aaron_do
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Hi baab,
the L and C form your tank. So the resonant frequency is found from ω2 = 1/LC.
1) C is made up of two parts. First the parasitic capacitance of your devices and the load. Second is however much additional capacitance you decide to add in parallel with the circuit. Often switchable capacitors are added to critical nodes in order to tune the resonant frequency (especially in a test chip). Otherwise, C should be as small as possible if you want the highest possible gain.
2) For the inductance, L, making it as large as possible while keeping the resonant frequency the same will generally get you the highest gain. You need to study about inductor design, and actually try to design inductors before you will really know the limitations here. But basically Rp = QωL, so the larger the L, the larger the Rp, and the higher your gain.
3) It should be obvious at this point that Rp is just the parasitic resistance of your inductor, but depending on the technology, the output resistance of your transistor might play some part. You don't want to deliberately add a resistor here. Also, your comment about large resistance = high noise is incorrect. you have to consider whether the resistance is in series or shunt to the signal path. If it is in series, then the more resistance, the higher the noise. But if it is in shunt, then the smaller the resistance, the higher the noise.
When I mentioned maximizing L, the goal was to get the highest possible gain. This will give you a nice big signal at the output that is insensitive to the next stage's noise. However, you don't want to saturate your LNA, so depending on your design, you may not want the highest possible gain anyway. Unfortunately, it isn't really possible to design the LNA without knowing what kind of load it is driving (i.e. the mixer).
regards, Aaron
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