Zitat:the Vgs(off) (or Vp pinch-off) is listed as a possible RANGE for the following devices:
2n5484 ... -0.3 v / -3.0 v
http://www.fairchildsemi.com/ds/2N/2N5484.pdf
J201 ... -0.3 v / -1.5 v
http://www.fairchildsemi.com/ds/J2/J201.pdf
2n5457 ... -0.5 v / -6.0 v
http://www.fairchildsemi.com/ds/2N/2N5459.pdf
these limits change with every type of jFET chosen ... let's assume for a minute that we have found a device for each type that so happens to have a Vgs(off) lying smack half-way in each limit
that is, we have a 2n5484 with a Vgs(off) of -1.65v, J201 with Vgs(off) of - 0.9v and a 2n5457 with Vgs(off) of -3.25v
if we assume that a "good" bias point is half-way down Vgs(off) then we would require a biasing voltage of - 0.825v, - 0.45v and - 1.625v respectively ... if by bias we refer to the idling current flowing through the device (in absence of signal) we then understand that a higher diode/source voltage would then reduce current and vive-versa ... with less idling current we get less device gain (transconductance) and less noise ... aside from an imporvement in noise also comes a variation on the wave bending (this requires Spice/Scope to observe)
also, we need to know what kind of voltages are available to us as far as diodes go (very roughly speaking):
Ge diode ... 0.2v (from transistor and not 1n34a or 1n60 "cat whisker")
Si diode ... 0.7v (1n4148 and 1n914 ok)
3mm LED ... 1.5v
5mm LED ... 1.8v
we can see that, half-way biasing can "roughly" be achieved for each device if we use 4x Ge for the 2n5484, 2x Ge for the J201 and 2x Si + 1x Ge for the 2n5457 ... (note that with the op-amp and trimmer approach we can dial things perfectly and continuously)
as far as lower-biasing goes (to the LEFT of Vp/2 in the input range) for the 2n5457 I would be tempted to try biasing with Si x3 ... for the 2n5484 I've found that Si x2 can work well //
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