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Questions about stability circle and impedance matching (Read 286 times)
Fr4gs7aR
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Berkeley, CA
Questions about stability circle and impedance matching
Jan 14th, 2019, 2:43pm
 
Hi all,

I'm a RF circuit beginner and got stuck in stability related issues.

According to Pozar's textbook, output stability circle is a contour in the "gamma_L" plane and its arc represents the "gamma_L" point that makes |gamma_IN|=1

so, from the above statement, it seems to me that the amplifier is stable when "gamma_L" is sitting outside of the output stability circle while the output stability circle DOES NOT encircle the origin of the Smith chart.

But, one of my friends (who is currently studying towards phd in RF circuit area) told me that I need to ensure that S11 is sitting outside of load stability circle for output stability.

I raised following questions but I couldn't really understand his logic when he answered my question. so I was wondering if anyone in this forum can help me to understand this topic better.

Questions were:

1) Is "load stability circle" equivalent to "output stability circle"? According to Keysight manual, it is equivalent but some papers
https://www.mwrf.com/components/ensure-stability-amplifier-designs
is saying completely opposite. this part is very confusing to me.

2) Why do we need to look at S11 instead of Gamma_L?
technically output stability circle is drawn on "Gamma_L" plane (Pozar Textbook). Looking at S11 for output stability circle does not make any sense to me since S11 is sitting its own plane and not related to Gamma_L. Even if they are related, its very hard to visualize on Smith chart how they are interacting when it comes to stability issue. Plus, output stability circle is already representing the gamma_L point corresponding to "|Gamma_IN|=1". What does S11 do with this?

please correct me if I am understanding something wrong!
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