single turn potentiometer can be used as a vernier.This would be a good place for a Kelvin-Varley divider.

author: MasterT
author_rank: Frequent Contributor
timestamp: February 16, 2023, 04:05:31 am
content: Leakage current of the C1 electrolytic would generate the biggest error, do math 1 uA x 10 kOhm

author: jorgemef
author_rank: Regular Contributor
timestamp: February 16, 2023, 07:52:56 am
content: Quote from: MasterT on February 16, 2023, 04:05:31 amYes, I realized that and used a mylar, the big orange.Forgot to update the schematic.

author: jorgemef
author_rank: Regular Contributor
timestamp: February 17, 2023, 11:56:51 am
content: Hello,I updated the circuit with the Darlington as suggested Imo and replace the carbon pot with multiturn metallic pot. Now things seems much better in terms of stability. The voltage divider pot was really the biggest source of noise before as Kleinstein told.Still some questions about 1/f noise, is this noise dependant on the current/voltage seen by the resistance? Can also the opamp compensate for this noise for downstream components?Asking as only the feedback loop R4 and R5 are metal film. The input filter i left it as carbon as I thought no current will be seen on these ones (uA level) and that for R6, R7, R8 and R9 the opamp OP07C could compensate, but I may be wrong.

author: iMo
author_rank: Super Contributor
timestamp: February 17, 2023, 12:19:26 pm
content: The components around the Darlington (R6/7/8/9 and the transistors) are "within the OP07 loop" and do not contribute much.R4 and R5 are critical, as their TC or better the "TC ratio" translates directly into the output voltage. You would need R4 and R5 to have as low as possible TC ratio.PS: with "perfect everything" and stable temperature the typical LM399 would fluctuate in your system such the output 10V would be within ~10uV short term (there is an aging drift), with occasional jumps like 4-5uV because of popcorn (if any).

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2023, 12:31:26 pm
content: The resistor excess noise (often also called current noise, but I don't like that name) depends of the voltage / current seen by the resistor. It is kind of the resistor value fluctuating with time. If used with AC on the resistors, it would do AM moulation.There is no way to remove the noise later - the 1/f noise is also nearly impossible to filter out with a low pass filter as there is always more at even lower frequencies.In other applications (e.g. audio) a high pass filter can help.R6, R7 don't contribute significant excess noise as changing the value would not change much. Similar a small voltage in series would be compensated by the OP-amp.So the 2 filter resistors R2/R3 are not critical as there is essentially no current flow and even carbon is OK. R3 is actually not needed.There is still the small amount of Johnson resistor noise, but that is not effected by the resistor type.There can be thermal EMF point of these resistors. If the temperature of both sides of the resistor is different it can gneratate a small thermal EMF voltage like a thermocouple. I don't know how bad carbon resistors are in this respect - I don't think they will specify it.

author: David Hess
author_rank: Super Contributor
timestamp: February 17, 2023, 09:32:07 pm
content: Noise will usually be dominated by the reference, and then the operational amplifier. Lower noise operational amplifiers like the OP-27 and multiple references in parallel or series can be used for lower noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2023, 10:12:56 pm
content: I would be a bit careful with the OP27: it has low voltage noise, but the current noise is quite high. This is especially relevant for the frequency range, as the 1/f cross over is usually higher for the current noise. In addition many datasheets are using a bit strange way to define the current noise, splitting the restance over both inputs. In the more common case with resistance mainly on one input the current noise is high. So the dataseet number for the current noise may be a bit on the optimistic side. So unless the resistance is relatively low (e.g. < some 3 K) the OP07 can be lower noise over all.A reasonable alternative to the OP07 can be the OPA207 or OPA202, with the advantage of lower power use, but SMD only.Metall type thick film resistors can have quite some 1/f noise and how much can vary between types and values. With some 3 or 5 V at the resistor they can have more 1/f noise than precision OP-amps like the OP07. So one may get more extra noise from the resistors than the OP-amp - this is of cause only with resistors with a significant effect on the gain. In addition the resistors can contribute to long term drift.Anyway the LM399 is way more noisy than an OP07. It can still make sense to get thin film resistors for critical resistors (e.g. gain setting).

author: Andreas
author_rank: Super Contributor
timestamp: February 17, 2023, 10:14:32 pm
content: Quote from: David Hess on February 17, 2023, 09:32:07 pmBut not in the range of the quantisation of his measurement device which has 100uV steps in the measurement.Lets have the reference ~4uVpp (or in worst case double of that) amplified factor 3 by a Op-Amp to 12-20 uVpp you will rarely see it on a instrument with 100 uV resolution. And usually a integrating DMM filters partly away the noise.So this "noise" of 400-500uVpp has other reasons.I would in first place replace the step up converter by batteries with suitable voltage.And as next point replace the pot by something with much lower T.C.with best regardsAndreas

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 18, 2023, 04:14:47 pm
content: I've tried to understand Bob Widlar's bipolar low input current buffer.Some insights are still missing. The input pair is running at Vce = 1 diode drop.Interesting combination of low input current, low noise, and where elsedo you get gain == 1.000?Paralleling more transistors in the usual way seems not to improve noiseany further but more current does.Q4, Q5 are just parking lots for easy change of transistor types.E1, V10, R3 create thermal noise on Vcc to see how much isallowed before the noise of the entire buffer gets worse.Cheers, Gerhard

author: MasterT
author_rank: Frequent Contributor
timestamp: February 18, 2023, 05:42:28 pm
content: Q2 is loaded by current source, so there is at least 1 more BJT not reflected on the circuits. Dynamic load could get you G >= 1000.Regarding noise, what I read, lowering Vce below ~2V does not get any improvements in noise level, but decreases h21.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2023, 06:17:18 pm
content: The choice of VCE = 0.6 V is not because of low noise, but it is a consequence of keeping the circuit simple.It is not clear that the gain is really 1.000, as there is some loading effect even from just the standing current. So the gain may be a bit small, like 0.99x.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 18, 2023, 06:30:08 pm
content: No, when Vbe close to Vce, there is no reason for currents between B and Cand it is safe to use super-beta transistors that cannot withstand much voltagebut need not much base current with beta up to 5000.The large standing current is so big to check sensitivities. Bob Widlarcould never afford 40 mA for the input stage of the LM(108 +- a few)

author: David Hess
author_rank: Super Contributor
timestamp: February 18, 2023, 06:43:04 pm
content: Quote from: Gerhard_dk4xp on February 18, 2023, 04:14:47 pmParalleling more transistors reduces the base spreading resistance which contributes its own noise, but paralleling more transistors without increasing the current does nothing for the shot noise."Low noise" transistors have low base spreading resistance, and integrated transistors can be very good in this respect. Ring or perforated emitter transistors are also reported to be very good.

author: jorgemef
author_rank: Regular Contributor
timestamp: February 21, 2023, 06:11:30 pm
content: Well, for the Opamp feedback resistors I replaced the film resistor with 3 parallel film resistors with the same equivalent resistance, thinking I would get better results.I then measured in the 5V range as I there is an extra digit on the BM869S with 500,000 counts. So can measure up to the 0,01mV.With the THT metal film I didn't got so good values so I set myself to measure the TCR from the stach of components I have. Best thing I could do. To the measures I soldered a 220K sample all together, hold it with some supporting clamps and blow with hot air station from fixed distance while measuring the temperature on the resistors level. Chart attached.It seems that the Chinese 1% SMD 0805 from a KIT I have are much more TCR stable than any other THT that I own.So I went and replace the feedback resistors with these SMD. Now I get results in a level acceptable for my equipment limitations.Don't want to pursue to use only batteries without DC-DC converters as I want it to be portable, hold up to 20V and last at least 6 hours with one charge.For getting these current features battery powered I would need a different box to hold 4x9V batteries or 8x3.7V 18065. Also i am on the edge of the limitation of my current multimeter in terms of measurements.One thing I observed is that with current darlington it takes around 1 hour for the output to stabilize/flat. I am wondering why, and if this is an effect of the darlington itself or the divisor pot to warm up, or the multi turn pot that is on the circuit for offset compensation. I believe this larger transient response got more pronounced after the darlington. I also notice if i start with 0V set, the output drifts from around two volts up to 0 volts-takes around 5 minutes in this drift, also started after the darlington was introduced.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2023, 07:11:53 pm
content: The transistor has essentially no effect on the output voltage and thus does not contribute to the warm up drift.The LM399 reference is normally quite fast, like 10-20 seconds to get a stable output.The point is more with the resistors. This can be the ones for the gain or the pot.One could at least have a switch to chose between the gain of close to 3 and the more stable gain of 1. This would also allow for finer adjustment in the lower range.A good way to get a reasonable stable gain is to use a set of equal resistors from the same batch. E.g. 3 x 10 K to set a gain of 3.

author: Andreas
author_rank: Super Contributor
timestamp: February 21, 2023, 07:55:39 pm
content: Quote from: jorgemef on February 21, 2023, 06:11:30 pmHello,perhaps you have to reduce the base resistor from 10k to something like 1K with a normal darlington.with best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2023, 04:26:17 am
content: Quote from: jorgemef on February 21, 2023, 06:11:30 pmThere are a number of things which can cause that, and it is definitely a problem.1. Is the output of the operational amplifier within range? Saturation, even partial saturation, causes loss of precision by limiting open loop gain among other things. If you use a series zener or resistor on the output for level shifting, then it should be capacitively bypassed for low AC output impedance.2. Are any return currents going through sensitive nodes? Layout is important.3. Loading on the operational amplifier output limits open loop gain limiting precision. I had good results with output currents up to maybe 50 microamps but maybe your Darlington is loading the output more than that? One sure solution to this is to use a JFET or MOSFET, or integrated buffer, to drive the output pass element.4. This goes with 3 above and should not apply in your case, but low power precision operational amplifier have a precision advantage because of less self heating.5. I would not use a 10 kilohm output series resistor without capacitive bypassing. That value is way larger than necessary to suppress parasitic oscillations, which are pretty unlikely with the built in output resistance of the operational amplifier anyway.

author: iMo
author_rank: Super Contributor
timestamp: February 22, 2023, 07:14:55 am
content: With the darlington you would not need 10k, you may go higher with its value.

author: jorgemef
author_rank: Regular Contributor
timestamp: February 23, 2023, 10:34:21 am
content: Fixed the problem for low voltage offset for the darlington when input voltage was set to zero. The power NPN was damaged. I checked and there was some leak between collector and base on the round of 2,5V measured with DMM in diode mode. Still for the voltages above 2,5V the darlington was working maybe compensated by the BC547.Anyway, after fixing the darlington with a new BD139 I still see too much stabilization stabilization time. Need to track the source of this startup non-linearity, and if is also affecting the long range oscillations or is a different problem from the feedback resistors. For that need to measure out the impact of voltage divider multi-turn pot with a lower gain to measure on the 5V range of my multimeter which has best precision.Edit: Just realized that I cannot go to 5V range to characterize the amplifier response without some resistance divider or other reference as LM399 is 7V and amplifier gain is 1+R5/R4.Will measure instead the output of voltage divider multi turn pot and see if is behaving non-linearly. I can later multiply the array by the gain and compare if a similar signal would result.

author: jorgemef
author_rank: Regular Contributor
timestamp: February 24, 2023, 11:07:33 am
content: Just to complement previous post with the transfer function of the multiturn pot. It seems it is the one affecting the initial non-linearity. Not sure if affects the long run. I see variations of 170uV from the previous post 5V chart but this happened during the night/day. This would mean variations of 56uV in the input if the amplifier would be linear. My multimeter resolves 1x10uV with error of two digits in last case, so should be able to distinguish these changes. Or I didnt measured long enough the input or there are long run linearities introduced by the amplifier feedback network which is the most likely. They are cheap SMD resistors.7v is output signal of the LM399. The other chart is the voltage measured at output of multiturn pot from cold start.Adding image of the multiturn in use.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2023, 12:28:47 pm
content: The nonlinear shape likely just follows the temperaure settling over time. The amplifier is still quite linear. The main part that is expected to cause the drift / settling are the resistors. This can be the resistors for the gain, or the 10 turn pot. The 10 Turn pot can be a big step up from the simple cheap one, but chances are it is still a weak point.The resistors for the gain are used for the ratio only. Parts from the same batch / series can be quite a bit better matching than the simple specs show.So for a gain of 3 it can be worth using 3 equal resistors.If done right, the amplifier and reference should be more stable than the handheld DMM with 10 µV resolution. Those handheld DMMs can affort the luxary of a heated reference and thus also may show warm up drift.Part of the drift may thus also be from the meter (warm up and the battery voltage going down).Even with a 6 digit DMM one can not easy tell appart the drift/noise of the LM399 DUT and the LM399 reference used in most of the 6 digit meters.

author: iMo
author_rank: Super Contributor
timestamp: February 27, 2023, 11:17:02 am
content: The TC of the wire wound potentiometers is usually +/-50ppm (ie Bourns Pot Guide), that is for its full resistivity. When acting as the divider TC should compensate (the ratio TC=0), except a large transient during a temperature change, which lasts typically up to one hour (ie. it depends on the potentiometer package size).The much bigger issue I saw are large fixed jumps after the temperature changes causing hysteresis by a voltage equivalent of 1 wiper step/notch (caused probably by a temperature induced dislocation of the wiper).Also you have to keep the current flow through the wiper as small as possible (as the wiper contact is the main source of the instabilities)..PS: a good reading (22MB, Bourns Potentiometer Handbook 1975) on thewire wound potentiometers