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S-Parameter for a FET - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


Guest
Post subject: S-parameter for a FET Posted: Wed Aug 10, 2005 2:37 pm
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? Thanks


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Jeanalmira
Post subject: Posted: Tue Aug 30, 2005 1:42 am

General


Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi :

I'm not too sure what do you mean by standard FET?

The S-parameters of device on smith chart depends on its impedance.
e.g. S11 relates to input impedance, while S22 relates to output impedance.

Different 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.

Regards,
Jean


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Guest
Post subject: Posted: Tue Aug 30, 2005 2:04 pm
I think 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.



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