dog1
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Hi
Thanks for the reply and sorry for the late response. However, I am still not convinced. Aaron, S11 = b1/a1 | b2 = 0, b1, a1 is referred to port 1, and to keep b2=0, I think it doesn't matter what is used as port 2, you have to ensure that the output is matched to the load. Yet you cannot change the two-port circuit itself, which means that the output load have to be changed (automatically) in order to match the output impedance of the circuit. So changing the loading port shouldn't matter. tm123, I think you are half right. however, I think that if the LNA can be described as a linear two port system, which I think is typically the case(actually I am just playing with an common source, and Using sp analysis in spectre, so I think linearity shouldn't be a problem), then the S parameter matrix should be enough to describe the behavior of the circuit. That is, S11= b1/a1 |a2=0 should stand as long as the LNA can be described as a linear two port system, even if in the test bench a2 isn't 0. put it another way, although in the test bench a2 isn't zero because I do not have output impedance match, when calculating S11, I can assume that the circuit is loaded by a matched load and use S11= b1/a1 and it should gives me the same result.
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