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Signal flow graph for two-port network (Read 2550 times)
baab
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Signal flow graph for two-port network
Feb 25th, 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

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aaron_do
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Re: Signal flow graph for two-port network
Reply #1 - Feb 25th, 2014, 9:14pm
 
Hi baab,


from the diagram, it looks like bs2 is a1*S21 under the condition that ΓL = *ΓOUTBut if that's the case, then

bs2 = S21.bs/(1-S11ΓS)

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, S12ΓSS21, and then you could even have multiple reflections etc. However, setting the condition ΓL = *ΓOUT 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
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baab
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Re: Signal flow graph for two-port network
Reply #2 - Mar 8th, 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_lect...

And you are right. That is a typo.
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