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-   -   How the negative feedback really works and an alternative feedback question (http://www.diyaudio.com/forums/solid-state/227818-how-negative-feedback-really-works-alternative-feedback-question.html)

Amit_112dB 14th January 2013 07:23 PM

How the negative feedback really works and an alternative feedback question
 
Here is what i think is the maths of negative feedback amplifiers.

http://s7.postimage.org/fnzgulbpz/untitled12121.jpg
http://s7.postimage.org/jkcsqkwpn/untitled12121.jpg

As you can see by the 5th iteration we reach close to the new gain as per the formula, which can be actually derived in the similar way. These iterations happen speed of electrons, close to speed of light and the final system equilibrium is reached.
Negative feedback : Operational Amplifiers

Please let me know if I am wrong :confused:

2ndly here is my alternative method of negative feedback.
using this method the gain of the amplifier will not reduce but i am not sure if it will work:confused:

instead of providing a -ve feedback = feedback factor * output

If i provide -ve feedback as = input - amplification factor * output
http://s7.postimage.org/5pge15ajb/untitled12121.jpg
http://s7.postimage.org/jkcsqkwpn/untitled12121.jpg

it seems the system oscillates between distortion and distortion free state.
What will happen.
Any ideas... great minds :D

-Amit_112db
-4yrs EE bachelor graduated and not so bad with maths

peranders 14th January 2013 07:26 PM

Negative feedback is nothing new and we have good theories for that. There is only one kind of negative feedback. I really recommend that you study the basic theory first. Wikipedia is one place.

sreten 14th January 2013 07:40 PM

Hi,

Feedback is not an interative process, it simply happens in realtime.
(Properly if you've met the stability criteria, and frequency dependent,
which varies according to the type of compensation scheme employed.)

Long time since I did my EEE degree but I really can't see
how you've come to approaching it in such a manner.

rgds, sreten.

myfirstamplifier 14th January 2013 08:33 PM

My old tube books state that adding a certain amount of negative feedbaack that the general sound quality is improved because the thd% becomes lower at higher volumes.

DF96 14th January 2013 09:04 PM

Sigh! Only a few hours ago in another thread I suggested that the OP needs to go away and read about algebra and electronics, then about feedback, after he presented an earlier version of his own 'new feedback theory'.

Amit_112dB 14th January 2013 09:12 PM

Quote:

Originally Posted by DF96 (Post 3326032)
Sigh! Only a few hours ago in another thread I suggested that the OP needs to go away and read about algebra and electronics, then about feedback, after he presented an earlier version of his own 'new feedback theory'.

My new feedback theory may not work as I said. its just an idea that popped up :D

But my theory of how the classical negative feedback works is not flawed. It gives absolutely the correct answer with the correct algebra. The output values of gains match with the gain formula from wikipedia.

sreten- my approach to feedback is not interactive at all.. just iterative

peranders- you may be absolutely correct. There may be only one kind of negative feedback. Its the way i am analyzing/viewing it suing iterations. I would love to know how its flawed.

Pure and simple math algebra of how a system is trying to achieve a stable state.
Howver you all may be right. I will definitely read op amps/amps n other stuff and think of it again.

myfirstamplifier 14th January 2013 09:24 PM

Quote:

Originally Posted by DF96 (Post 3326032)
Sigh! Only a few hours ago in another thread I suggested that the OP needs to go away and read about algebra and electronics, then about feedback, after he presented an earlier version of his own 'new feedback theory'.

Thats really funny... but i always sick up for friends that want to learn and don't know how.

Dave Zan 14th January 2013 09:54 PM

@Amit
The best reference on feedback theory is probably still the fundamental work by H.W. Bode. It is available on-line.
There is so much half-baked nonsense written about feedback that you will have to be careful if you want to be taken seriously. Your OP has both links the same, so it is not clear what the difference is supposed to be. The thumbnails are too small to be much help.

Best wishes
David

GoatGuy 14th January 2013 09:57 PM

Just want to toss this into the ring:

First, feedback theory is well worked out, and isn't really hard to understand, once you read completely through a couple of different sources. In the very easiest (ironically) presentation, the effect of feedback on a theoretically perfect operational amplifier makes it almost completely clear how it flies.

A "near perfect" operational amplifier has near-infinite gain, no stability issues, and infintessimally fast response time. It is obvious that if any signal at all is put on the "+" pin with the "-" pin connected to ground - at near-infinite gain, the output will swing instantly to the + or - rail. Not a very useful device (as an amplifier. A great comparator!)

But simply connect the output back to the - input ... and it becomes clear that the output will exactly track the input: if it becomes even a tiny bit more positive, that will be fed to the negative input, and it'll reverse back to matching. Likewise, if it is a tiny bit shy of exactly the same, then the + pin is greater than the - input, and the output will again rapidly swing more positive.

Lesson "2" is that the "really large gain" aspect - as well as slew time - cannot be infinite, or an undefined condition will exist leading to no solution. The math works out with a few more complicated terms, but "really large gain" serves to make the smaller compensation terms so small as to be generally negligible.

Lesson "3" is that combinations of voltage-dividers can be used in the feedback loop to give the overall device a numerically consistent, rational gain figure from the ratios of the external resistance elements themselves. Again, the op-amp's gain hardly figures in before the 4th or 5th decimal place of accuracy.

Lesson "4" is that time-domain reactive components (inductors, and especially capacitors) serve to make the feedback far less simple mathematically. Things rapidly get complex. Unusual circuit topologies can exploit the near-perfect integration abilities of capacitors to simulate inductors, to effect filter elements and more. Point though is, you realized after this that "damn, it can get complicated".

Lesson "5" introduces notions of bandwidth limitations, stability, input differential bias, and so on. By this time, one can break away from the op-amp learnin', and start to apply the theory to conventional multistage discrete component amplifier design.

I recommend this path-to-understanding to one and all. The lessons are rendered so much clearer by the strikingly unique and outstanding mathematical simplification of the op-amp model. Real-world is different of course, but the foundation ... is 75% of the battle.

GoatGuy

Amit_112dB 14th January 2013 10:06 PM

An article i read said its not an iterative process.

Although iterations lead to same result. I am yet not sure how iterations are wrong.

Then the article goes on to say that
"In our ideal device, the change is instant, in a real device it is possible to measure the time it takes for the correction(i call iterations) to be made. "

So the iterations are instant. As i said at the speed of electrons.

Wikipedia didn't help much and I will try to find something with more maths

Quoting from
Distortion and Feedback (This is the URL)
This is not an iterative process, which is to say that the amplifier does not keep feeding the input signal (meaning a significant part of the input waveform) into the inverting input to be re-amplified, re-distorted and re-compared. This is where some of those who criticise negative feedback have made their first error.

The output of the amplifier simply keeps changing in the appropriate direction until the error amp detects that the voltages are again identical, at which point the output of the error amp ideally just stops where it is, and so does the rest of the chain. In reality, there will be a small amount of instantaneous correction as the two voltages approach equality, but this must happen much faster than the input signal can change with normal programme material.

The fact that the correction is usually done well before the input voltage has even changed significantly clearly means that no part of the feedback signal is fed through the amplifier over and over again - that just doesn't happen. In our ideal device, the change is instant, in a real device it is possible to measure the time it takes for the correction to be made. For an audio amplifier, the correction must be completed faster than the highest frequency of interest can change - how much faster is open to some conjecture, and that will be looked at later in this article.


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