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Post subject: Stability circles
Unread postPosted: Tue Mar
29, 2005 8:52 am
I have 2 questions regarding
For example the input stability circle contains
the input impedances that cause to Gin<1. To make the network stable
in the input domain means to find passive load impedances that would
cause to Gin<1.
The intersection of the Gload domain with
Gin domain splits the Gload domain into 2 regions: Unstable one and
stable one. I saw in a tutorial that the discussion over stability is
divided into 2 cases: One for S11<1 and the other for S11>1. According
to S-parameters definitions:
Gin=S11 If ZO=Zload. If Zload=Zo then
S11=0 and that must be a stable region. For each of these 2 conditions
2 possible situations are possible:
1) The Gin circle covers
the center of the Smith chart
2) The Gin circle doesn't cover the
center of the Smith chart
For the first condition the area of
the Smith chart outside Gin circle is the unstable region.
second condition the entire Gin circle is defined as the unstable region.
My queation is regarding the second condition: Why is the entire Gin
circle is unstable and not the area outside the Gin circle as in the
Also why the network is defined as unconditionally
stable when there is no intersection between the Gin circle and the
Gload circle (Smith chart).