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Matching transistors & measuring the results
Matching transistors & measuring the results
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Old 28th October 2017, 08:37 PM   #101
Diego Mike is offline Diego Mike  United States
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Quote:
Originally Posted by Mark Johnson View Post
Some differential-pair testers have very low or zero base series resistance; they only measure delta_VBE.

Other testers have significantly non-zero base series resistances (which are closely matched). These testers measure the combined effects of delta_VBE and delta_Beta.
In case I sound like I know what I'm doing: I don't. Can you elaborate a bit for the uneducated?

And FWIW, the Atlas DCA55 (that I use for my initial sorting into "beta" groups) tests single transistors only.
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Old 28th October 2017, 10:38 PM   #102
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Mike,
You have to use tweezers to handle transistors on your presort. Otherwise you will have less well matched pools to work with. This comes from experience. You also can't breathe on them until after you have the beta reading.

On close beta readings I still get wide differences between collectors, but the close matches tend to measure at the same beta if you can keep them at the same temperature. That is nigh impossible to do with single testing.

Even when I have the same beta readings, they may come in with 10 mV difference or more. The meter beta readings are very approximate. I'll test all the parts within a range of beta, and many different beta readings are actually excellent matches.

Depending on your test current, a 1 mV difference is a wildly good match, and I tend to have most of mine come in there. I do tend to allow them a longer time to equalize thermally, like about 10 minutes or longer.

I line the transistors beside their listed beta, or stick them in foam and use numbered references that tie to beta. I'll test one against the pile of close readings, then on to the next against the rest. It is slow going at the beginning, but it goes a lot quicker as you progress. If I have a balance below 1/2 mV I'll just pair the two and on to the next.

Understand that with matches this close, you must match the degeneration resistors in the circuit (if used). If you don't, the resistances will throw your match off. These matches are silly good.

-Chris
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Old 30th October 2017, 03:39 AM   #103
Phloodpants is offline Phloodpants
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I sort them out on a sheet of paper that's got a grid drawn on it, with zero in the middle, and squares numbered 1-99 to the left and right. I pick a reference transistor with a beta somewhere in the middle of the pack. (Using a cheap transistor tester) The transistors are tested in the jig, and then placed on the grid according to how far away from the reference they are. After a while, you start to get little piles of transistors that are close to each other, and these candidates can be tested more thoroughly for matching.
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Old 30th October 2017, 12:16 PM   #104
Mark Johnson is offline Mark Johnson  United States
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Matching transistors & measuring the results
The good news is that when you buy qty=10 pieces of 1% resistors, you are guaranteed to find two of them that match each other within (1/10)th of 1%. Assuming you have an ohmmeter and test fixture that can repeatably resolve an 0.1% difference.

And, as it turns out, ten 1% resistors cost less than two 0.1% resistors. Plus, 1% resistors are available in many, many more resistance values.
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Old 30th October 2017, 07:06 PM   #105
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Mark,
True, I use an HP 3457A with Kelvin clips to match resistors. But I'll still buy the 0.1% resistors and just use them. It's a lot faster.

-Chris
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Old 31st October 2017, 03:14 AM   #106
Phloodpants is offline Phloodpants
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OK, I have a more practical idea—than an insulated glovebox—for regulating the temperature of the DUTs, and bringing them to stability quickly, at a consistent, repeatable temperature. Expounding on Diego Mike's heatsink-die approach... Start with a cylinder of aluminum or copper, and machine a recess, precisely the shape of two TO-92 transistors pressed face-to-face. An oval recess deep enough to fit down over the transistors. (The circuit board would need minor modification, flipping one of the transistors 180 degrees.) If the machining is precise, and the physical tolerance is close, then the two DUTs should stay very close to the temperature of the surrounding metal, without any need for thermal paste. A way is needed to heat the die to a temperature that would be typical in the equipment its intended for. (My experiments suggest that the temperature is not critical as long as it's above 30C or so.) There are thermostat/relay boards available for cheap on ebay. A small power resistor can be affixed to the die as a heater, and the thermostat's thermocouple embedded in the die near the transistors.
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Old 31st October 2017, 03:49 AM   #107
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Chris,
Mount the driver transistor on the other side and make use of its heat output too. It should give two heat sources, one on each side to heat things more evenly.

I know of lots of equipment that does run around 25°C. What is it about 30°C and above that is so special? In better designs the diff pair may only have 10 ~ 15 V across them. Self heating is next to non-existent in that situation. If you're looking at J-Fets, then expect 7V or less.

-Chris
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Old 31st October 2017, 04:28 AM   #108
Phloodpants is offline Phloodpants
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The cylinder would be vertical, with the transiostor holes in the bottom, and I thought to mount the heater resistor on top of the cylinder, centered perfectly symmetrically. Maybe in a machined groove filled with thermal epoxy.

Nothing special about 30C, but in my observations, the DUTs track really well between about 25C and 80C, and not so well below 25C. I reckon 30C is a good spot along that continuum.

I think this jibes with your observation that they match quicker at higher currents like 3ma. More self-heating and so their temperatures get up into this more linear region above 25C.
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Old 31st October 2017, 04:52 AM   #109
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Chris,
Certainly possible. I've been more focused on using something reliable than exploring all possible effects.

I still have my first one thrown together on perf board as I needed a better way ASAP. Later I added the PNP section. Of course, not before I needed it. One thing for sure, there is a huge difference between a tightly matched pair and a pair that isn't very close. So much so that it doesn't make sense to install a pair that isn't matched.

-Chris
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Old 31st October 2017, 12:36 PM   #110
31697B is offline 31697B  Thailand
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used to be many dual monolithic devices in one package. What exist today as matched pair for audio ---- npn and/or pnp pair??


-Richard
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