How to explain how negative feedback lowers noise?


 

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

The other thing I'm struggling with is why negative feedback lowers
noise in an amplifier. That's actually an effect that's relevant to
the work we're doing (it's some maths code stuff) and I just don't
know how to explain it. What's the best way you can explain how this
works?

Thanks


Tom Lee
 

Hi,

"You cannot control what you do not measure" is how I introduce negative feedback to my freshmen. An example that they all seem to get right away is a thermostat. I go from there to explaining how negative feedback solves the tracking problem ("How op-amps won WWII"), which explains the lowering of distortion, (possible) reductions in noise, etc.

But the oft-repeated claim that negative feedback reduces noise needs to be treated with great care, because it is mostly false, or at least "not even wrong". Negative feedback won't clean up a noisy input (other than by filtering, which can be provided by open-loop means with even lower noise). Because neg. fb is a tracking system, the output can't be less noisy than the input; indeed, it can only be noisier. At best, neg. fb can appear to suppress noise entering into the system near the output (because it is then tracked out). I say "appear" because an open-loop system possesses the same property, so it has nothing to do with neg. fb, per se. So, perhaps the reason you are having difficulty showing neg. fb's noise-reduction abilities is that they don't actually exist!

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/22/2021 10:40, cheater cheater wrote:
Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

The other thing I'm struggling with is why negative feedback lowers
noise in an amplifier. That's actually an effect that's relevant to
the work we're doing (it's some maths code stuff) and I just don't
know how to explain it. What's the best way you can explain how this
works?

Thanks




Jean-Paul
 

Bonjour à tous

Great topic, and as Dr Lee mentioned, historical links.

We must deal separately the noise of the input signal vs internals noise of the amplifier opamp, resistors, etc.

There is also the "noise gain" of the amplifier.

One can write books about this, and I reference the OP to the manual excellent texts on opamps and noise in electronic systems

if the objective is to extract a signal from a noisy input, an amplifier with or sans negative feedback will produce the same results, changes in amplitude but no effect on the SNR.

I hope this note can be useful

Bon courage

Jon


Harvey White
 

An employee does something wrong.  Management tells them about it.  They try to do better.  Management isn't satisfied, so they have another talk.  This cycle continues until management is satisfied.  The talks are the negative feedback

Other examples:  Think governor on a gasoline (or steam) engine. Especially the old whirling ball one.

Think adjusting the heat in the house until you feel comfortable.

Harvey

On 3/22/2021 1:40 PM, cheater cheater wrote:
Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

The other thing I'm struggling with is why negative feedback lowers
noise in an amplifier. That's actually an effect that's relevant to
the work we're doing (it's some maths code stuff) and I just don't
know how to explain it. What's the best way you can explain how this
works?

Thanks





 

Negative feedback reduces the amplitude of signals that do not appear at the input point of the feedback circuit. Thus, it will reduce noise generated within the feedback loop through degeneration, but it cannot reduce noise already present at the feedback input point.

Bruce, KG6OJI


 

Negative feedback reduces the amplitude of signals that do not appear at the input point of the feedback circuit.
sorry, I don't really follow - would you mind rephrasing? thanks!

On Mon, Mar 22, 2021 at 8:21 PM ebrucehunter via groups.io
<Brucekareen=aol.com@groups.io> wrote:

Negative feedback reduces the amplitude of signals that do not appear at the input point of the feedback circuit. Thus, it will reduce noise generated within the feedback loop through degeneration, but it cannot reduce noise already present at the feedback input point.

Bruce, KG6OJI






Göran Krusell
 

A noisy amplifier or a noisy oscillator connected to a reference source will do whatever it takes to follow the reference source signal. This applies for slow reference noise only which means noise is within the loop bandwidth. Every amplifier or oscillator circuitry has a defined bandwidth. Now if the source is very low noise then the noise from the amplifier/oscillator will be reduced by the negative feedback action. But noise above the loop bandwidth will be unaffected.
Loop bandwidth! If you are driving your car a sunny Sunday morning in a rural country side at 30 mph you can easily follow the highway, this is within the loop bandwidth. On the other hand if you would try to drive your car through the desert at 100 mph this would be outside the loop bandwidth. And you will have troubles the rest of the day.
Göran


Keith
 

Here’s my explanation of negative feedback.

1. A group of happy cheerful children are playing noisily as they enter the house through the door (input)
2. The beleaguered father is just leaving the other door (output) to go to work and he’s extremely annoyed by these children’s sudden and noisy but happy appearance, spoiling as it does his perfect departure.
3. The irritated and angry father goes back around to the front door (input) and yells at the children to stop making so much noise and go clean up their rooms (inverted signal fed into input signal).
4. The children, now somewhat subdued but still basically happy, continue their play but at a lower volume level, partially subdued by their father’s chastisement.

🙂
Cheers,
CBG


 

I will jump in with my explanation.  Negative feedback does nothing for noise on the input.  That input noise looks just like a valid signal to the circuit input so it gets amplified at the same rate as the input, their ratio stays the same.  However, noise that "sneaks in" to the amplifier is magnified by a very large number because that is what amplifiers do.  Then negative feedback subtracts a constant part of the output from the original input,  But there was no "sneak in noise" at the input so the full amplitude of the sneak in noise is "fed back" and subtracted to get near zero total noise at the output.  Even the math says so but it gets messy to do.

On Monday, March 22, 2021, 05:35:14 PM CDT, Keith <coolblueglow@gmail.com> wrote:

Here’s my explanation of negative feedback.

1. A group of happy cheerful children are playing noisily as they enter the house through the door (input)
2. The beleaguered father is just leaving the other door (output) to go to work and he’s extremely annoyed by these children’s sudden and noisy but happy appearance, spoiling as it does his perfect departure.
3. The irritated and angry father goes back around to the front door (input) and yells at the children to stop making so much noise and go clean up their rooms (inverted signal fed into input signal).
4. The children, now somewhat subdued but still basically happy, continue their play but at a lower volume level, partially subdued by their father’s chastisement.

🙂
Cheers,
CBG


Tom Lee
 

That's exactly the "not even wrong" attribution that I warned about. Your example has nothing at all to do with negative feedback, and it horribly muddies the waters to shoehorn negative feedback into the explanation.

I repeat: Negative feedback does not reduce noise.

If I have an open-loop system, exactly the same math holds. Noise injected near the output matters less than noise injected near the input. The reason has nothing whatsoever to do with the presence or absence of feedback. It is only due to the fact that there is gain between those different points into which noise could enter the system. Wrapping or not wrapping a loop around all of that makes not a bit of difference.

Please let's not repeat this commonly held error. Negative feedback is magical, but not in that way.

Tom

Sent from an iThing, so please forgive the typos and brevity

On Mar 22, 2021, at 17:45, "machineguy59 via groups.io" <machineguy59=yahoo.com@groups.io> wrote:

I will jump in with my explanation. Negative feedback does nothing for noise on the input. That input noise looks just like a valid signal to the circuit input so it gets amplified at the same rate as the input, their ratio stays the same. However, noise that "sneaks in" to the amplifier is magnified by a very large number because that is what amplifiers do. Then negative feedback subtracts a constant part of the output from the original input, But there was no "sneak in noise" at the input so the full amplitude of the sneak in noise is "fed back" and subtracted to get near zero total noise at the output. Even the math says so but it gets messy to do.
On Monday, March 22, 2021, 05:35:14 PM CDT, Keith <coolblueglow@gmail.com> wrote:

Here’s my explanation of negative feedback.

1. A group of happy cheerful children are playing noisily as they enter the house through the door (input)
2. The beleaguered father is just leaving the other door (output) to go to work and he’s extremely annoyed by these children’s sudden and noisy but happy appearance, spoiling as it does his perfect departure.
3. The irritated and angry father goes back around to the front door (input) and yells at the children to stop making so much noise and go clean up their rooms (inverted signal fed into input signal).
4. The children, now somewhat subdued but still basically happy, continue their play but at a lower volume level, partially subdued by their father’s chastisement.

🙂
Cheers,
CBG










Chris Wilkson
 

Tom is right. Negative feedback can only reduce the noise introduced by the amplifier itself. If the system works properly, the input will be perfectly amplified - with no noise added by the amplifier. But if noise is part of the input, then that noise is amplified by exactly the same amount as the intended input signal.

Hey Tom,
I once heard the argument that negative feedback could eliminate noise and it was supported by live measurements. Actually, more than once from more than one source. It usually went something like this...

Here's a noisy input signal (shown by live measurement - very fuzzy) and here's the amplified output with a larger amplitude, but less noise content (also shown on the scope - not fuzzy at all). It was clear that the output had *less* noise than the input. Therefore, the negative feedback must be reducing the noise! Many attendees just accepted the result...and continued to propagate the idea. The problem of course was the input noise signal was high frequency, beyond the bandwidth of the system. It was just being filtered out by the inherently lowpass universe. Have you seen any talks like this?

I think there are a lot of similar situations out there that contribute to common misconceptions like this one. And this one is really common.


Tom Lee
 

Hi Chris,

Yes, there are many "proofs" that reveal various misconceptions about negative feedback. Many of them start with a kernel of truth, but then veer off into the weeds. Unfortunately, it is often the case that the misconceptions can be traced to textbooks. Teaching is hard enough, but unteaching is nigh impossible.

-- Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/22/2021 22:35, Chris Wilkson via groups.io wrote:
Tom is right. Negative feedback can only reduce the noise introduced by the amplifier itself. If the system works properly, the input will be perfectly amplified - with no noise added by the amplifier. But if noise is part of the input, then that noise is amplified by exactly the same amount as the intended input signal.

Hey Tom,
I once heard the argument that negative feedback could eliminate noise and it was supported by live measurements. Actually, more than once from more than one source. It usually went something like this...

Here's a noisy input signal (shown by live measurement - very fuzzy) and here's the amplified output with a larger amplitude, but less noise content (also shown on the scope - not fuzzy at all). It was clear that the output had *less* noise than the input. Therefore, the negative feedback must be reducing the noise! Many attendees just accepted the result...and continued to propagate the idea. The problem of course was the input noise signal was high frequency, beyond the bandwidth of the system. It was just being filtered out by the inherently lowpass universe. Have you seen any talks like this?

I think there are a lot of similar situations out there that contribute to common misconceptions like this one. And this one is really common.




 

Thanks everyone for the very interesting contributions. I have one
more question about the feedback/noise debacle. Without feedback we
have less linearity, therefore the original signal comes with a bunch
of distortion products. Meanwhile the noise which is much lower in
amplitude and therefore remains in the linear part of the curve can
keep raising in power longer before it starts clipping. So in that
case, once you amplify your signal enough that you get to the
non-linear segment, if you turn up amplification even more, you're
actually increasing noise, and increasing distortion products, while
the signal doesn't increase just as much as the noise does. Therefore
you can get more noise with the same amount of original signal. So
more linearity means less noise in this case. Is this logic correct?

On Tue, Mar 23, 2021 at 6:36 AM Chris Wilkson via groups.io
<cwilkson=yahoo.com@groups.io> wrote:

Tom is right. Negative feedback can only reduce the noise introduced by the amplifier itself. If the system works properly, the input will be perfectly amplified - with no noise added by the amplifier. But if noise is part of the input, then that noise is amplified by exactly the same amount as the intended input signal.

Hey Tom,
I once heard the argument that negative feedback could eliminate noise and it was supported by live measurements. Actually, more than once from more than one source. It usually went something like this...

Here's a noisy input signal (shown by live measurement - very fuzzy) and here's the amplified output with a larger amplitude, but less noise content (also shown on the scope - not fuzzy at all). It was clear that the output had *less* noise than the input. Therefore, the negative feedback must be reducing the noise! Many attendees just accepted the result...and continued to propagate the idea. The problem of course was the input noise signal was high frequency, beyond the bandwidth of the system. It was just being filtered out by the inherently lowpass universe. Have you seen any talks like this?

I think there are a lot of similar situations out there that contribute to common misconceptions like this one. And this one is really common.





Keith
 

re: Negative Feedback example, etc.

Folks tell me that a gentle answer turns away wrath, so I hope this is a gentle reply. I mean it in that spirit.

Tom, with respect to your criticism of my example, I would suggest that you do a quick review of the original post #1. Here is the pertinent part of it, for your convenience:

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

This is question 1 in the OP...period. The core of it is simple. "...how negative feedback works in a system..." Only in the next sentence (not quoted above) does the OP then use the word "another", and only then does he bring up a second question about feedback and noise. So, the original post is really two questions. Question 1 is the issue of a real world example of negative feedback in a system. Question 2 is the interaction of feedback and noise.

Nothing in my example is intended or stated to address that second question. I had nothing to add to that discussion, and so attempted to provide the OP with his example for question one. Now I admit that the use of the word "noise" in my example does unintentionally blur the line - since I say "noisy happy children". I see how that might cause confusion, so I will attempt to edit my post to remove that word. Thanks for that.

But, to be clear here, my example was only intended to apply to question #1 in the OP. Question #1 was the only part for which I felt I had an example that met his requirements, specifically that it be;

1. "non technical" - (which I admit I assumed would mean for persons who have no electronic background)
2. use "everyday terms" (everyday means things that average people from all walks of life could grasp)
3. provide a "simple / physical / negative feedback" example.

Of course every analogy breaks down at some point, but in learning and teaching, it is quite common to go from the simple to the complex in a series of stepped examples - first simple and familiar, and therefore necessarily incomplete at some level. Then more subtle, complex, and therefore more narrow and demanding in proofs and adherence to reality.

Thanks for reading my reply in a mild spirit. I mean no disrespect, but at the moment I stand by my example (modified to remove the word "noisy" of course) as meeting the requirements of OP's question one only. Of course, if you see it differently, then perhaps we'll just have to agree to disagree on this one? In any case, thanks for your contributions to the forum. You're a valuable resource here.

Warmly,

Keith


Mike Merigliano
 

As  a non-EE, maybe my take on this could be useful, because the OP was looking for a non-technical analogy or explanation. But it should still be accurate.

The kids going into the house example does not work because the kids are the input - they are already noisy, so negative feedback that compares input to output would not eliminate it. Another way to say it is: nice quiet kids come into the house, other kids already in there yell at them and makes the once-quiet house noisy. The dad asks the noisy kids to stop and be just like the quiet kids, and the house stays quiet.

I don't think this is such a simple, easy analogy, but it seems more accurate (the feedback is not truly instantaneous, for one thing). The key is that negative feedback makes a comparison between input and output, and subtracts what is not in the input.

Unteaching is very difficult - as a researcher dealing with managers, I deal with that almost everyday.

I hope I was gentle enough.

On 3/23/2021 7:04 AM, Keith wrote:
re: Negative Feedback example, etc.

Folks tell me that a gentle answer turns away wrath, so I hope this is a gentle reply. I mean it in that spirit.

Tom, with respect to your criticism of my example, I would suggest that you do a quick review of the original post #1. Here is the pertinent part of it, for your convenience:

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

This is question 1 in the OP...period. The core of it is simple. "...how negative feedback works in a system..." Only in the next sentence (not quoted above) does the OP then use the word "another", and only then does he bring up a second question about feedback and noise. So, the original post is really two questions. Question 1 is the issue of a real world example of negative feedback in a system. Question 2 is the interaction of feedback and noise.

Nothing in my example is intended or stated to address that second question. I had nothing to add to that discussion, and so attempted to provide the OP with his example for question one. Now I admit that the use of the word "noise" in my example does unintentionally blur the line - since I say "noisy happy children". I see how that might cause confusion, so I will attempt to edit my post to remove that word. Thanks for that.

But, to be clear here, my example was only intended to apply to question #1 in the OP. Question #1 was the only part for which I felt I had an example that met his requirements, specifically that it be;

1. "non technical" - (which I admit I assumed would mean for persons who have no electronic background)
2. use "everyday terms" (everyday means things that average people from all walks of life could grasp)
3. provide a "simple / physical / negative feedback" example.

Of course every analogy breaks down at some point, but in learning and teaching, it is quite common to go from the simple to the complex in a series of stepped examples - first simple and familiar, and therefore necessarily incomplete at some level. Then more subtle, complex, and therefore more narrow and demanding in proofs and adherence to reality.

Thanks for reading my reply in a mild spirit. I mean no disrespect, but at the moment I stand by my example (modified to remove the word "noisy" of course) as meeting the requirements of OP's question one only. Of course, if you see it differently, then perhaps we'll just have to agree to disagree on this one? In any case, thanks for your contributions to the forum. You're a valuable resource here.

Warmly,

Keith




Matt
 

Here is my attempt to explain in a real-world example the concepts of negative feedback and whether or not it lowers noise.

The is for my buddy Keith.

I came up with a simple real-world example of negative feedback and noise.

Imagine you are driving your car. There is a control system involved with keeping the car on the road. Your vision is the sensor that keeps track of whether or not you are driving on the road. If your car starts to deviate from the center of your lane, let’s say you start to head toward the edge of the road on the right, you pick that up as you see the car headed toward the stripe on the right side of the road. Your control system, consisting of your eyes and your arms through the steering wheel, applies a correction in the OPPOSITE direction of the current output (wheel direction) which is negative feedback.

Now let’s say you are in your 4-wheel drive Jeep and the road is full of very large potholes which can have a tendency to throw the vehicle from one side to the other as you travel along. The potholes represent noise or distortion in your system. Now you can ignore the potholes and not correct for them, but your vehicle might go off the road as a result. Or, you can steer to correct for each pothole as you hit it (no anticipatory correction), applying negative feedback to your vehicle in an attempt to keep it on the road. By doing this, you are lowering the noise in terms of the path that the vehicle will take. Note that the potholes are essentially noise within the steering control loop and the effect on your vehicle’s path will be reduced as corrections are applied – negative feedback lowers noise in the loop.

Let’s get off the road with the potholes and back onto a smooth paved road. Now let’s say your spouse is in the passenger seat and is “helping” to navigate by giving you directions on where to go. Any errors in her directions that you follow essentially are noise on the input of your control system. Let’s say your spouse told you to make a “wrong” turn (or multiple wrong turns), which constitutes noise in the steering system input and none of your steering control system will lower that noise. So negative feedback in our steering control system won’t help to lower noise because the noise is not in the control loop.

Matt

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Keith
Sent: Tuesday, March 23, 2021 9:04 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] How to explain how negative feedback lowers noise?

re: Negative Feedback example, etc.

Folks tell me that a gentle answer turns away wrath, so I hope this is a gentle reply. I mean it in that spirit.

Tom, with respect to your criticism of my example, I would suggest that you do a quick review of the original post #1. Here is the pertinent part of it, for your convenience:

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

This is question 1 in the OP...period. The core of it is simple. "...how negative feedback works in a system..." Only in the next sentence (not quoted above) does the OP then use the word "another", and only then does he bring up a second question about feedback and noise. So, the original post is really two questions. Question 1 is the issue of a real world example of negative feedback in a system. Question 2 is the interaction of feedback and noise.

Nothing in my example is intended or stated to address that second question. I had nothing to add to that discussion, and so attempted to provide the OP with his example for question one. Now I admit that the use of the word "noise" in my example does unintentionally blur the line - since I say "noisy happy children". I see how that might cause confusion, so I will attempt to edit my post to remove that word. Thanks for that.

But, to be clear here, my example was only intended to apply to question #1 in the OP. Question #1 was the only part for which I felt I had an example that met his requirements, specifically that it be;

1. "non technical" - (which I admit I assumed would mean for persons who have no electronic background)
2. use "everyday terms" (everyday means things that average people from all walks of life could grasp)
3. provide a "simple / physical / negative feedback" example.

Of course every analogy breaks down at some point, but in learning and teaching, it is quite common to go from the simple to the complex in a series of stepped examples - first simple and familiar, and therefore necessarily incomplete at some level. Then more subtle, complex, and therefore more narrow and demanding in proofs and adherence to reality.

Thanks for reading my reply in a mild spirit. I mean no disrespect, but at the moment I stand by my example (modified to remove the word "noisy" of course) as meeting the requirements of OP's question one only. Of course, if you see it differently, then perhaps we'll just have to agree to disagree on this one? In any case, thanks for your contributions to the forum. You're a valuable resource here.

Warmly,

Keith


Tom Lee
 

Hi Keith,

Don't conflate my being didactic with being angry. Email is a lousy medium because the reader will attach an emotion track based on scant clues. That is a noisy, error-prone process.

That said, I would refer you to my initial response to the OP. Not surprisingly, I prefer my example to yours. I have taught feedback to a large collection of quite diverse audiences, and have settled on a particular set of explanations as a result of many pedagogical experiments.  I craft the particulars to match the background of the intended listener. It is very tricky to provide simple answers that also do not introduce fundamental errors.

The reason I began my response with the quote, "You cannot control what you do not measure," is that many folks have heard it before, although likely in other contexts. Even if they are unfamiliar with it, it makes intuitive sense, and it captures the essence of what negative feedback is about: You need to measure the variable to be controlled. That's necessary (but not sufficient). So in that likely familiar quote is the notion of control based on some sort of measurement of the thing to be controlled, which also then implies the existence of some sort of reference that conveys when that control is successfully achieved.

The example I offered was the thermostat. Everyone has one in the home. Everyone knows what it's supposed to do, even if the particulars of how it does what it does may be mysterious. It's a commonly encountered negative feedback system. All you have to do is point out a few of its features, and how they map to features of the quote. This simple answer to the OP's first question has all of the attributes that you enumerate, and has the added distinction of not introducing fundamental errors in the process. I teach a freshman class that has many non-EE/non-STEM students. They all grasp negative feedback's essence from the thermostat example. I had an occasion to test that explanation again this past term. And it triggers the right set of follow-on questions.

The discussion about noise wasn't a digression unrelated to the first. A proper description of negative feedback, simple or not, should set one up for answering more sophisticated questions. A poor simple explanation will align neurons in a way that actively militates against an intuitive understanding of the OP's second question (or is it the first?). Once you've got the right block diagram implicitly or explicitly implanted in their crania, adding a couple more inputs (noise or signal) to the system poses no cognitive problem. If you haven't given them the right block diagram, answering the noise question becomes nigh impossible.

So, mere simplicity is not a virtue. There's a lot of engineering that should go into crafting an answer that is both simple and correct. That kind of simple answer is scalable to address more sophisticated questions. I would go further and argue that a good simple answer stimulates precisely those types of question. A random simple answer is often a "lie that we tell to children" to get them to stop asking questions. That's bad enough, but even worse is that answering the more sophisticated questions that do get asked requires undoing the simple explanation. Why do this when there are demonstrably better alternatives?

-- Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/23/2021 06:04, Keith wrote:
re: Negative Feedback example, etc.

Folks tell me that a gentle answer turns away wrath, so I hope this is a gentle reply. I mean it in that spirit.

Tom, with respect to your criticism of my example, I would suggest that you do a quick review of the original post #1. Here is the pertinent part of it, for your convenience:

Hi all,
I'm trying to explain to people at my company (none of whom are EEs or
statisticians) how negative feedback works in a system. That's one
thing that I'm trying to get across, and I can't come up with an
explanation of it in every day terms. All the examples I find in
biology etc seem kind of dubious and not very straightforward -
there's a lot of "trust me on this" as to why it's actually negative
feedback and not some form of other regulation. What's a simple
/physical/ negative feedback?

This is question 1 in the OP...period. The core of it is simple. "...how negative feedback works in a system..." Only in the next sentence (not quoted above) does the OP then use the word "another", and only then does he bring up a second question about feedback and noise. So, the original post is really two questions. Question 1 is the issue of a real world example of negative feedback in a system. Question 2 is the interaction of feedback and noise.

Nothing in my example is intended or stated to address that second question. I had nothing to add to that discussion, and so attempted to provide the OP with his example for question one. Now I admit that the use of the word "noise" in my example does unintentionally blur the line - since I say "noisy happy children". I see how that might cause confusion, so I will attempt to edit my post to remove that word. Thanks for that.

But, to be clear here, my example was only intended to apply to question #1 in the OP. Question #1 was the only part for which I felt I had an example that met his requirements, specifically that it be;

1. "non technical" - (which I admit I assumed would mean for persons who have no electronic background)
2. use "everyday terms" (everyday means things that average people from all walks of life could grasp)
3. provide a "simple / physical / negative feedback" example.

Of course every analogy breaks down at some point, but in learning and teaching, it is quite common to go from the simple to the complex in a series of stepped examples - first simple and familiar, and therefore necessarily incomplete at some level. Then more subtle, complex, and therefore more narrow and demanding in proofs and adherence to reality.

Thanks for reading my reply in a mild spirit. I mean no disrespect, but at the moment I stand by my example (modified to remove the word "noisy" of course) as meeting the requirements of OP's question one only. Of course, if you see it differently, then perhaps we'll just have to agree to disagree on this one? In any case, thanks for your contributions to the forum. You're a valuable resource here.

Warmly,

Keith




Keith
 

Hi Tom,

Thanks for your gracious reply. I do see your point now and it makes sense to me. Also, I agree, there are too many problems with the example I gave. Also, somehow I missed your "thermostat" example in my read of the thread. That is clearly a great example, and when I read it I regretted even more even posting mine. Should have just said "Thermostat example is the one!" :-) Also, my friend Matt's example of automobile steering seems like a useful deeper dive into the idea, and seems to hold up well - at least to me.

Wish I could delete my post about the kids...guess I'll have to figure out how to do that - but for future readers, if you still see it, skip it and read Tom and Matt's examples.

Love this forum. Not just people shouting nonsense back and forth, but actual dialogue and learning. (at least I'm learning :-)

Cheers to all,

Keith
CBG


Joe
 

Hello folks,
I enjoy reading here very much. Indeed everyone can learn something.
One of my past job duties was to teach young professionals a basic understanding of aerodynamics dealing with mining ventilation systems. Trying to do so without starting with lots of maths put me often into trouble to find easy examples. I have burnt much midnight oil but frequently ended spreading calculations all over the board as there was no better proof of theory than figures.
I had to tell the guys: "You must stand this now ..."
Regards, Joe


Torch
 

1 + -1 = 0

Unwanted noise is a side effect of amplifying a desired signal. The ratio of internal noise to desired signal is greatest at the first stage of amplification and magnified in further stages. If a portion of the signal coming from the first amplification stage is inverted and fed back to the input, it will tend to cancel the internally generated noise more so than the desired signal, improving the ratio seen by the input of the next stage.