How close do the two transistors need to be to be assured they’re close enough in temperature? If they need to be within a couple centimeters that could create a problem.
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Heck if I know 
If the temp in the room is even then I’d guess they can be any distance apart. But if you’ve got a lot of moving air at different temps like I do with the AC on…I figured having them pretty close and in the same airstream made the most sense.
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I also used this circuit to match BC547’s. Almost all of them matched I think I only had like 2 or 3 that were different. I did it on breadboard didn’t think to make a permanent board😢
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If you touch one of the transistors for 1 second your multimeter will jump up immediately and then slowly taper back. In other words…they are super temp sensitive. If they are all on the same bench together I’m sure it’s fine. Some designs even position them so they are thermally fixed to each other
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Yeah, They all seemed to start about 12-15mv then spike 1-2 mv more before slowly drifting down and settling. I did a bunch of 2N3906’s for the ARP VCF (which I still need to wire the panel on) and found tons of matches. I had values from .2mv to 1.7mv so they were all within 2mv of each other (which is what I’ve seen a few designs specify) but since I took the time to match them I figured why not take a few minute more and REALLY match them
The 2N3906’s were from a big $6 delivered box of 200 from Amazon. I also had a strip of 15 BC 547’s from Tayda that I matched and I only found two “perfect” pairs out of all 15. But again they were all within 2mv of each other.
Oh - and for those who may want to try etching their own PCB’s…I will be doing a detailed post on the process I’m using now that I’ve got it working consistently. I took photos of every step on the last board I made…just haven’t sat down and made the post yet.
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With 4 pin headers you could have made them EBCE, fitting both EBC and BCE transistors.
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And CBE and ECB. Or with 5 pin headers, EBCEB gets you all six permutations.
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See, this is why I like to run things by people smarter than me before putting too much effort into them
Heck, I only used the 4 pin headers because I didn’t realize I didn’t have any 3 pin. I was originally planning on using DIP sockets…but decided that would be too cramped for my chunky fingers.
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