Re: Q122/222 Jfet J300 subs for 2215A scope ?

dave G8SFU

Sorry I got the impression from earlier in the thread that these were a totem pole pair of fets .
My bad
Must be more careful to look at the manual.

Regards dave

⁣Sent from BlueMail ​

On 18 Nov 2020, 20:19, at 20:19, Tom Lee <> wrote:

I’ve analyzed the circuit. Any IDSS below about 30mA will satisfy the
loop, as I’ve noted twice before. If much lower than that, dynamics may
suffer, but the upshot is that there’s a very wide window.

It’s that accommodating nature that leads me to suspect that the OP may
have other problems.


Sent from my iThing, so please forgive brevity and typos

On Nov 18, 2020, at 12:15, Ed Breya via
<> wrote:

There may be some confusion over "selecting" versus "matching," due
to the different input amp topologies also included in the discussion.
In this case, it's an opamp-stabilized system, with only one JFET up
front, for each channel. There is no need for matched JFETs, but
apparently there is a need for the JFETs used to fall in a narrower
Idss range than the generic part types they come from. It looks like
the official Tek part number used is provided by the vendor from J310s
or similar, selected for some range of Idss according to Tek's spec for
this part. The problem is that we don't know the selection criteria
applied, but we do know the one good original J310 found in the circuit
appears to work, so finding another VHF JFET that has similar Idss
should work too. We don't know if the original is an ideal example - it
may be at an edge of the desired range, or right in the middle, but at
least getting close to whatever it is should be in the ballpark.


Join to automatically receive all group messages.