Stability factor of differential LNA

I don't think this makes any sense. Is it even meaningful to measure stability when you have an ideal voltage-controlled-voltage-source in the circuit?

Andrew.

Hello Mr. Andrew,

I do want to remove it and add buffers instead, I just use it to avoid loading effect. 

The issue is that I have a Balun LNA, the CG side voltage gain goes from 12dB to 4 dB when I remove the VCVS, also most of the references use "infinite output impedance", therefore I assumed a VCVS.

To check for my stability, is it ok to add buffers (which will drop the gain by -4dB) check my stability factor? or does a VCVS make the circuit unconditionally stable?

if not, what do you recommend I do for stability analysis? (Completely remove VCVS and add buffers and make that my new circuit?)

P.S: I am very happy that you answer me Mr. Andrew, I see you all over the forum helping people.

Hello Mr. Andrew,

I do want to remove it and add buffers instead, I just use it to avoid loading effect. 

The issue is that I have a Balun LNA, the CG side voltage gain goes from 12dB to 4 dB when I remove the VCVS, also most of the references use "infinite output impedance", therefore I assumed a VCVS.

To check for my stability, is it ok to add buffers (which will drop the gain by -4dB) check my stability factor? or does a VCVS make the circuit unconditionally stable?

if not, what do you recommend I do for stability analysis? (Completely remove VCVS and add buffers and make that my new circuit?)

P.S: I am very happy that you answer me Mr. Andrew, I see you all over the forum helping people.

I'll leave it to others to answer this - I don't have practical experience myself of designing using the Rollett K factor for stability (my design background was more analog than RF design, and so for that stability was more about feedback loops for which stb analysis is more appropriate). However, looking at the definitions in various texts and sites such as this: https://www.microwaves101.com/encyclopedias/stability-factor you may find some useful info. I'll ping one of my colleagues to see if they can give more insight on the right way to approach this. Maybe you should read some Microwave textbooks on this or even John Rollett's original paper.

Regards,

Andrew.

Thank you Mr. Andrew

Any help would be extremely appreciated.

Regards,

Qusai

Hello Mr Andrew,

Any update on this matter?

I would really appreciate any tips or tricks to test stability with VCVS as the buffers are decreasing my gain a lot and the CG gain with buffers is 4dB down from 12dB.

Regards,

Qusai

Qusai,

No, as I said in my earlier reply, I can't really help with this. To be honest, I can't really see how Kf would work with a vcvs used (and I'm not sure how that helps in a real circuit anyway).

Andrew.

Dear Qusai,

I was not aware of your post until I read Andrews response to your request for additional thoughts. I would have added my comments earlier - sorry!

The methodology you are using to establish absolute stability appears to differ from the methodology I believe most LNA designers use. Typically, after the optimum match (which may not be perfect) to your source impedance has been determined, the impact of load impedance is considered in light of the stability. Using an ideal vcvs as the load seems totally unrealistic as a load as you can never realize it. If your load is known, that load is used to assess stability. If not, one determines the stability as a function of the range of expected possible load impedance. The load impedance will clearly impact gain of the LNA, so if a specific gain is required, this will limit the possible load impedances. In summary, finding the stability using an ideal vcvs load impedance is not really a relevant measure of stability. If it is useful, a presentation made by Avago on a similar topic might be useful at URL:

www.ntms.org/.../MUD_W5LUA_LNAs_Web.pdf

I hope, Qusai, I understood your question correctly...and my thoughts help a little.

Shawn