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Old 21st August 2007, 10:09 AM   #1
suzyj is offline suzyj  Australia-Aboriginal
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Default Matched JFETs

Hi guys,

Siliconix have ditched my favorite matched dual JFET, the SST404. I knew it was too good to be true. A new mob, Calogic, appear to be continuing to run the fab and making parts, but the end result is that I can't find them in small quantities.

I'm chasing up a (reasonably) bulk lot of fifty SST404s, which should do me a goodly time, but this whole business has me frightened about using packaged matched duals.

So I'm looking at single JFETs, as there's a reasonable range of different parts that I can design with. I can hand-match singles, in say a SOT-23 package for Vgs at some arbitrary Id. But I'm concerned about how closely I'll have to couple the devices thermally to maintain that match to within reasonable tolerances.

What are people's experiences? If you match a pair, what do you need to do to ensure they stay that way over a reasonable operating range for an amp? Is locating them next to one another on a PCB adequate? Or do you need to bridge them thermally, using for example a bit of (horrendously expensive) gap pad and a little heat sink?

Cheers,

Suzy (sobbing in her beer)
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Old 21st August 2007, 12:27 PM   #2
AMV8 is offline AMV8  United Kingdom
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Hi Suzy

I have been trying similar things.

I still have some 2sj109 and 2sk389 but I find it difficult to buy more at "reasonable" prices. So I have been experimenting with 2x2sj74 and 2x2sk170 pairs. I began to experiment after noticing that the Borbelly amps and kits often used the pairs of fets instead of the dual fets.

I have been building two complementary symmetrical fet pre amps. The input stage is a complementary differential stage not unlike the Borbelly starter designs but without the folded cascode on the output stage. ( Although I might add a folded cascode later as I read that Borbelly and John Curl both seem to prefer that arrangement. ) The output stage jfets are connected at their drains to act as constant current sources to each other.

I expected some difficulty with dc on the output due to temperature effects on the fets. To my surprise once I have set the output dc to zero I am finding little or no dc on the output on the pre amps.

I did match the fets when building the pre amps. I had a 100 2sk170bl and 100 2sj74bl and used matched sets of each. I am using 2sk170bl and 2sj74bl on the input with 3ma drain current on each jfet. The output stage has 8ma drain current.

The temperature in the pre amp case does not vary much so maybe that is why I get no dc drift. However I think it is largely due to device matching and complementary symmetrical layout as moving the pre amp from room to room does not seem to have any effect. I have not found it necessary to connect the fets with heatsinks - although I did lay them out so that I could do so if necessary.

I am planning to try some fets in the input syage of a class A power amp next so I let you know what happens then.

Don
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Old 21st August 2007, 04:37 PM   #3
CBS240 is offline CBS240  United States
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Hi

With the differential j-fet input and gnf, I find that matching of the devices is related to DC offset. In one of my "ugly amps", I very closely matched one channel, but deliberately matched the other not quite as close just to see the effect. There were a few more mV of offset, right about were I figured. I haven't used duel devices, they cost a lot more and are harder to find, as you know. There is more variety of singles anyway. If you choose to go with singles and hand match, you should get a decent quantity of them. There will be much greater difference in matching Vgs in j-fets than beta or Vbe in bipolars. My method is to sort them in general groups measuring at whatever Id you plan to operate them, and then from each group closely match them into pairs when you go to build the amp. Get some small sticker labels to keep the closely matched pairs together. I'm sure there are other methods, but this simple setup works for me. Accuracy depends on how much time and effort you invest in it. As for SOT-23, I would think if they are mounted in the same place, the temp should be the same if you cascode the input j-fets so Pd is the same. I haven't experienced any real problem with this.
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Old 21st August 2007, 05:10 PM   #4
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thermalloy used to make TO-92 "duallie" clips, but i'd have to look up to see if they still do. they're oval aluminum clips that hold 2 TO-92 transistors face to face for diff amps
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Old 21st August 2007, 08:08 PM   #5
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it might also be wise to sort JFETs by noise figure also. put them into a SE common source circuit, and measure the output with no input (terminated with 1k) with a noise meter.
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Old 21st August 2007, 08:37 PM   #6
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Quote:
Originally posted by unclejed613
thermalloy used to make TO-92 "duallie" clips, but i'd have to look up to see if they still do. they're oval aluminum clips that hold 2 TO-92 transistors face to face for diff amps
You can use copper foil tape -- 3M makes it and it's always handy to have around.
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Old 22nd August 2007, 01:08 AM   #7
Ipanema is offline Ipanema  Malaysia
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Has anyone tried this?

http://docs-asia.electrocomponents.c...6b8082607c.pdf
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Old 22nd August 2007, 01:21 AM   #8
dimitri is offline dimitri  United States
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PMBFJ620_RN20032309_noise.xls

Bias condition: Vds=10V, ID=10mA, RG=0ohm

kHz nV/vHz

0.01 11.06
0.120 8.82
1 5.95
10 2.84
100 2.06
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Old 22nd August 2007, 02:10 AM   #9
Ipanema is offline Ipanema  Malaysia
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Hi Dimitri,

thanks for the measurement data. Your noise measurement is a bit higher than Philips datasheet. I'm interested to know how the noise measurement is done. :-)

BTW, has anyone built circuit with this dual nfet?

Regards.
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