lo noise jfets

[tempus]

I know a lot of you are working with JFETS, and I thought I'd post this because these (2SK170) are really low noise:
http://ampslab.com/Freewares/2sk170.pdf

They sell them here http://ampslab.com/trans_2sk170bl.htm; I'm sure there must be others that sell em too

[alanlan]

Thanks for the link, they look pretty good - low VGS(OFF) as well - makes them useful.

[R.G.]

It's interesting to compare those to the 2N5088 and 2N5089. The bipolars have noise figures of 3 and 2db maximum, no specified typical or minimum. The JFET has a noise spec of 1 typical and 10max, and also 0.5 typical and 2 max - but the test conditions are specified as the same. So one of these is wrong.  Also, the bipolars have their noise sensitivity curves supplied so you can match them for lowest noise in terms of current loading and source resistance.

The 2SK170 is a low noise device all right - but whether it's lower than something like a garden variety 5089 depends on the source resistance, drain/collector current, bandwidth, and biasing. It's incredibly easy to foul up a 0.5db noise figure device by clumsy biasing. How do I know this? 

The 170 is a good device, but it demands careful design to deliver its promised goodness.

The best part of the 170 is its low 1/F break frequency and performance with low source resistances. Generally the JFETs are most excell-ent with high source resistances as compared to bipolars.

[Cliff Schecht]

Look up the U310. It's really more of an RF part but it's got a great noise spec throughout the frequency spectrum (up to about 450 MHz). I've been wanting to try some in a preamp design because of the low noise/high gain nature. I think Siliconix (Vishay) and On Semi still make them.

[sjaltenb]

Cliff,

I found them on Mouser...13$ and change per device?? is this correct?

[Cliff Schecht]

Try $0.15 a piece for the On Semi variant:
http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMv34NK1v5E8iZ%252b2ynzXX7p3FThvbMBtDWA%3d

[JDoyle]

Look up the U310. It's really more of an RF part but it's got a great noise spec throughout the frequency spectrum (up to about 450 MHz). I've been wanting to try some in a preamp design because of the low noise/high gain nature. I think Siliconix (Vishay) and On Semi still make them.

The 24 mA min Idss make them useless for a 9V stompbox, in my opinion. But then again the 'BL' and 'V' suffixed 2SK170s are in the same boat...

One last personal opinion - if you want to be able to get to the 'good stuff' of JFETs, and are working within the typical stompbox constraints of a 9V battery, you need a JFET that can drop it's entire Idss over it's load and not hit a rail before that happens.

And to continue what R.G. was saying - it is VERY hard to get any transconductor to 'be all it can be' in regards to a noise spec, and remember that any resistance added to the source terminal, for noise purposes, is like adding it in series with the gate...

Regards,

Jay Doyle