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Stability circles questions - RF Cafe Forums

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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:
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.
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).

Many thanks,

Posted  11/12/2012
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