# More Stability Circles Questions - RF Cafe Forums

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IR1
Post subject: Stability Circles
Unread postPosted: Tue Jul 13, 2004 9:28 am

Hi,

I have a question regarding stability circles: Do these circles include the impedances which gives reflection coefficients greater than 1: S11>1, S22>1?

The intersection of these circles with the Smith chart gives the areas of instability, matching of the network (e.g Amplifier) to these regions should be avoided...Does this fit to my question above?

Many thanks,
IR1

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Jeanalmira
Post subject:
Unread postPosted: Tue Mar 15, 2005 5:20 am

Hi,

For designing amplifers and oscillators, the stability of the two
port network must be considered. Oscillation is possible if either
the input or output port impedance has a negative real par ;this
would imply that gamma in >1 or gamma out >1. In case of assume S12=0 : S11 >1 and S22 >1.

While there are 2 types of stable conditions :
1. Unconditionally stable : where gamma in and gamma out are less or equal to 1; K <1 and delta <1.

2. Potentially unstable (Conditional stability)
The network is conditionally stable if
gamma in is less or equal to 1 and gamma out is less or equal to 1 only for a certain range of passive source and load impedances.

Then, back to your questions on stability circles.
1. Output stability circles can be determined by set gamma in =1.
2. Input stability circles can be determined by set gamma out =1.

the stability circles can represent S11 >1 and S22 >1. It depends on the conditions. But one thing to remember that inside smith chart, S11 and S22 <1.

any doubts or feedback are welcome.
Hope it helps.. :D

Sincerely,
Jean

Posted  11/12/2012

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