On Nov 6, 12:44 am, wrote:
Hello all and thanks for all the answers,
...

On 6 Nov, 03:20, K7ITM wrote:

....
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.

Hello Francesco,

But I suppose that will be a problem no matter whether Id is 10mA or
7mA. ;-)


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.

At least the BF991 data sheet lists a recommended source impedance for
optimal noise figure. I would guess that the BF981 will be quite
similar. Of course, it will always be best to adjust for lowest noise
figure.

I am not surprised that going between 9V and 10V made very little
difference in the drain current. You certainly must be operating
there in the "saturation" region of the FET (not to be confused with
saturation in a bipolar...), where Id is very nearly independent of
Vds, for constant gate voltages. Also, if you have a source resistor,
that provides negative DC feedback and will stabilize the drain
current--if the current decreases, the voltage drop across that
resistor decreases, and Vg1s (and Vg2s) become more positive, thus
tending to keep Id even more constant than it would otherwise be.

I would not do it myself, but if you feel a need to get Id up to 10mA,
you could remove the "cold" end of the inductor from ground, bypass it
to ground with a small capacitor, and apply a slightly positive
voltage from a voltage divider. I think it is much better to just go
with the 7mA or whatever it turns out to be for your particular part,
and concentrate on input matching adjustments.

You didn't mention what frequency this preamp is for, did you? I am
curious--is it for 144MHz or 440MHz or some other band?

Cheers,
Tom

Hello Tom,

On 6 Nov, 15:52, K7ITM wrote:

...
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.

Hello Francesco,

But I suppose that will be a problem no matter whether Id is 10mA or
7mA. ;-)

yes indeed, unfortunately I don't have a NF test equipment to play
with, so I can't experiment too much.



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.

At least the BF991 data sheet lists a recommended source impedance for
optimal noise figure. I would guess that the BF981 will be quite
similar. Of course, it will always be best to adjust for lowest noise
figure.

having a way to measure it, of course.


I am not surprised that going between 9V and 10V made very little
difference in the drain current. You certainly must be operating
there in the "saturation" region of the FET (not to be confused with
saturation in a bipolar...), where Id is very nearly independent of
Vds, for constant gate voltages. Also, if you have a source resistor,
that provides negative DC feedback and will stabilize the drain
current--if the current decreases, the voltage drop across that
resistor decreases, and Vg1s (and Vg2s) become more positive, thus
tending to keep Id even more constant than it would otherwise be.

That's right, I just checked what happened rising the Vd because it
was easy to do, and checking theory always helps.


I would not do it myself, but if you feel a need to get Id up to 10mA,
you could remove the "cold" end of the inductor from ground, bypass it
to ground with a small capacitor, and apply a slightly positive
voltage from a voltage divider. I think it is much better to just go
with the 7mA or whatever it turns out to be for your particular part,
and concentrate on input matching adjustments.

I won't do it either, I just have the feeling that the particular
mosfet I have is maybe failing, but I would have to steal another 981
from a working preamp to be sure, which I'm reluctant to do, or use a
different dual gate mosfet like the 960 or 966 (I have plenty of them
from TV tuners).


You didn't mention what frequency this preamp is for, did you? I am
curious--is it for 144MHz or 440MHz or some other band?

70 Mhz, not a really quiet band actually, but I live in a quiet place
anyway. With a transverter tuned by ear (currently I don't even have a
spectrum analyzer) and a 6 element yagi home made I can hear very
frequent MS pings from beacons I normally don't hear. It's often
enough to tune a beacon frequency and turn to the right direction and
wait some minutes to get some pings and be sure everything is working.
If we get permission to operate on this band in the next years we
might be able to make the first TEP contact on this band. I don't
think a preamp would make a difference in my setup, but it surely
won't harm.

Thanks again and best 73
Francesco IZ8DWF