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Post subject: Stability circles
Unread postPosted: Tue Mar 29, 2005 8:52 am
I have 2 questions regarding stability circles:
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:
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.
For the 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 first condition?
Also why the network is defined as unconditionally stable
when there is no intersection between the Gin circle and the Gload circle (Smith chart).