Why Is Zin function in ADS not suitable for measuring active circuit?

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orz

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I have tried to use Zin function of S-para to measure passive circuit (ex. just R of 100 Ohm) and result is right(no matter how i changed the impedance of Term's).
but when i use it to measure active circuit, the result seems to be the function of impedance of Term's. Why?
Thanks!
Best wishes!
 

I presume that your active circuit includes DC sources to bias the circuit and set up the appropriate device operating points. Is the circuit DC or AC coupled? If you have a DC coupled circuit, with no DC blocking capacitors, then if you change the resistance of the Term then this will change the loading on the circuit and change the operating point. This could lead to changes in Zin. If the circuit is AC coupled this should be less of an issue.
 
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    orz

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You're right! My circuit has dc current sources and voltage source to bias mosfet
I don't quite understand DC coupled.
Would you please explain it? Thanks!
Best regards!
 

Do you have DC blocks/series capacitors at the input and output pins of the schematic to prevent DC current flowing out of the circuit and through the Term components? You need them. Any DC current path through the Terms will load the circuit and change the operating point as the resistance is varied.
 
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    orz

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Well, I didn't have it.
You are right, if current flow to the outer circuit from inner sources, the outer circuit became part of the active circuit.
Then if I changed the impedance of term , mosfet's bias would change.
I really appreciate your help!
 

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