# Official Definition of Noise Gain

P

#### Paul Hovnanian P.E.

Jan 1, 1970
0
Jim said:
To- said:
On Sun, 13 Jan 2008 16:29:06 -0800, Joerg

Jim Thompson wrote:
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

On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)

I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

J

#### Jim Thompson

Jan 1, 1970
0
Jim said:
[snip]
"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

On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)

I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

And everyone continues to miss the usefulness of Noise Gain as a
stability analysis tool, for example...

FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE composite amplifiers
Graeme sboa015.pdf

...Jim Thompson

J

#### Joerg

Jan 1, 1970
0
Jim said:
Jim said:
[snip]
"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
On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)
I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

The CFO of a client calls them dog and pony shows.

R

#### RST Engineering $$jw$$

Jan 1, 1970
0
Back a LOOOOOng time ago when I was in the corporate engineering world, we'd
always ask if we had a barking part in a proposed dog and pony show.

Jim

J

#### Jim Thompson

Jan 1, 1970
0
Jim said:
Jim Thompson wrote:
[snip]
"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
On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)
I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

Yep, Bought myself a ViewSonic PJ256D Portable DLP Projector for
Christmas ;-)

(The lightest projector available.)
The CFO of a client calls them dog and pony shows.

Yep. But that's what works at design reviews... PROVIDED you can

...Jim Thompson

J

#### Joerg

Jan 1, 1970
0
Jim said:
Jim said:
On Tue, 15 Jan 2008 11:06:12 -0800, "Paul Hovnanian P.E."

Jim Thompson wrote:

[snip]
"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
On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)
I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

Yep, Bought myself a ViewSonic PJ256D Portable DLP Projector for
Christmas ;-)

(The lightest projector available.)

Might want to make sure there is a spare bulb

Yep. But that's what works at design reviews... PROVIDED you can

Yes, one "oh dang" answer can ruin the credibility rather fast.

P

#### Phil Allison

Jan 1, 1970
0
"Eeysore"
Since when was the gain for noise any different to the gain for signal ?

** Shows an appalling lack of the basics of op-amp operation.

Or have you designed a special circuit that amplifies noise less than
signal ?

** Lotsa audio circuits do that.

Graphic EQs with parallel connected rows of faders for example.

...... Phil

P

#### Paul Hovnanian P.E.

Jan 1, 1970
0
Jim said:
Jim said:
[snip]

"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

On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)

I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

And everyone continues to miss the usefulness of Noise Gain as a
stability analysis tool, for example...

Well, I know what gain and phase margin are. But how does one use noise
gain in evaluating system stability?
FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE composite amplifiers
Graeme sboa015.pdf

A lot of what I see in that paper just looks like classic Bode plots of
open and closed loop gain.

J

#### Joerg

Jan 1, 1970
0
Paul said:
Jim said:
Jim Thompson wrote:
[snip]
"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
On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)
I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.
The Presentation is EVERYTHING ;-)

And everyone continues to miss the usefulness of Noise Gain as a
stability analysis tool, for example...

Well, I know what gain and phase margin are. But how does one use noise
gain in evaluating system stability?
FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE composite amplifiers
Graeme sboa015.pdf

A lot of what I see in that paper just looks like classic Bode plots of
open and closed loop gain.

Maybe this helps, 2nd page:
http://www.edn.com/contents/images/231570.pdf

P

#### Paul Hovnanian P.E.

Jan 1, 1970
0
Joerg said:
Jim said:
On Tue, 15 Jan 2008 11:06:12 -0800, "Paul Hovnanian P.E."

Jim Thompson wrote:

[snip]
"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
On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)
I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

And everyone continues to miss the usefulness of Noise Gain as a
stability analysis tool, for example...

Well, I know what gain and phase margin are. But how does one use noise
gain in evaluating system stability?
FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE composite amplifiers
Graeme sboa015.pdf

A lot of what I see in that paper just looks like classic Bode plots of
open and closed loop gain.

Maybe this helps, 2nd page:
http://www.edn.com/contents/images/231570.pdf

Yet one more rule of thumb relating noise gain (and open loop gain) to
stability. What I'm after is an explanation of how noise gain relates to
the underlying causes of instability.

P

#### Phil Hobbs

Jan 1, 1970
0
Paul said:
Joerg said:
Paul said:
Jim Thompson wrote:
On Tue, 15 Jan 2008 11:06:12 -0800, "Paul Hovnanian P.E."

Jim Thompson wrote:

[snip]
"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
On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain,
loop phase and closed-loop gain with data from a 3-pass AC analysis
macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like
I need another pass... blink of an eye but it gets me everything
displayed at once... the customers like that ;-)
I'm guessing that the noise gain figure is something your customers
requested.

This sort of begs the questions of why you need it and whether there are
better ways of analyzing the system. I'd just give them a figure that
fits what the majority of the various sources seem to think it is and
then include a footnote explaining how you derived the figures you gave
them.

The Presentation is EVERYTHING ;-)

And everyone continues to miss the usefulness of Noise Gain as a
stability analysis tool, for example...
Well, I know what gain and phase margin are. But how does one use noise
gain in evaluating system stability?

FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE composite amplifiers
Graeme sboa015.pdf
A lot of what I see in that paper just looks like classic Bode plots of
open and closed loop gain.
Maybe this helps, 2nd page:
http://www.edn.com/contents/images/231570.pdf

Yet one more rule of thumb relating noise gain (and open loop gain) to
stability. What I'm after is an explanation of how noise gain relates to
the underlying causes of instability.

The noise gain is also the absolute ratio |AVol/(1+AVloop)|. Increasing
the noise gain is the same as reducing the loop gain, which directly
impacts stability as we know. Which to focus on is a matter of taste
and of what's in view. For stability, the loop gain is where it's at,
as you say, especially since in speaking of noise gain we usually ignore
phase. For noise, crosstalk, EMI and so forth, the noise gain is
conceptually more appealing, at least to me.

BTW it isn't necessary to specify which of the op amp inputs the noise
gain is defined at--they're fully differential, so that makes zero
difference--but it has to be calculated right at the op amp input, not
at the input to the stage.

Cheers,

Phil Hobbs

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