On Nov 5, 7:19 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

Hi Francesco,

The data sheet I was able to quickly find suggests optimum noise
figure at a particular Vds, Vg2s, and Id, but does not mention the
Vg1s required to achieve that Id. I suppose that for your particular
part, you may need to run gate 1 slightly positive with respect to the
source to get the 10mA. However, I doubt that there will be a
significant advantage to doing so with respect to the noise figure.
It will almost certainly be more important to adjust the input
matching to achieve lowest noise figure.

The data sheet I found was old and only seems to have summary data. I
looked on the NXP web site, and found what seems to be a similar part,
the BF991. The data sheet for it is complete, but it seems still very
sparse. I am used to data sheets for low noise RF amplifier
transistors that include graphs of things like noise figure versus
input matching at some particular bias, and possibly for two or three
bias conditions. It was a disappointment to not see something like
that for the BF9x1 parts, though they may be good parts anyway.

Cheers,
Tom