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SP simulation and negative real Y (Read 6445 times)
xristigi
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SP simulation and negative real Y
Aug 13th, 2008, 6:56am
 
Hello everybody,

I have been running sp simulation to find the equivalent parallel impedance in a node of my circuits and it seems that :

Yreal=-1m
and Yimag=3m

Should I worry for this negative real part?? Does it imply instability??
What does it mean exactly?

Thanks in advance
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pancho_hideboo
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Re: SP simulation and negative real Y
Reply #1 - Aug 13th, 2008, 8:38am
 
xristigi wrote on Aug 13th, 2008, 6:56am:
I have been running sp simulation to find the equivalent parallel impedance in a node of my circuits and it seems that :
Yreal=-1m
and Yimag=3m
Should I worry for this negative real part?? Does it imply instability??
What does it mean exactly?

It could be conditionally stable.
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« Last Edit: Aug 13th, 2008, 5:31pm by pancho_hideboo »  
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xristigi
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Re: SP simulation and negative real Y
Reply #2 - Aug 13th, 2008, 9:02am
 
Thanks,

I also came to that conclusion... I also realized that it is due to the emitter followers that I have connected there. I guess I should check stability for the given source impedance.

Thanks again
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pancho_hideboo
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Re: SP simulation and negative real Y
Reply #3 - Aug 13th, 2008, 9:19am
 
xristigi wrote on Aug 13th, 2008, 9:02am:
I guess I should check stability for the given source impedance.

You have to check stability both for source and load impedances.
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buddypoor
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Re: SP simulation and negative real Y
Reply #4 - Aug 13th, 2008, 10:12am
 
Quote pancho_hideboo: It's conditionally stable


Conditional stability is a specific term from the control theory describing a system which can be stabilized by increasing the loop gain.
Presently, I cannot see how one can come to the conclusion that a negative real part of the input conductance of a system indicates this kind of "conditional stability". Perhaps something else is ment ?
Question to pancho_hideboo: What does this mean for the system under discussion ? What has to be done to destabilize the system or to keep it stable ?    
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LvW (buddypoor: In memory of the great late Buddy Rich)
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pancho_hideboo
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Re: SP simulation and negative real Y
Reply #5 - Aug 13th, 2008, 5:04pm
 
buddypoor wrote on Aug 13th, 2008, 10:12am:
Conditional stability is a specific term from the control theory describing a system which can be stabilized by increasing the loop gain.

No, you are wrong. Maybe you are not familiar with RF circuit theory.
Terms, "Conditionally Stable" and "Unconditionally Stable" are also used in RF circuit.
But these can not be completely related to stability theorem in control system theory.

http://www.designers-guide.org/Forum/YaBB.pl?num=1190272820

We can judge whether an amplifier is unconditionally stable or not by well-known K-factor or Mu-factor.

See stab_fact() and mu() in "Chapter 9: S-Parameter Analysis Functions" of the following.
http://cp.literature.agilent.com/litweb/pdf/ads2005a/pdf/expmeas.pdf

buddypoor wrote on Aug 13th, 2008, 10:12am:
Presently, I cannot see how one can come to the conclusion that a negative real part of the input conductance of a system indicates this kind of "conditional stability". Perhaps something else is ment ?

Assume a simple amplifier. Here I define followings.
  Gamma_S=Source Reflection Coefficient
  Gamma_In=Input Reflection Coefficient
  Gamma_L=Load Reflection Coefficient
  Gamma_Out=Output Reflection Coefficient

  StabFactor_S=real(Gamma_S*Gamma_In)
  StabFactor_L=real(Gamma_L*Gamma_Out)

If both StabFactor_S<1.0 and StabFactor_L<1.0 are satisfied, this amplifier is stable. This is conditionally stability.
And this conditionally stability could be satisfied even if input impedance or ouput impedance have negative real part.
This stability is dependent on source and load impedances.
For example, when an input of amplifier is left open or short, abs(Gamma_S)=1.0. If this amplifier has real(Yin)<0 or real(Zin)<0, abs(Gamma_In)>1.0. So this amplifier could not be stable.

buddypoor wrote on Aug 13th, 2008, 10:12am:
What does this mean for the system under discussion ? What has to be done to destabilize the system or to keep it stable ?    

For example, an amplifier of real(Yin)<0 could be stable if shunt conductance is inserted at input of amplifier so that it cancels real(Yin)<0.
Of cource, this too simple stabilizer degrades gain of amplifier.
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« Last Edit: Aug 14th, 2008, 12:12am by pancho_hideboo »  
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