Q122/222 Jfet J300 subs for 2215A scope ?

Tom Lee

Yes, a J310’s IDSS range is centered high for this particular circuit. The J309’s IDSS range is actually about right, as the buffer circuit aims to operate the FET at a VGS of -0.6 (to compensate for the VBE of the bipolar follower that the FET drives), so the drain current for a J309/J310 is perhaps 8-10mA lower than IDSS, assuming a typical gm of 15mS.

AFAIK, the J309 and J310 come from the same process recipe.


Sent from my iThing, so please forgive typos and brevity.

On Nov 18, 2020, at 1:53 PM, Ed Breya via <> wrote:

For some reason I've been referring to the original working JFET as a J310, but looking back all the way to the OP, I'm not sure what it is - all kind of parts are floating around this thread. So Charles, can you identify the one official, working JFET that I've been referring to?

The reason I ask is that I looked up the J310, and found it is a very heavy beast - VHF/UHF, with Idss 24-60 mA - way too big and fast for this little scope amplifier, and way too big for Tom's estimated max of 30 mA for circuit operation. The correct part must be something else more "normal," among many possible choices. The J309 may work, but even it seems a bit much.The J300 seems about right, and the J210 or preferably J211 may be a good option. Also, I mentioned the 2N5485 and 2N5486 earlier, which I'm pretty sure were used a lot under various Tek part numbers. I think especially the 2N5486 was used extensively in scope input amplifiers.

Anyway, there are types that it could be, and types it shouldn't be.


Ed Breya

Yup, I see it. I finally looked at the schematic for the 2215A. Looks like the quiescent drain current should be around 11 mA, and the source at Vbe.I'm looking at the J310/J309 data sheet here:

If I'm interpreting Fig. 1 properly, the lower curve set is for the J309, and the Id is about right for Vgs= -Vbe. The upper curves for the J310 are about 3 X higher - too far out there for the nominal circuit conditions, although Q15 could probably pull it hard enough to DC balance (but things would start getting weird). The JFET runs with Vds about 8V, so pretty close to the 10V used in the specs. The Vgs should be fairly constant since the required output excursion is only about +/-100 mV.

This circuit reminds me of the ones in all the other 2200 series, and may be essentially exactly the same. More info may be discerned by looking at those others to see what JFETs were used. The 2N5486 keeps sticking in my brain.


Ed Breya

Oops - I meant to say Vds here, not Vgs:

The Vds should be fairly constant since the required output excursion is only about +/-100 mV.