A naive BCY87 experiment - the ref diode - Page 1
Forum section: electronics / general

author: iMo
author_rank: Super Contributor
timestamp: September 16, 2022, 10:59:38 am
content: This is perhaps an anecdotal experiment I've made today - I've found a 6 legs transistor in my junkbox - the BCY87 dual npn transistor.Out of curiosity I've wired it up into a form of the 2DW2xx diode (T1 the zener, T2 the diode) and I've quickly measured the voltages and currents (I've not fired up my 34401A so rather a simple meter used here). There is a 3k resistor in series with the ref diode.It fires up at about 40uA, the tempco at 80uA Irbe1 is rather high, I've not tried to search for the zero tempco current as the Irbe1 currents over 1mA could damage the transistor, imho.. Not sure it is worth of more experimenting with it, however..PS: I've put it here as the whole circuit is roughly equivalent to the LTZ1000 with cheapo +/-500ppm/K divider resistors in its 7->10V stage..

author: miro123
author_rank: Frequent Contributor
timestamp: September 16, 2022, 12:57:24 pm
content: Funny Friday evening experiment.What about the AC noise. If I remember correctly some retro circuits used the same configuration as white noise generator.

author: iMo
author_rank: Super Contributor
timestamp: September 16, 2022, 01:03:44 pm
content: Not only as a noise generator, I was using that circuit for blinking an LED, as a child, when I got my first LED (R+C+reverse biased base emitter junction+LED)..

author: MK
author_rank: Regular Contributor
timestamp: September 16, 2022, 04:30:35 pm
content: dont forget that it degrades the transistor mode. Also if you want it noisy run it at a low current. A while ago I came across a cryptography site where they wanted a hardware white noise source and had plots to show the randomness of the eventual generator.

author: Andreas
author_rank: Super Contributor
timestamp: September 16, 2022, 05:57:42 pm
content: Hello,I made a similar experiment with aBCV61B(current mirror) switched as zener and diode.UBat was 14.1V and a 1K resistor gave ~9.14V (so ~5mA Zener current).1/f Noise: 0.1-10 Hz ~23-28 uVpp (100 sec). so higher than typical buried zeners at 10V (4-6 uVpp)T.C. not measured.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: September 16, 2022, 07:37:16 pm
content: The BCY87 is also a matched pair for diff amps and mirrors, but in metal case. I wonder what could be the max reverse BE current and whether there could be (at least theoretically) a zero tc somewhere within that current.

author: Andreas
author_rank: Super Contributor
timestamp: September 16, 2022, 07:58:53 pm
content: Mhm,with 150 mW max @ 25 deg C I would not get above 50-75 mW. (or 5-7mA)With best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: September 16, 2022, 08:06:21 pm
content: Hmm, but that is the reverse base-emitter current in the avalanche region. I've not found any data on that - like max current or TC.. The max current in that region might be limited by other processes, not just power loss..PS: here is a pdf on the gain degradation caused by BE avalanche breakdown.. Looks like anything higher than 100uA is not good, even though it is not clear for me what could be the impact of the gain degradation on the zener voltage stability..https://ur.booksc.me/book/2340496/325edeAlso there are papers on hot electrons creating charges at the surfaces, etc.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 17, 2022, 05:03:35 pm
content: It seems a better solution would be to start with a purpose-made zener and a series diode. Soldered close together they should track closely in temperature, or maybe pot them together. That way you don't need to worry about degradation of a junction not designed for reverse bias. You can also choose the zener voltage for best performance.

author: iMo
author_rank: Super Contributor
timestamp: September 17, 2022, 06:16:50 pm
content: @Conrad: Frankly, my only intention has been to try with the pair of matched transistors on the same die in metal package, to mimic the 2DW2xx ref diodes. Basically it works, there are some open questions, as always - the most important one being whether there is a reasonable current for the zero TC.The 4-5uV rms noise  is not bad at all, it is at the same level as the REF01 and similar refs.The current gain degradation of the transistor working in the avalanche breakdown at higher reverse base-emitter currents - that could be an interesting research topic for the experimenters here as well. The above paper is almost 50years old, thus they may refresh its key message with some new measurements too..

author: julian1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 09:51:55 pm
content: Interesting experiments. I considered using a matched bjt package (that300) to emulate ltz1000 type circuit, whereby one bjt provides gain amplification). It should be possible to use the bjt's Vebo/Vceo in Zener reverse breakdown. While taking advantage of the tight temperature coupling of the on-die matching. I watched a bunch of videos, explaining the bipolar process since one of the pnp PN junctions looks suspiciously like it could be a buried zener, before concluding that it wouldn't work. Although maybe there's another part out there with an accessible substrate connection one could use to get a buried zener.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2022, 07:24:27 am
content: The transistor degradation did not only happen at high currents, but also a rather low currents like 10 µA. The article also mentions a change in the breakdown voltage of up to 20 mV, which would be bad for a reference. So there seems to be no truely safe level, just slower degradation. Another interesting point mentioned in the article is that the gain degration is reversible with annealing. So there may be positive effect of operating at a higher temperature, as it is faster to espeblish an equilibrium between the generation of new defects and the annealing.Another problem with using transistors is that there is quite some scattering on voltage were they actually break down. So chances are only a small percentage may work and testing may be kind of destructive, as the avalanching also tends to increase the transistor noise.Dual BJTs are getting rare and hard to get. IN comparison the 2DW23x are relatively easy to get.The termal coupling is one of the smaller problems - so just single transistors should be good enough. No real need to destroy expensive matched pairs for this.A somewhat interresing point may be annealing part. Here the use of a transistor can be an advantage, as one can measure the annealing part via the gain and thus without generating new defects.

author: Andreas
author_rank: Super Contributor
timestamp: October 04, 2022, 03:12:22 pm
content: Update on BCV61B (after having found it in my drawer).I made some ageing measurement over ~10 days (16000 minutes).Attached to one of my ADCs together with a wind shield to reduce noise.The result is several mV drift per day even after 10 days.So this device is not suitable for my volt-nut needs.with best regardsAndreas