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

Tom Lee


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.


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