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

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.


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