BJT Matching Circuit Design Questions - Page 1
Forum section: electronics / beginners

author: tomthepom
author_rank: Newbie
timestamp: May 29, 2023, 06:32:19 pm
content: Hi all,I'm a noob trying to design a circuit for matching BJTs. I had a look at the Moog transistor matching circuit below, but I had a few questions about the design.1. What's the purpose of using a second transistor as a current source? Couldn't the op-amp be used to bias the DUT directly?2. Would it not be better to use a voltage reference at the positive terminal of the op-amp, rather than dividing the rails? I'm more concerned with repeatability than with hitting exactly 100 uA.Any recommendations regarding keeping the temperature of the whole thing stable during testing are also welcome.Thanks!

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 29, 2023, 07:44:12 pm
content: for question 2: Yes it would be better. Perhaps there wasn't a proper voltage reference back when this circuit likely was designed. Zeners have lots of Tempco.To maintain constant temperature, nothing beats a stirred liquid bath. The liquid being used is generally isopropil alcohol or glycol. Now, to mantain that bath at a precise and constant temperature, that is a whole new discussion here.When I have seen these baths, the liquid is mantained at a slightly higher temperature than the maximum expected ambient. That is, if the ambient will vary between 18 and 26C, you would keep the liquid at 30C.Question for you; where did you find that circuit?

author: floobydust
author_rank: Super Contributor
timestamp: May 29, 2023, 07:50:01 pm
content: I useDr. Leach circuitfor matching (edit: hFE) at the operating VCE or something above a few volts.Handling and putting transistors on a breadboard heats them up due to your fingers, and sometimes RF pickup can cause confusing results.

author: tomthepom
author_rank: Newbie
timestamp: May 29, 2023, 08:52:56 pm
content: Ah, I originally found the circuit here:https://dragonflyalley.com/synth/constructionTransistorMatching.htmThanks for the recommendations. A stirred liquid bath does seem like a good idea; maybe I could grab a hotplate stirrer off of eBay... though I'd need to find one with acceptable temperature stability.Another idea would be to use the stirred liquid bath, then make some sort of comparator circuit for comparing two BJTs at a time. That way, even if the temperature of the bath is unstable, the devices under comparison will be at the same temperature at the moment the measurement is taken. I'd still prefer a stable temperature though, since I'm guessing that the difference in Vbe between two transistors is not the same at all temperatures (even if the transistors in question are the same temperature).I'll draft a circuit I had in mind that skips the use of another transistor when I get home.

author: Benta
author_rank: Super Contributor
timestamp: May 29, 2023, 09:29:42 pm
content: Just to be precise, this is not a circuit for "matching transistors".It's a circuit for measuring VBEat an emitter current of ~100 uA. Probably designed for a very specific purpose.There are lots of other parameters in transistors.The "liquid bath" thing must be a joke. The only parameter that needs to be kept under control is that all 20 transistors are at the same temperature. Leave them on your work bench for 30 min. and only handle them with tweezers after that, then that's fixed.

author: tomthepom
author_rank: Newbie
timestamp: May 29, 2023, 10:16:54 pm
content: Of course. In this case, the BJTs are to be used in a linear to exponential converter for a musical VCO.The circuit application is explained in this article:https://www.schmitzbits.de/expo_tutorial/Regarding the liquid bath: for this particular application, I am not tracing the whole IV curve and the transistors are only loosely matched, so I get where you are coming from as far as thinking it is overkill.Just getting the transistors to room temp is probably sufficient.However, I was still grateful for the suggestion. I think the liquid bath still has applications, as it reduces the need to wait excessively for the transistor temperatures to equalize. I'm also a student living in an apartment without a controlled bench environment. The temperature of my apartment fluctuates quite a bit during the day, so the bath isn't an idea I'd throw out straight away.I may be a noob, but I did at least make sure to understand the functioning of the circuit I posted. I also don't want to discourage or admonish suggestions - I appreciate you pointing out that the bath could be unnecessary, but there's no need to be patronizing.Let's try not to be rude to each other!

author: Benta
author_rank: Super Contributor
timestamp: May 29, 2023, 10:51:19 pm
content: No rudeness intended, sorry if it came over like that.But engineering is about not going over the top.If all your transistors are in the same place (= same temperature), and you spend 5 minutes to measure them, your apartment temperature over the day doesn't come into play. If you want to measure one transistor per hour, that's different and will take 20 hours.Somehow, this is getting silly... prolly my fault.

author: tomthepom
author_rank: Newbie
timestamp: May 31, 2023, 04:21:26 am
content: That's alright; sometimes it's hard to tell tone through text!Anyways, I've gone ahead and played around a bit in LTSpice. In the real circuit I'll probably use some sort of simple zener shunt reference like the LM4040 to get my 5V reference.What's the practical difference between the circuit on the left (the original reference design) and a circuit which treats the BJT being biased by the op-amp itself as the DUT?It seems to me that the circuit on the right should be more practical. It doesn't require a negative power rail, and the DUT actually has less power through it (about .25 mW instead of 1 mW) which I would think would help reduce part heating as measurements are taken.There is a slight increase in base current, but I think the leakage is pretty negligible through a 51k load.So, why was the original circuit designed with a separate transistor acting as a current sink?

author: tomthepom
author_rank: Newbie
timestamp: May 31, 2023, 04:30:40 am
content: Now that I think about it, the leakage (base current) in this case is determined by the fact that we want Ic to source approx 100 uA. The difference comes from the fact that in the original circuit, the collector current of Q2 is actually Q1's collector current minus Q1's base current. I believe this is probably inconsequential, but please correct me if I'm wrong. I'm still wondering if there's some practical reason for not biasing the DUT directly.

author: pqass
author_rank: Super Contributor
timestamp: May 31, 2023, 06:52:58 am
content: You can match transistors without actually measuring their Vbe.You just need three resistors, a multi-turn trimmer pot, a diode, 2x 3pin sockets, voltmeter, and bipolar power supply.http://ijfritz.byethost4.com/MiscProj/transmat001.pdf

author: MathWizard
author_rank: Super Contributor
timestamp: May 31, 2023, 07:56:10 am
content: In general, how much better matched/more equal spec's, are BJT array chips ? There are jelly bean DIP-8 or SMD version, BJT array chips, with 2x 2n2222 or 2n3904. Aside from all being in the same package, and being made at the same time, how much more matched would they be, then any given discrete BJT's taken off the assembly line ?I know they don't have to be better matched, but in general are they ?

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2023, 02:22:17 pm
content: Quote from: tomthepom on May 29, 2023, 06:32:19 pmAn operational amplifier could operate as a current source without the transistor, but this requires either matched resistors, or the transistor to be floating without the ground connection to the base which will make for a noisier measurement.QuoteSince the Vbe varies logarithmically with the current, the sensitivity to the reference voltage is not great.Quote from: MathWizard on May 31, 2023, 07:56:10 amWithout testing, only count on them being matched to within their matching specification.

author: jwet
author_rank: Frequent Contributor
timestamp: May 31, 2023, 03:04:16 pm
content: Agree with everything said- classic current sink and source circuit. A few tips maybe-1. Use the version of the circuits with the transistor, though the op-amp can do it bare, its difficult to isolate the feedback, etc- Art of Electronics covers this circuit- a workhorse.2. The beta of the external transistor (not your dut) is an error term, you're measuring Ie and assusing Ic is same- these are different by beta/beta+1. This is 1% at beta of 100. The 2N5087 are a favorite Hi Beta Q (~350) or a MOSFET in this position can improve accuracy- mostly just worth knowing.3. You can use single supply circuits for both circuits if you pay attention- even an LM358 would work fine. The sink circuit should be no problem with a ground sensing amp. For the sourcing circuit, you have to stay about 2v below the HV rail on the top for the CMR of the amps like the LM358- you could use a 2.5v LM4040 or any bandgap at 2.5 to solve this.4.Temp won't matter much as said - all tests should be a the same temp, if you're in a drafty room, you can use cutoff paper cup over the dut or the whole circuit while testing. Sytrofoam works better but is not welcome in any kind of ESD environment without special precautions- grounding.I've found Art of Electronics to be very good on basic theory of transistors- Ebers-Moll etc.You can still buy dual and matched transistors and arrays- do a little search on Digikey. These were common in the past- check Ebay- MAT01, CA3086 were classics. Some don't spec matching directly but any two adjacent transistors from a die will match pretty darn well.Have fun- there is a lot of education embedded in your project.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2023, 03:32:43 pm
content: Quote from: schmitt trigger on May 29, 2023, 07:44:12 pmWhere a lower temperature coefficient is required, reference zener diodes are available which even back then were competitive with common and even premium references. For instance the 1N821 series is available with temperature coefficients down to at least 5ppm/C, at a cost competitive with a modern LT1236 or LT1021.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 31, 2023, 04:28:50 pm
content: Correct! I had forgotten about the “reference” zeners.

author: MathWizard
author_rank: Super Contributor
timestamp: May 31, 2023, 10:08:43 pm
content: If it was me with my jelly bean parts, for the voltage divider reference, I'd use an adjustable reference TL431.1-2 months ago I got a bunch of nice brand name op-amps from LSCS including some pretty low offset voltage instrumentation ones. But I haven't even tried any of them at all, too busy with the day job, and just doing math by night.In the past I made an adjustable diff-pair circuit to match Vbe w/ my 6.5D DMM. Next time I'll try something with an op-amp to drive them and to compare them.

author: tomthepom
author_rank: Newbie
timestamp: June 03, 2023, 04:34:12 am
content: Thanks for the input guys! I'll probably try out Ian Fritz's method, and come back if that doesn't work