Re: How to explain how negative feedback lowers noise?

Chuck Harris <cfharris@...>

The noise that is coming back through the feedback channel
and being negatively combined at the differential amplifier
on the input is not the same noise that is coming in the
input. It is random, so it is just as likely that the
negative feedback will be increasing the noise as it is
that it will be decreasing it.

I think where the unwashed masses get confused is between
noise and distortion. Negative feedback can decrease the
distortion in an amplifier, so the THD can decrease... but
that is distortion, it may annoy you, but it is not noise.

-Chuck Harris

On Thu, 25 Mar 2021 16:27:44 -0400 (EDT) "n4buq" <>
Yes, I understand how these work. My question was more about why the
explanation where noise from the first stage of an amplifier is fed
back (negated, inverted, or, ?) to the input which seems much like a
noise-canceling headset to me. I inferred from the sighs that the
previous explanation was incorrect and (sorry) but I don't understand
why that's the case.

Were the "sighs" meant to indicate that the example given was
incorrect or, perhaps, just more of the same already-covered
explanations? Sorry - sighs can a bit more meaningful when combined
with an eyeroll.

Barry - N4BUQ

----- Original Message -----
From: "Göran Krusell" <>
Sent: Thursday, March 25, 2021 3:18:58 PM
Subject: Re: [TekScopes] How to explain how negative feedback
lowers noise?

Hi Barry,
No sighs from my desk, your question is a good one. I think these
modern headphones work in the following manner, a small amount of
noise comes through the leather in your headphones. At the same
time the external noise is picked up by a small microphone in the
headset, amplified by an inverting amplifier and added to the
direct noise to you ear in equal amounts. The noise is thus
attenuated. And it does work, I have one such headphone from a
well-known company. Göran

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