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"
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.
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.