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

Tom Lee

Yup, the 2215A uses an op-amp bias loop to get away from the older arrangement that required a JFET matched pair. The ~30mA max IDSS figure I quoted is from looking at the schematic. The current source transistor goes into saturation at that point. For some common-mode range, you want the JFET's IDSS to be below that maximum, hence my recommendation for something in the 15-20mA range.

If IDSS is much lower than that, the bias loop still closes fine, but the output impedance of the buffer starts to rise to the point where bandwidth might be affected.


Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/11/2020 07:14, Ed Breya via wrote:
I don't know about the 2215A, but the other 2200s use an opamp. The opamp (JFET input type) provides the DC/LF path.It's overall DC-balanced by the feedback, so the JFET bias is taken care of. It has to be fast enough for the bandwidth required, and the DC conditions are not too critical, as long as the loop can keep it all balanced.


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