[Jeroen Belleman]

Phil Hobbs wrote:
John Larkin wrote:

The problem with jfets is the huge part-to-part variation in Idss and
transfer curves. A 10:1 datasheet spread in Idss isn't unusual.





+10v
|
|
|
d
gnd---------g
s
|
|
+------ Vs
|
|
10K
|
|
|
gnd

For a typical vanilla jfet in this circuit, Vs might be +1 to +4 volts
maybe.

John



BF862s are much better--cutoff to full on in half a volt or so. Their
transconductance is so high that even with a 2.5:1 range of I_DSS, the
spread of V_GS is pretty reasonable.

They're also very quiet, go about 700 MHz, and cost 20 cents. Other
than that, they stink.

Indeed, when I discovered their existence back in 2001, I could hardly
believe the datasheet. I'd never seen a JFET with a ratio of yfs/Cin
that high. That, and with only 0.8nV/rtHz input-referred noise. I love
it.

Jeroen Belleman