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input coupling capacitor and noise figure

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ansonyeap

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Hi all. I doing an design on LNA and my LNA utilized a capacitor as DC block to isolate the DC and AC signal at the input of the LNA. The schematic is as show in the picture attached.

the Cin suppose to be shorted when operating at 2.14GHz frequency but, I notice that If I put the required capacitance in the design, the noise figure of the system also increased. but if i change to a lower capacitance, the noise figure drop. Can any one tell me why this happening? any ideal? or explanation to this?

thank in advance.
 

How did you simulate it without the capacitance? A DC connected signal would alter the biasing.

Keith.
 

you can consider your input matching. Cin is part of your input matching and better the matching, closer will be your NF to the NFmin (Minimum Noise Figure). so, may be your input matching improves when you reduce your Cin ..
 

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