small signal device matching procedure (as in LSK170) - Page 2 - diyAudio
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Old 9th October 2008, 03:46 PM   #11
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Default I've used this successfully.....

http://www.geofex.com/Article_Folder...h/fetmatch.gif

This is an active FET matching fixture. I've made a PCB layout in expresspcb for it, and it works well. If anyone wants it, send me an e-mail for the file.
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Old 9th October 2008, 04:17 PM   #12
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Old 9th October 2008, 04:19 PM   #13
AndrewT is offline AndrewT  Scotland
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absolute accuracy is not too important, unless you are sharing data with others.
Repeatability is important.
Today's results should compare to yesterday's and last year's and next year's.
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Old 10th October 2008, 02:30 AM   #14
anatech is offline anatech  Canada
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Hi Andrew,
Quote:
Today's results should compare to yesterday's and last year's and next year's.
You need to control the device temperature in that case.

-Chris
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Old 10th October 2008, 04:39 AM   #15
flg is offline flg  United States
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Matching Idss amongst bottom and top transistors should assure a median voltage at the output, I guess it's cap coupled though anyway But, this also gives max + and - voltage swing at the outpuit It also suggests gain to be very similar between the 2 devices.
I've read were because both transistors are not operting in the same configuration, that matching dosent matter??? It dose not take much mismatch to cauase the output to go to one rail. Therby having 0 volt swing to one rail
Output queicent offset voltage may not be best at midpoint of the supplies though. It is good not to draw to much gate current or saturate the device to much
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Old 10th October 2008, 05:57 AM   #16
steenoe is offline steenoe  Denmark
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Default Re: I've used this successfully.....

Quote:
Originally posted by FUCHSAUDIO
http://www.geofex.com/Article_Folder...h/fetmatch.gif

This is an active FET matching fixture. I've made a PCB layout in expresspcb for it, and it works well. If anyone wants it, send me an e-mail for the file.
Fuchs;
Looks interesting. Is that rig temperature stable, it looks like it? The usual way with a power-resistor is very edgy, temp-wise.
Please turn on your email. (or mail me)

Cool looking amps on your homepage. Just spotted that my hero Joe Bonamassa has one



BTW, nice thread.
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Old 10th October 2008, 02:32 PM   #17
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Default Yes it is.

I buy a bag of 100 or 200 Fets for production, and group them in bins by the test numbers. Makes life so much easier. I believe the rig can be considered temperature stable, as the FET is not under any stress that might heat it up or make it vary it's operating point.

I'll check my settings to make sure I'm open for getting e-mails.
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Old 10th October 2008, 07:33 PM   #18
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Default Re: I've used this successfully.....

Quote:
Originally posted by FUCHSAUDIO
[url]This is an active FET matching fixture. I've made a PCB layout in expresspcb for it, and it works well. If anyone wants it, send me an e-mail for the file.
Welcome Andy!

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Old 10th October 2008, 07:45 PM   #19
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Default Back at cha !

Howdy Nelson. Thanks for the welcome !

FET Matcher artwork is attached. www.expresspcb.com

Attached Files
File Type: zip fet matcher.zip (4.7 KB, 340 views)
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Old 10th October 2008, 08:05 PM   #20
anatech is offline anatech  Canada
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Hi FUCHSAUDIO,
You need to control your ambient temperature. For me in the basement of my house, that's a problem. For a business office, might not be. I still had to watch the vent air in my business.

I just wanted to point out that the FET may measure differently if the ambient drifts. It's the same with BJTs.

I take this one step further and thermally tie transistors together in a diff pair jig (posted in a couple places here). As long as the temperature is the same on each part, you can see how they balance in the testing jig. It's just a common emitter/source through a current source with collector/drain and base/gate sensing resistors. It works very well for matching parts closely. It does take time to allow the parts to reach thermal equilibrium.

-Chris
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