Signal flow graph for two-port network

[anhnha]

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/app-notes/index.mvp/id/1849

https://www.physicsforums.com/attachment.php?attachmentid=67016&stc=1&d=1393322023

 

[Baluncore]

Can you please give us a little more information.
Which figure or diagram are you referring to ?
What is your actual question ?

 

[anhnha]

Thank you. I thought I figured it out.
My confusion was from why the left diagram is equal to the right one in Figure 2.4. I was told that node a2 doesn't depend on the input bs and therefore to change from left diagram to the right one all I need is to calculate b2 as a function of bs.