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Old 8th October 2010, 08:23 PM   #1
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Default More FET noise measurements (for EUVL)

Sorry for the delay, but I decided to build my own test jig to get a better picture of the noise spectrum.


There are two plots here. I had to go to a double shielded enclosure with the outside shield connected back to signal ground on my A/D. There was still some slight mains hum depending on where I put the box on my bench (out in the middle of the room was best). In any case it was reduced to 2nV or so. I tracked it down to the input cap. I thought I connected the outside foil to ground, but it seems the end termination and FET lead is enough. Next rev will have extra care here. The 997Hz tone is a mystery. I thought it might be the cal oscillator on my scope but it wasnít.
The noise is plotted on a log scale with -87.2 dB = 1nV/rt-Hz. This was established by taking the best FET and adding a reference resistor in the gate, a Vishay 10k .001% oil immersed precision resistor. I then scaled the plot down to 1nV. This theoretically even corrects the measurements for temperature but this is not necessary under normal lab conditions. I like this better than inserting a tone especially when using FFTís, everything is scaled without the tedium of keeping track of bin equivalent noise BW.
All the FETís were measured at 1.2mA Id. The lowest plot is my reference line. It varies just a little and I took an estimate around 1kHz of -87.2 dB. The next two plots (moving up) are a 2SK372 (Idss = 24.19mA) and 2SK369 (Idss = 14.75) respectively. Note the scale, we are only looking at .5nV Ė 3nV or so. The last plot is a random 2SK170. This is a very nice example of GR noise that would not show up well on a Quantech since it is only 3nV at 10Hz. These noise levels should scale as the fourth root of Id (one of the things that I want to do eventually).
The second plot is all three FETís with Rg = 10k. It looks like the slight rise at the low end on the 2SK170 is due to the GR noise. The roll off at the high end is probably the A/D. The unexpected result is that the other two plots rise at the high end. This could be induced gate noise but probably not impact ionization since we are at low Vgs. The lack of this in the 2SK170 and the fact that the 2SK372 is in a very small package leads me to believe this is a new/different process, this defiantly needs more study. For now these two FETís look great for MC for sure.

The last plot is the BF862, pretty impressive for an RF FET!
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File Type: jpg noise2.JPG (95.3 KB, 1891 views)
File Type: jpg noise4.JPG (70.6 KB, 1883 views)
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Old 9th October 2010, 12:39 AM   #2
EUVL is offline EUVL  Europe
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Hello Scott,

Thousand thanks.
I almost thought you had forgotten in tons of other work that you had.

And very interesting result indeed. I never expected that 2SK372 to be so good, indeed better than the two. And BF862 is really impressive.

If you are willing, I can send you some more FETs -- 2SK366, 2SJ107, 2SK163, 2SJ44, 2SK2223, .... The trouble is that these were bought as matched pairs at a premium price, so I definitely would like to have them back this time.

At long last we have some results from one very reliable source for direct comparison.

Would you care to explain your test setup a bit more, to see if it can be duplicated with reasonable effort ?


Best regards,
Patrick


PS You may of course email me if you prefer.

Last edited by EUVL; 9th October 2010 at 12:42 AM.
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Old 9th October 2010, 02:27 AM   #3
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Quote:
Originally Posted by scott wurcer View Post
The 997Hz tone is a mystery. I thought it might be the cal oscillator on my scope but it wasnít.
Coulda been your grand mum's hearing aid oscillating in the other room. Or someone left the Krohn Hite 4400A on.

Wonderful work, thanks! We have no low noise environment in NJ, what with Snooki all over the telly.
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Old 9th October 2010, 02:38 AM   #4
EUVL is offline EUVL  Europe
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For those using the current-sourced JFET source follower (alias B1) for preamp or buffer for Sallen Key filters, the 2SK372 is IMHO a much better choice. It has twice the transconductance of the 2SK370/170, with about double the capacitance. The output impedance in that circuit is thus halved. And now we know it also has quite a bit less noise.

You can easily get them (V grade) at ampslab, amongst others, at reasonable price. The V grade with +/-9V supply will run hot, so a heatsink is recommeded. (Pictures next week, as I am not in town till 15 October.)


Patrick

Last edited by EUVL; 9th October 2010 at 03:01 AM.
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Old 9th October 2010, 11:21 AM   #5
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will continue from HERE. Datasheets for 2SK372 and 2SK363 shows this is the same part but different housing.
Yes, they are very good for source follower configurations (biased with CCS), kind of punchier sound

@scott
thank you for those measurements. Great work

Last edited by stormsonic; 9th October 2010 at 11:26 AM.
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Old 9th October 2010, 12:32 PM   #6
EUVL is offline EUVL  Europe
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Datasheet also suggest that 2SK170 & 2SK370 to be the same part, except that they are most likely the same specification but manufactured with different processes, with the 2SK370 probably having a smaller die, and certainly a smaller package, to save costs.

The smaller package has a dissipation disadvantage when used free air standing. It has however a big advantage when glued together on the flat face as a matched pair. The physical distance between the two silicon dies are much smaller, with about half the plastic in between. This allows much better thermal coupling between the two, and hence DC tracking. Also, if you now enclose the two with an aluminium heatsink with a cut-out of the right size and shape, you also conduct the heat from the silicon die to the aluminium much better than the standard TO92 housing.

Of course you can argue standard TO92 will do fine. But if you want to run say 15V 25mA through the device, it is very close to its rated maximum. I personally would never want to run my devices more than 25% of rated maximum, for reliability reasons.

I shall open a new thread new week to show you what I meant.
We should keep this thread for JFET noise measurement discussions only.


Patrick

Last edited by EUVL; 9th October 2010 at 12:52 PM.
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Old 9th October 2010, 02:05 PM   #7
juma is offline juma  Serbia
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Quote:
Originally Posted by EUVL View Post
... 2SK372 is IMHO a much better choice. ...
The only problem i see is Pd_max=200mW at 25 C (150mW at 50C).
Heatsinking, as you said, might be mandatory.
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Old 9th October 2010, 02:09 PM   #8
EUVL is offline EUVL  Europe
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You can always use 2SK372BL, if you could get them of course.
Transconductance still 2x that of 2SK170BL.

Patrick
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Old 9th October 2010, 02:27 PM   #9
juma is offline juma  Serbia
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I think I'll stick to BF862
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Old 9th October 2010, 02:36 PM   #10
EUVL is offline EUVL  Europe
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Look at the data above carefully.

BF862 is not quite up to it in comparison.
And it is even more difficult to get rid of the heat.


Patrick
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