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matching BJT hFE for β24

beta24

matching BJT hFE for β24

Postby bk_856er » February 24th, 2018, 9:24 pm

I recently started my b24 build after initially acquiring all the parts in 2013. It's coming along quite nicely.

Following pieterh's lead, I decided to get extra BJTs to match hFE, perhaps making initial setup easier. The matching tool is a low-cost "AY-AT" transistor tester upgraded with some precision parts to align with the original design intent. I bought 200 BC550C and 200 BC56C from Digi-Key (ON-Semi manufactured). I also tested the parts that I got from the AMB Shop in 2013.

Below are some histograms. Scales and bins are not identical, but it still captures everything.

What is the optimal grouping to use for the b24? Is it noteworthy that the AMB Shop parts measure differently @ ~600 for both types?

BK

BC550C.JPG

BC560C.JPG

compare.JPG
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Re: matching BJT hFE for β24

Postby amb » February 24th, 2018, 9:44 pm

My stock of BC550C/BC560C are from Fairchild. That may account for some differences. Now that On Semi has acquired Fairchild, I don’t know how the redundant products are being handled. Also, the transistors I ship are consecutive from a tape, which are more likely to match each other than random loose units.
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Re: matching BJT hFE for β24

Postby BesPav » February 24th, 2018, 9:53 pm

bk_856er wrote:What is the optimal grouping to use for the b24?


Pick 10-15 as high as possible hFE BC560 samples.
Heat them at ~50-55 Celsium degrees, like them will be heated in a real amp.
Measure again.
Then pick two-three BC550 from each different hFE range.
Heat them also.
Measure and pick most equal parts to earlier picked BC560.
If needed pick another BC550 from that hFE range, heat them and check.

For best results you need two precise quads, one per channel.
Place them at Q9-Q12 positions.
Other bjt’s doesn’t need that precise matching.

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Re: matching BJT hFE for β24

Postby bk_856er » February 24th, 2018, 10:05 pm

I always expect the best from the AMB Shop! My "little tester" isn't calibrated or certified, but taking the numbers on face value and going by the specification sheet, nearly half of my Digi-Key BC560C don't meet the minimum Classification C hFE of 420. All the parts were supplied on continuous tape. Perhaps the test conditions are slightly different, etc.

Is there a preferred hFE for the b24? Any benefit to having B550C and B560C hFE close matched, vs. all B550C one value and all B560C another (different) value?

BesPav, just saw your post thank you. This matching business is a slippery slope! Q9-Q12 I should be able to handle. Need to think about the best way to get a consistent elevated temp that'll work with my tester...

BK
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Re: matching BJT hFE for β24

Postby BesPav » February 24th, 2018, 10:13 pm

bk_856er wrote:Need to think about the best way to get a consistent elevated temp that'll work with my tester...


Your home radiating battery in winter most likely are heated to that level.
Usually +25-35 degrees upper than home air.

Just place them on top, add thin-lead tweezers, wear thin gloves and try to be fast enough.

If your multimeter are showing realtime value you will easily see hFE drop at temperature falling.
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Re: matching BJT hFE for β24

Postby bk_856er » February 24th, 2018, 10:36 pm

Below is the setup I used for hFE measurement. It's an "AY-AT" clone assembled from parts and using key 0.1% resistors and better regulators. Shown with two of the AMB unicorn parts ;-).

BK

1-IMG_1055.jpg


2-IMG_1057.jpg
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Re: matching BJT hFE for β24

Postby amb » February 24th, 2018, 10:44 pm

:D
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Re: matching BJT hFE for β24

Postby bk_856er » March 4th, 2018, 1:02 am

So I ended up using my initial hFE measurements to coarsely bin and sort the devices and chose about 50 BC550C and 50 BC560C with the best overlap. Then I used a much more careful measurement method (no handling, timer, very systematic) to get new numbers on this smaller batch for the Q9-Q12 "super matches" and best effort for everything else. I didn't have a good way to heat the devices, so I just used room temperature. Prior to this I measured one device about 20 times to get a feel for the repeatability and also experimented with the consequence of handling with fingers and warming them up (yep, handling with fingers matters). As far as Ti is concerned all this is unnecessary for the b24, at least using his Store devices, but what else am I going to do with 400+ devices, a new measurement toy and some free time? :geek:

BK

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