Measurement results
As saying goes - it's better late then never.... So here it goes.
First graph (see attached) shows how a current production JFET’s stack up in comparison to mighty BF862.
Seems like BF862 still holds ground as having lowest overall noise level, but 2SK2394 has much better 1f corner! Actually it’s best I have ever seen from a JFET or BJT for that matter, a really spectacular part! And it’s noise floor is maybe some 0.1nV/rtHz higher. I say maybe, because I must admit this falls into my calibration/measurement uncertainty. So for my intended applications 2SK2394 seems like a clear overall winner and my new favorite. I also have 2SK3557 on order. It looks like the same silicon as 2394 just lower trans-conductance, so I don’t expect any improvements. I will make an update when I measure them (don’t hold your breath though).
Second graph shows how these modern parts look compared to older favorites like 2SK170.
NSVJ3910 seems like a good match for 2SK170. Almost identical noise spectrum’s, but it has twice as much trans-conductance (22 vs. 40 mS) and five times lower input capacitance! (6 vs. 30pF). So a huge overall improvement over now legendary 2SK170. Also, negligible NSVJ3910 input capacitance makes this part most attractive in terms of paralleling then any other JFET measured here. Just a word of caution – it was the ONLY part that decided to be ~350Mhz oscillator instead of amplifier in my measurements. So care must be taken to isolate gate appropriately for the application.
Most strangest result is the difference between NSVJ3910 and CPH3910. It’s suppose to be same silicon in different package, but CPH3910 has this hump from 100Hz to 9Khz. Now I’m not an semiconductor process expert, but it looks like some manufacturing impurity to me. This could be just my batch (Farnell 2020.06 stock) or it could be how this part really is.
I made some distribution measurements to see if this is consistent defect in my batch (third attachment). It's a noise spectrum of 10 random samples from a batch of 40 pcs. Seems like 70% of samples are affected to some degree.
For comparison: fourth attachment is 10 samples of 2SK2394 (from a batch of 40). Almost ideal parameter spread except one “fluke”. This just shows again that for a really noise-critical applications, hand selection is the only way to go.
I have made much more measurements with different currents/older parts and not only JFET's. If interested they can be found at my website:
Modern JFET noise measurements - MV Audio Labs
