Re: Differential Pair Input Noise Issue

thanks, your explanation is the best even though I read your post for
a couple of times.
2k ohm is a big value based on my understanding of thermal noise
that's why I posted this question here. Right now I understand the
author's words-he thinks 2k ohm is small enough to minimize thermal
noise. In stead I think the author should pick a op-amp running under
lower supply voltages then pick a smaller resistor.




On Aug 17, 6:20 pm, bill.slo...@xxxxxxxx wrote:
On Aug 18, 9:06 am, bigca...@xxxxxxxxx wrote:

Here is picture:

http://www.flickr.com/photos/26914086@N05/2772737582/sizes/o/

I didn't figure out why "run a lot of current ot reduce the voltage
noise".

Noise current is proportional to the square root of the total current,
and resistor (Johnson) noise is proportional to the square root of the
resistance value; by going for a higher current and lower resistors at
that point, the designer has minimised the noise contribution atthat
particular point.

And how to pick the circled resistor?

Small enough the op amp doesn't swing to the rails (and saturate its
internal gain paths) under normal operating conditions.

--
Bill Sloman, Nijmegen

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