The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> Signal flow graph for two-port network
https://designers-guide.org/forum/YaBB.pl?num=1393321689

Message started by baab on Feb 25^th, 2014, 1:48am

Title: Signal flow graph for two-port network
Post by baab on Feb 25^th, 2014, 1:48am

Hi, this is part of an application note entitled "Low-Noise Amplifier Stability Concept to Practical Considerations" from Maxim.
Could you help me with the question in the picture? Thanks.

Source: http://www.maximintegrated.com/appnotes/index.mvp/id/1849

Title: Re: Signal flow graph for two-port network
Post by aaron_do on Feb 25^th, 2014, 9:14pm

Hi baab,

from the diagram, it looks like b_s2 is a₁*S₂₁ under the condition that Γ_L = *Γ_OUTBut if that's the case, then

b_s2 = S₂₁.b_s/(1-S₁₁Γ_S)

so either I'm wrong or there's a typing error...

normally if there is reflection at the load, the power will go back to the device, and some of the power will reflect back to the load through S₂₂, and some will take the longer path, S₁₂Γ_SS₂₁, and then you could even have multiple reflections etc. However, setting the condition Γ_L = *Γ_OUT ensures that any power reflected at the load will never find its way back to the load again. So I'm thinking you can calculate the power to the load with the S₁₂ path removed. That's how I got the formula above. Unfortunately, I don't have any of my RF references handy and I haven't looked at this stuff in a while...

regards,
Aaron

Title: Re: Signal flow graph for two-port network
Post by baab on Mar 8^th, 2014, 10:58pm

Thank you.
I have spent a long time reading about S parameters and got basic understanding about it.
I found the great lecture here:
http://www.ittc.ku.edu/~jstiles/723/handouts/section_4_5_Signal_Flow_Graphs_lecture.pdf

And you are right. That is a typo.