JFET source follower gain problem

[philwinder]

jfet buffer gain

That's very interesting, I haven't seen that topology before. How does it work? Does the transistor provide a current source?

Phil

 

[FvM]

jfet constant current source

I mainly see it as feedback amplifier. The loop gain brings the gain closer to unity and also reduces the output impedance. As a kind of current source operation, the drain current, set by R1, is kept constant by feedback action.

An important parameter, depending on the drain current hasn't been yet mentioned in the discussion, it's JFET noise voltage. Unfortunately, it's rather high with a small area electrometer FET, but still increasing with decreasing Id. But as far as I see, the usual 2N4117 SPICE model has no realistic parameters for the current dependancy of low frequency flicker noise. So you have to evaluate it in a real circuit.

From a noise optimization point of view, a simple source follower with a +/-15 V supply and a e.g. 500k resistor load, as suggested in the datasheet, is still the best circuit (gain of 0.97).