A couple of things come to mind...
The best noise figure also has another parameter: Vds=10V. It might
take that much to push 10mA through the channel.
One other thing to remember is that as Rs increases, Vg1s and Vg2s
decrease, which tends to further pinch the channel and reduce Id.
I'd try Vd=12V and Rs=200ohms, and use either a 10k resistor or
(better) an RF Choke between g1 and s...that will force Vg1s=0 no
matter what the bias condition. That will set Vs=2V, so then an equal-
resistor divider from Vd gives 6V which is Vg2s=4V.
If you really can't pull 10mA then it might be worth trying out anyway
(as previously suggested), but I'd suspect the device is no good.


On Nov 5, 8:19 am, wrote:
Hello,

I'm assembling a classic preamplifier with the BF-981 dual-gate
mosfet. The datasheet says the best noise figure is obtained with
Vg2S= 4V, Vg1S= 0V and Id=10 mA.
So I ended with a Vdd of 9V (because I had a spare 7809) and obtained
4V for Vg2 with a 68k+56k divider, Vg1= 0V through the coil of the
tuned input filter, Rs first replaced with a multiturn 500 ohm trimmer
to find the correct 10 mA point. Rs and the 56k to ground bypassed
with 10 nF ceramic.
Easy, right?
The problem is I can't get the mosfet to drain more than 7 mA (at Rs=0
ohm). I also tried to increase slightly Vg2S by replacing the 68k with
another 56k but I end with about 7.5 mA max. Bringing Vd up to 12V
makes almost no difference.
Do I have a broken BF-981 (I don't have another one handy to test) or
it could be a normal value spread for this device?

Thanks

Francesco IZ8DWF