3-port S-parameter question

[jl010i]

If I have a resitively loaded common-source amplifer and I have 3 ports (one connected to the gate, one connected to the source and one connected to the output) all terminated with 50Ohms. What would be the refection coefficient be at each port?

I would assume that they all would equal 0 because each is terminated in 50Ohms, however, I am not sure. Thanks for any help in advance.

 

[flatulent]

S11, S22, and S33 are determined by the transistor, not the external loads on the transmission lines to the transistor. Since this is not a passive, lossless system, The usual sum of different parameter equaling 1.000 does not hold true.

 

[jl010i]

So if I want to find the gain from one port to another, I need to know the reflection coeficients from each port, even though they are all terminated in 50Ohms? Is that what you are saying?

 

[flatulent]

You will need to know all nine S parameters. You can make it easy on yourself by turning it into a two port network by leaving one of the transistor terminals inside and connecting it to whatever is necessary. Do not forget to put the bias on the transistor.

 

[jl010i]

I guess what I am asking is how do find the gamma at each port? For instance, if I inject a signal at the source, what will be the gamma(s) do to the reflection at each of the other two ports?

 

[flatulent]

You measure it in the same way as any transmission line problem. There are specialized instruments for getting the magnitude and phase. If you just want the magnitude, you can use a directional coupler.