BF256B Discontinuance- are there any decent JFETs left?

[H713]

It's fine for now, but I still question if I should be designing equipment around a part that On-Semi wants to obsolete. Most likely it's only still in production because a government client told them to keep it in production... so we'll see how long that lasts.

 

[U230421]

The increase in leakage can be 5 orders of magnitude, a real trap for the unwary.

Thanks Mark, this explains a lot I didn't realise. I noticed the increase, but not the cause.

The moral is use p-channel JFETs if possible...

P jfets...availability... now that's a challenge!

...cascoding being a common technique used...

It serves more issues.

 

[U230421]

...because a government client told them to keep it in production...

Is that a checked fact or an assumption?
I rather think it's market and shift from discretes.
Availability here is ok, but not as twenty years ago, admitted.

 

[H713]

Is that a checked fact or an assumption?
I rather think it's market and shift from discretes.
Availability here is ok, but not as twenty years ago, admitted.

Purely a guess, but my conclusion was reached based on the fact that On-Semi redacted their decision to cease production of the BF256B a few months after the initial discontinuance notification. I have a hard time believing that there was that much of a market demand to make them do that.

 

[H713]

Also, it's all well and good to us P-channel JFETs, but can anyone list any good low-noise ones that are still in production?

 

[john_ellis]

Could it be that the decision to keep the BF256B going was aided by ON Semi taking over Fairchild? Perhaps Fairchild had a cheaper line manufacturing them?
But yes I agree, there might be some military app that needed them...

 

[H713]

Could it be that the decision to keep the BF256B going was aided by ON Semi taking over Fairchild? Perhaps Fairchild had a cheaper line manufacturing them?
But yes I agree, there might be some military app that needed them...

I believe that the On-Semi acquisition of Fairchild pre-dates the announcement of obsolescence by about three years.

 

[steveu]

Entering ~BF256B parameters into the On-semi selector gives BF256B and MMBF4416.
MMBF4416A is slightly cheaper and 35V instead of 30V.

 

[H713]

I question the actual availability of the MMBF4416A since all the major distributors have 0 stock. The 2SK3557 is a lower voltage part IRRC, but it is relatively available still.

 

[john_ellis]

Farnell in the UK now have some BF256B devices on order. ON Semi reactivated these in October last year.

 

[Mark Tillotson]

Another approach if you want future-proofed matched JFET pairs is simply to leverage the best JFET input opamps like the ADA4625-1. 3.2nV/√Hz, 4.5fA/√Hz, < 80µV offset.
Yes you've only got single-ended output, but FET input opamps aren't going away anytime soon and the matching is not going to disappoint!