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BSEE - KB3UON
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S-parameter for a FET - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: S-parameter for a FET Posted: Wed Aug 10, 2005
I've a quick question; let's say i'm looking at a smith
chart, how would i know if it's a standard FET's response. How does
a standard FET look like on smith chart? How about MESFET above 5 GHz?
Post subject: Posted: Tue
Aug 30, 2005 1:42 am
Joined: Tue Mar 15,
2005 11:43 pm
not too sure what do you mean by standard FET?
of device on smith chart depends on its impedance.
e.g. S11 relates
to input impedance, while S22 relates to output impedance.
type FET has different S-parameters, even same FET has different S-parameters
under different bias points.
But generally, the S-parameters
for both input and output will be inside smith chart; |S11| <1 and
|S22|<1, that will give you most of the cases, potentially unstable
conditions, although, maybe there could be unconditionally stable also....
I hope it helps.
Post subject: Posted: Tue Aug 30, 2005 2:04 pm
you might want to take a look at the small signal model of a fet from
a electronic circuit book.
Basically you see the series inductances,
and series and parallel capacitance parasitics in front of the gate
source resistance, and on the drain the dynamic resistance of the fet.
The electrical length to the reference plane might shift all this
a bit also.