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Design >> Analog Design >> Regarding noise
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Message started by raja.cedt on Dec 15^th, 2011, 3:15am

Title: Regarding noise
Post by raja.cedt on Dec 15^th, 2011, 3:15am

hello every one,
i have question regarding noise of an cascade stages. Let us say you have two stages, so to find the noise generally find the total TF of the system and multiply the spectral density and find the integral. This is very common and every one do this. But think about an opamp, at the input assume you have small gate resistance and gate capacitance (which is very small of course), so integrated noise at the opamp input very high (KT/C, due to small cap). any thing wrong in this?

Thanks,
Raj.

Title: Re: Regarding noise
Post by Sanjay Rajasekhar on Dec 15^th, 2011, 5:04am

The noise power of an RC circuit is kT/C because R sets the bandwidth and the noise density as well. So it cancels out. You'll see the full power of kT/C when you have a system which does not filter this noise out and reduce its power.

In a practical scenario when you have a very small RC at the input of the opamp, the noise power, although high, is spread over such an incredibly high bandwidth that most of the power is going to get filtered out by other time constants in the system, like the gm/C of the opamp itself.

Regards,
Sanjay

Title: Re: Regarding noise
Post by raja.cedt on Dec 15^th, 2011, 6:30am

hello sanjay,
thanks for reply, ya i forgot about that filtering. Thanks for reminding. I would like read about more noise filtering in cascade. Do you have reference on that?

Thanks,
Raj.