EMRFD Message Archive 1786

Message Date From Subject
1786 2008-07-09 05:19:45 Jason Milldrum Dual Gate MOSFET Amp - Gain and AGC
Hi Guys,

I've written up part 2 of my investigation in the BF998 dual gate
MOSFET. This installment deals with the gain of the amplifier in a
typical configuration and the AGC characteristics of the amp. There's
nothing too earth-shattering in here, but it's pleasing to see that
the experimental data tends to agree with the manufacturer datasheet
and application notes. As always, your feedback and comments are
welcome.

<http://www.nt7s.com/blog/2008/07/dual-gate-mosfet-investigations-gain-and-agc/>

73,
Jason Milldrum, NT7S
=====
Amateur Radio Station NT7S - <http://www.nt7s.com>
NT7S Blog - <http://www.nt7s.com/blog>
qrp-l.org Flickr Group - <http://flickr.com/groups/qrp-l/>
1787 2008-07-09 08:50:34 Weddig, Henning-C... Re: Dual Gate MOSFET Amp - Gain and AGC
Jason,

You missed one important "trick" please put always a resistor from the
drain to the output resonator!
This will prevent the amp to oscillate even on UHF frequencies!
Secondly bypassing the gate 2 is also important, as to avoid unwwanted
RF coupling.

Regards

Henning Weddig
DK5LV

Jason Milldrum schrieb:
>
>
> Hi Guys,
>
> I've written up part 2 of my investigation in the BF998 dual gate
> MOSFET. This installment deals with the gain of the amplifier in a
> typical configuration and the AGC characteristics of the amp. There's
> nothing too earth-shattering in here, but it's pleasing to see that
> the experimental data tends to agree with the manufacturer datasheet
> and application notes. As always, your feedback and comments are
> welcome.
>
> <http://www.nt7s.com/blog/2008/07/dual-gate-mosfet-investigations-gain-and-agc/
> <http://www.nt7s.com/blog/2008/07/dual-gate-mosfet-investigations-gain-and-agc/>>
>
> 73,
> Jason Milldrum, NT7S
> =====
> Amateur Radio Station NT7S - <http://www.nt7s.com <http://www.nt7s.com>>
> NT7S Blog - <http://www.nt7s.com/blog <http://www.nt7s.com/blog>>
> qrp-l.org Flickr Group - <http://flickr.com/groups/qrp-l/
> <http://flickr.com/groups/qrp-l/>>
>
>
1788 2008-07-09 09:50:15 Silva Re: Dual Gate MOSFET Amp - Gain and AGC
Hi Henning
please post the your suggestions on a schematic...
It is very interesting for all us...

Best regards from Brazil!!!

73´s,
Silva.

At 12:30 9/7/2008, you wrote:
>Jason,
>
>You missed one important "trick" please put always a resistor from the
>drain to the output resonator!
>This will prevent the amp to oscillate even on UHF frequencies!
>Secondly bypassing the gate 2 is also important, as to avoid unwwanted
>RF coupling.
>
>Regards
>
>Henning Weddig
>DK5LV
>
>Jason Milldrum schrieb:
> >
> >
> > Hi Guys,
> >
> > I've written up part 2 of my investigation in the BF998 dual gate
> > MOSFET. This installment deals with the gain of the amplifier in a
> > typical configuration and the AGC characteristics of the amp. There's
> > nothing too earth-shattering in here, but it's pleasing to see that
> > the experimental data tends to agree with the manufacturer datasheet
> > and application notes. As always, your feedback and comments are
> > welcome.
> >
> >
> <http://www.nt7s.com/blog/2008/07/dual-gate-mosfet-investigations-gain-and-agc/
> >
> <http://www.nt7s.com/blog/2008/07/dual-gate-mosfet-investigations-gain-and-agc/>>
> >
> > 73,
> > Jason Milldrum, NT7S
> > =====
> > Amateur Radio Station NT7S - <http://www.nt7s.com <http://www.nt7s.com>>
> > NT7S Blog - <http://www.nt7s.com/blog <http://www.nt7s.com/blog>>
> > qrp-l.org Flickr Group - <http://flickr.com/groups/qrp-l/ >
> <http://flickr.com/groups/qrp-l/>>
> >
> >
>
>------------------------------------
>
>Yahoo! Groups Links
>
>
>
**************************************************
Brazilian Ham Radio - ARRL Member
Callsign: PY8ELO
Name: Silva
Grid Locator: GI25ar
Address: POBOX 151 CEP 68180030
ITAITUBA - PA - BRAZIL
**************************************************
AMAZON FOREST, THE WORLD´S LUNG!
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1789 2008-07-09 16:21:37 Jason Milldrum Re: Dual Gate MOSFET Amp - Gain and AGC
Hello Henning,

Thank you very much for the feedback. I've seen that swamping resistor
used on the drain in many, but not all circuits. I decided to leave it
out of the circuit for now to see how stable the amplifier would be,
but I intend to do some more experimentation with it in the circuit
later. This is one of those instances where I wish I had a spectrum
analyzer good to a few GHz to see what kind of parasitic oscillations
that I get off of the amp. Also, it's my understanding that the
addition of this swamping resistor will degrade the noise figure, is
that correct? I would like to make some measurements with the resistor
in and out of the circuit to find the difference.

Regarding gate 2 bypassing, are you saying that the bypass cap needs
to be connected directly to the gate instead of its current location
at the AGC voltage input? Would that really make a significant
difference?

Thanks again for the comments, keep them coming!

73,
Jason NT7S

1794 2008-07-10 13:29:54 Wes Hayward Re: Dual Gate MOSFET Amp - Gain and AGC
Hi Jason,

Yes, it can make a profound difference. The capacitor really need
to be right at the gate. If you don't bypass gate 2, you will dump
the noise voltage from the resistor driving gate 2 right into the
FET. With 100K, that voltage will be high. This could really trash
the amplifier NF performance, turning a stellar performer into
something that is pretty bad for NF. But use a really good bypass
cap that is going to do a good job at VHF and UHF. A 1000 pF with
really short leads is good. This is a good place to use a chip cap
even if you are among the folks who don't like SMT parts.

There are two resistors that we often see in the drain circuits.
One is a swamping resistor that is directly across the primary (drain
winding) of the output transformer. This merely constrains the gain
to a smaller value. It also serves to provide a clean output R.
This loading R will help to stabilize the amplifier, but will not do
a lot at UHF.

The other resistor is one that is right in series with the drain.
This is often in the region of 20 to 100 Ohms. This serves to
provide a wideband load that kills UHF oscillations. The utility of
this resistor can be studied with a microwave stability analysis,
easily done with numerous programs, or from scratch if you are
willing to do some analysis. You will see examples of the small
drain R in some of the amplifiers in emrfd.

73, Wes
w7zoi
1805 2008-07-12 00:31:31 Jason Milldrum Re: Dual Gate MOSFET Amp - Gain and AGC
Thanks Wes,

Sometime I feel incredibly dense! Of course that 100k resistor is
going to develop a large voltage across it with the bypass cap on the
other side (Ohm's Law strikes again). Like you stated in your 50 MHz
preamp, there's no reason for that thing to be in there except for
convenience. I'm taking it out when I continue with my experimentation
and sticking that 1nF bypass cap right on gate 2. Thanks for setting
me straight...couldn't see the forest for the trees. :)

Also many thanks for the explanations of the two resistors. I'm going
to have to break out LTSpice to see what's going on with that 100 ohm
VDD resistor. I've been trying to wean myself off of so much
simulation, but this seems like an excellent area for simulation given
that I don't have the test gear to really see these effects in the
real world.

73,
Jason NT7S