Francesco,
I have some better answers now, as I have a very complete BF981
datasheet in front of me.
1. For fixed Vg1s and Vg2s, changing Vds has no effect on Id. Just
like you observed.
2. The *typical* Id for [Vg1s=0, Vg2s=+4, Vds=10V] is 11mA. The
minimum value of Id for the same bias point is 4mA. So your observed
7mA is within the process variation.
3. No the gates are not interchangable.
4. The datasheet does not give any Vg2s/Id curves, and Vg1s cannot be
positive (max gate current is exceeded nearly immediately) so you're
stuck with 7mA for this part.
5. Id *does* depends very strongly on Vg1s...for the typical part,
Vg1s=0V gives 11mA, but Vg1s=-0.5V gives Id=4mA.
6. Noise data is given at Vds=10V, Vg1s=0V, Vg2s=+4V: Fmin=0.6dB,
Gs,opt=0.6mmho, Bs,opt=-1.25mmho @ 100MHz.

The BF991 does look to be the same die as the BF981, just in the SMT
package.
GL,

On Nov 6, 1:44 am, wrote:
Hello all and thanks for all the answers,

as I already said, I tried to raise Vd up to 12V and Id almost doesn't
change, as Vg2s remains constant.
g1 is always at 0V in this design since there's an inductor to ground
(part of a parallel resonant LC), so actually Vg1s is slightly
negative, less than -0.1V with a 15 ohm Rs. I doubt this really
affects the Id. I actually tried to keep Vg2s at 0V and change Vg1s
and what I found is a weaker effect on Id with respect to Vg2s, with
Vg2s=0V and Vg1s=4V the Id was less than 3mA. Clearly the two gates
aren't interchangeable.

On 6 Nov, 03:20, K7ITM wrote:



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.

righ, but on the datasheet there's no data about the right source
impedance for best NF.

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.

I'll have a look at the BF991 datasheet too, thanks for hint.
I have actually other spare mosfets like the BF960 and BF966, but they
seems worse than the BF981.

Thanks again
Francesco IZ8DWF