Have to calculate gain for typical small JFET preamplifier

[supernet]

I would like to made small and simple JFET preamplifier with 6 dB gain.

I found simple circuit like this below, but I can not find out how to recalcultate required gain. Source resistor is key to go, but ...

If anybody here got some experience with JFETs please help


Thanks

[john curl]

Vg=R(1)/{R(2)+1/Gm} or R(1)/R(2) approximately

[supernet]

Thanks

Is Gm transductance?

[davidallancole]

yes

[Tim__x]

It should noted that Gm is very inconsistent from batch to batch, so if you need precise gain it'll need trimming.