Find the input impedence of LNA

[kwanmajun]

Recently i got a interview question to find the input impedence of LNA, the question is
" The rf input is connect with a cap C, the bias resistance is R, a inductance is connect from the cmos source side to ground, the cmos pull up voltage is 1.8V. then what is the input impedence"

thanks, please answer with details, thanks~

 

[kspalla]

probably, mostlikely, he would be intereted to know that how do you proceed in case of RLC circuit.
you should have told about how to calculate Zth, assuming some antenna impedance X+jY.

Because it is difficult to find the impedance without measurements and modeling.

 

[Element_115]

You could be a wise guy and say it's S11(Looking in to the Cap).

You need to know the S-parameters of the LNA and the Frequency of interest.
I would assume that "C" is large ~-j0.1X & "L" is a RF Choke so ~ +j1000X.
So it would be like {(LNA S11) // R} - 1/ωC

And kspalla is right, he would most likely want to see how you handle something
out of left field.