# Official Definition of Noise Gain

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#### Jim Thompson

Jan 1, 1970
0
What is the "Official Definition of Noise Gain"?

I'm looking for a good clean analytical method I can add into PSpice.

Pease's article is, as usual, clear as mud ;-)

...Jim Thompson

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Jan 1, 1970
0
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#### John Larkin

Jan 1, 1970
0
What is the "Official Definition of Noise Gain"?

I'm looking for a good clean analytical method I can add into PSpice.

Pease's article is, as usual, clear as mud ;-)

...Jim Thompson

UMM, "how much the opamp's input voltage noise is amplified by the
time it gets to the output"?

Or maybe

"If, in a given circuit, one added a floating AC voltage source of
unit amplitude in series with an amplifier's input pin, how many units
of that signal would appear at the output"?

Noise gain must of course be a function of frequency, although one
could default to the midband value.

John

J

#### Jim Thompson

Jan 1, 1970
0
That's silly. Noise gain has nothing to do with noise.

John

Of course not, but did you actually READ the paper ?

...Jim Thompson

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#### Jim Thompson

Jan 1, 1970
0
UMM, "how much the opamp's input voltage noise is amplified by the
time it gets to the output"?

Or maybe

"If, in a given circuit, one added a floating AC voltage source of
unit amplitude in series with an amplifier's input pin, how many units
of that signal would appear at the output"?

Noise gain must of course be a function of frequency, although one
could default to the midband value.

John

You are clearly not trained in the area ;-)

...Jim Thompson

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#### John Larkin

Jan 1, 1970
0
Of course not, but did you actually READ the paper ?

...Jim Thompson

Skimmed it. Too complicated to actually read.

John

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#### John Larkin

Jan 1, 1970
0
You are clearly not trained in the area ;-)

...Jim Thompson

Of course nobody "trained" me to design electronics. I'm not a dog.

John

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#### Phil Hobbs

Jan 1, 1970
0
Jim said:
You are clearly not trained in the area ;-)

...Jim Thompson

I'd say it was partial dVout/dVin, measured at the noninverting input.

Cheers,

Phil Hobbs

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#### Jim Thompson

Jan 1, 1970
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I'd say it was partial dVout/dVin, measured at the noninverting input.

Cheers,

Phil Hobbs

That's what I'm concluding, also. Is that in general?

...Jim Thompson

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#### Tim Wescott

Jan 1, 1970
0
Thanks, Paul!

I also found a similar reference.

I'm hoping there's a "catch-all" definition, but maybe it doesn't exist.

...Jim Thompson

It sounds like the sort of term for which a catch all definition doesn't
exist. This is bad because -- well, there's no definition. But then,
it's good because you can invent your own!

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

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#### Joerg

Jan 1, 1970
0
Jim said:
What is the "Official Definition of Noise Gain"?

Definition: A gain that happens during the months before major
elections. Typically affects technical newsgroups the most ;-)

[...]

P

#### Phil Hobbs

Jan 1, 1970
0
Jim said:
That's what I'm concluding, also. Is that in general?

...Jim Thompson

Well, in real life it's a convolution, of course, but if you don't mind
defining it in the transform domain, then I think that's the definition.

To do a noise analysis, you refer all the noise sources to the inputs.
If you've chosen the right ones, they're all uncorrelated, so they add
in RMS (in general there's a cross term, but it's almost never needed).

Since an op amp is almost perfectly differential, you can put the noise
source in either input, but choosing the + input makes it trivial to
analyze--it's just the closed loop noninverting gain.

Cheers,

Phil Hobbs

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#### Jim Thompson

Jan 1, 1970
0
Jim said:
What is the "Official Definition of Noise Gain"?

Definition: A gain that happens during the months before major
elections. Typically affects technical newsgroups the most ;-)

[...]

Sno-o-o-o-ort!

Just got back from "Bucket List" and dinner.

Great movie if you're old enough to have experienced (and appreciated)
some of the representations ;-)

...Jim Thompson

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#### Paul Hovnanian P.E.

Jan 1, 1970
0
Tim said:
It sounds like the sort of term for which a catch all definition doesn't
exist. This is bad because -- well, there's no definition. But then,
it's good because you can invent your own!

The paper I found has some useful stuff about calculating both a noise
gain and an equivalent noise source. They show a case in which the noise
voltage is fed to the op amp non-inverting input while the signal is fed
to the inverting, giving different noise and signal gains.

But I've always found it easier to calculate an equivalent noise source
connected to the same input as the signal and as a result subject to the
same gain as the signal. It makes S/N ratios at various points along the

J

#### Jitt

Jan 1, 1970
0
To- said:
Jim said:
What is the "Official Definition of Noise Gain"?

Definition: A gain that happens during the months before major
elections. Typically affects technical newsgroups the most ;-)

[...]

Sno-o-o-o-ort!

Just got back from "Bucket List" and dinner.

Great movie if you're old enough to have experienced (and appreciated)
some of the representations ;-)

...Jim Thompson
"Noise gain (1/B): The voltage response given by the
inverse of feedback-loop attenuation" --Walter Jung p.31

and, "The term that includes error factors due to nonideal
gain is the _true noise gain_ or _closed-loop gain_... "
on p.30

E

#### Eeyore

Jan 1, 1970
0
Paul Hovnanian P.E. said:
But I've always found it easier to calculate an equivalent noise source
connected to the same input as the signal and as a result subject to the
same gain as the signal. It makes S/N ratios at various points along the

I've been intruiged by this thread since I'm puzzled how anyone could think the
gain for noise would be any different to the gain for a signal (measuring
bandwidth issues aside) !

The method you mention Paul is of course exactly how audio designers do it. I
can't imagine why anyone would do it any other way since they'll simply be WRONG
!

Graham

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#### Eeyore

Jan 1, 1970
0
Jim said:
What is the "Official Definition of Noise Gain"?

Since when was the gain for noise any different to the gain for signal ?
Or have you designed a special circuit that amplifies noise less than
signal ? They're called FILTERS btw.

Graham

E

#### Eeyore

Jan 1, 1970
0
Jim said:
You are clearly not trained in the area ;-)

You are clearly mentally defective.

I've been saying for ages that the Alzheimers was destroying your
credibility but you insist on posting nonsense to prove the point.

Graham

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