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.

In analog designer's word, Positive feedback.

In microwave designer's word, potentially unstable.

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

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

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.

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.

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

K is a natural number. You want to plot it vs. frequency?
In ADS there is an icon for stability in S-Simulation, possibly you can write an equation to extract it.

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

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

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.

agree with djalli

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

Mazz
Yes, clear now.
Thanks

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.

Hi dsjomo,

Thanks again.

some useful applications

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

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,

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