[Akku]

Having fried every jfet of my PPA (It was working pefectly, my fault, I shorted something) I am ordering new sets but now a have a doubt. In some recent posts and even in Tangent web site the 2N5457 is said to be a substitute for 2N5486.
When my PPA was working, following the datasheets, I used 2N5457 for Q1 instead of 2N5484 and not for Q2

I was wrong?
The IDss range is the same for 57 and 84 while 86 has much higher ratings (8-20 mA).
Do I misread the sheets?

 

[Dreamslacker]

You have not misread the sheets. You can use 2n5457 in both Q1 and Q2 as long as you have measured the IDss of the jfet in Q1 to be lower than that of the jfet in Q2.
By using the 2n5457 in Q1 and the 2n5486 in Q2, you eliminate having to measure the IDss as the latter would have an IDss higher than the former.

 

[blipblop]

I went ahead and bought the different FET values - I couldn' t be bothered to match high and low values manually (not to mention that this amateur wants to take all room for error out of the equation wherever possible!)

 

[Akku]

The frase in Tangent site that I found misleading, I quote, is the following:

"Alternately, you can use 12 JFETs, each with a minimum IDSS around 10mA. (Example: 2N5486, 2N5457)"

He is talking about rail isolation jfets and not about cascodes but 2N5457 doesn't have a minimum IDSS around 10 mA (It's a barely 1 mA).
Do I missed somenthing?

 

[guzzler]

2N5457 is 10mA Idss

 

[Nisbeth]

Quote:

Originally posted by guzzler 2N5457 is 10mA Idss

The datasheet I have say min 1, typ 3 and max 5. Exactly the same as the 2N5484? What's going on?


/U.

PS: Tangent could have written 2N5457 by mistake? The 2N5459 (which is on the same datasheet) would match the 10 mA Idss. Is that the problem?

 

[guzzler]

This has been solved over in the PPA calculation thread, in a nutshell it was me that was wrong (should have looked at the datasheet, although the Rapid catalogue i have here says an Idss max of 10mA for the '57), so European DIYers, we can get the '86 and the '57 instead of the '84 and be happy!

g

 

[tangent]

I'll make all this clearer in the docs. I was fuzzy one some of it myself.

The main message remains the same: the exact types of the JFETs aren't critical, it's the Idss that matters. You Europeans can probably find JFETs that will work that we can't get here in the US.

 

[Akku]

Originally posted by Nisbeth
The datasheet I have say min 1, typ 3 and max 5. Exactly the same as the 2N5484? What's going on?

That's what exactly the Fairchild Semi datasheet say.
Guzzler: where did you pick that value?

BTW the same datasheet rate 5459 for a minimum Idss of 4 mA, I am getting more and more confused

Edited:

Now it's clear. Sorry, I haven't seen those messages before posting