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Why is it that in a cascode LNA, only the input transistor contribute to the noise?

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boshiouke

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Why is it that in a cascode LNA, only the input transistor contribute to the noise, while the cascode transistor doesn't? Thank you.
 

vfone

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Re: cascode LNA

If the parasitic capacitance of the node between cascaded LNA’s is high, the noise contribution of the second LNA could be significant at high frequencies.
 

boshiouke

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Re: cascode LNA

Thanks for the reply, vfone.

In CMOS LNA, since noise current is proportional to gm, the cascode transistor's gm is degenerated by input transistor, so I suppose that explains why it contributes less. But how about bipolar LNA? The shot noise is 2qI. How does it work over here? Thanks.
 

zigbee2005

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cascode LNA

you can find the answer in the Razavi's book.
 

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