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The stability of LNA?

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lna stability

Many said that if S12 is too large will turn an LNA to an osillator?

How come? is there any theory or qualitative analysis on this issue?

Thanks.
 

stability factor lna

In analog designer's word, Positive feedback.

In microwave designer's word, potentially unstable.
 

stern stability factor

dsjomo said:
In analog designer's word, Positive feedback.

In microwave designer's word, potentially unstable.
Click to expand...
i agree wih u,but he may choose another amplifier with small s12,or using techniques that explained in gonzaliz book of amplifer design,that using 1/s12 insted of s12
regards
 

stability in lna

The definition of stability factor (from ADS help)
Given a 2 x 2 scattering matrix between the input and measurement ports, this function calculates the stability factor.

The Rollett stability factor is given by

k = {1- |S11|**2 - |S22|**2 + |S11*S22 - S12*S21| **2} / {2*|S12*S21|}

The necessary and sufficient conditions for unconditional stability are that the stability factor is greater than unity and the stability measure is positive.

Reference

[1] Guillermo Gonzales, Microwave Transistor Amplifiers, second edition, Prentice-Hall, 1997.

As you can see, the S12 is directly involved in this function, and, as the S12 increases k decreases.

I hope it can help.

Mazz
 

lna stability considerations

A non-zero S12 represents a signal path from the transistor output to the input. This feedback path is an opportunity for oscillation to occur.
 

stability of lna

In physics: Stability Factor (Stern Stability Factor) is says nothing but a measure (some propability factor measure indeed) of occurence of phenomena of oscillation which are EM waves in all directions transverse far from the source, zero intensity along the current axis of EM wave normal-mode frequency. Considering EM wave is harmonic.

Oscillation can be damped, underdamped, or overdamped.
 

stability measure ads

Mazz said:
The definition of stability factor (from @DS help)
Given a 2 x 2 scattering matrix between the input and measurement ports, this function calculates the stability factor.

The Rollett stability factor is given by

k = {1- |S11|**2 - |S22|**2 + |S11*S22 - S12*S21| **2} / {2*|S12*S21|}

The necessary and sufficient conditions for unconditional stability are that the stability factor is greater than unity and the stability measure is positive.

Reference

[1] Guillermo Gonzales, Microwave Transistor Amplifiers, second edition, Prentice-Hall, 1997.

As you can see, the S12 is directly involved in this function, and, as the S12 increases k decreases.

I hope it can help.

Mazz
Click to expand...

Hi, how to plot the K factor in ADS? Thanks.
 

stability factor mu

k is a real number (please see the definition).
you can write the equation in the @DS data display or you can use the icon "stabilty" (or something similar..) in the S parameter menu. in the second case you'll find "k" in the dataset, that's it.
I hope it can help.
Mazz
 

rollett stability s parameter

As a rule of thumb for any microwave amplifier, that if it simply finds an open circuit load it should oscillate
 

lna oscillation phenomena

djalli said:
K is a natural number. You want to plot it vs. frequency??
In @DS there is an icon for stability in S-Simulation, possibly you can write an equation to extract it.
Click to expand...

Yes, what I want to know is a convenient way to plot K vs. frequency in ADS. When designing an LNA, we can not only focus on the desired frequency, we also need to ensure that it will not oscillate at other frequency. See figure 5 in the attachement.
 

stability measure gonzalez

agree with djalli
 

stability factor from s-parameter plot

cmosbjt
there is something not clear with my previous explanation?
Mazz
 

s-parameter k factor stability

Mazz
Yes, clear now.
Thanks
 

guillermo gonzales

cmosbjt, there is a instance named StabFact under s-parameter simulation tag under ADS, use that instance you can plot k factor in ADS. That is the convinient way you want.
 

k-stability factor and mu factor

dsjomo said:
cmosbjt, there is a instance named StabFact under s-parameter simulation tag under @DS, use that instance you can plot k factor in @DS. That is the convinient way you want.
Click to expand...

Hi dsjomo,

Thanks again.
 

stability mu factor vs k factor

some useful applications
 

lna k-factor stability

hcseu said:
some useful applications
Click to expand...

I have read the file, it is very useful to me ,Thank you vmuch!
 

ads stability factor

cmosbjt said:
Hi, how to plot the K factor in ADS? Thanks.
Click to expand...

Form the data display window, add new equation like this: k=Stab_Fact(S) and plot it on rectangular plot..

This should be done in an S-Parameter simulation..

I hope that helps,
Regards,
 

what is unconditional stability of lna

you can also use a necessery and sufficient condition for stability in ADS (Mu factor)
 

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