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Connection of Feedback and noise?

deflow

Jan 16, 2012
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Hi, i had a practical today about amplifiers and im lecturer asked me two questions that i could not asnwer.
he asked "how can feedback affect noise in opamp?" and "how can feedback affect the distortion"
can anyone answer this for me? im very curious
 
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Harald Kapp

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Nov 17, 2011
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Sorry, no complete solution, this is not the place for it. But I think I can help you find the answers yourself.

You'd have to think along several lines:

1) If the noise or distortion is alreaday part of the input signal, what will happen to the amplified output signal? Can, and if so, how, any amplifier selectively amplify the signal but not the noise?

2) If the noise or distortion come from the amplifier, the sources are normally refered to the input (equivalent input noise voltage and current). Thus you can superimpose the signal and the noise and compute the resulting noisy output signal. And since the transfer characteristic of the OpAmp is clearly defined by the feedback characteristic you have that answer, too.

You may want to look up additional detail here: http://www.ti.com/lit/an/slod006b/slod006b.pdf, chapter 10.

Regards,
Harald
 
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Rleo6965

Jan 22, 2012
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If I remember right my radio tv technical course many years ago.. Noise signals can be cancelled by feeding back portion of noise signals 180° out of phase to the input of amplifier.
 

Laplace

Apr 4, 2010
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Noise signals can be cancelled by feeding back portion of noise signals....

Well, yeah, but how do you pick out just the noise from the signal at the correct phase in order to feed back the noise?
 

Rleo6965

Jan 22, 2012
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An rc circuit with same time constant of noise will allow only the noise signal pass ( not the audio ) and amplified and inverted if necessary and feedback to the input of amplifier. I can't find rc noise feedback circuit in the internet. But i'm sure I learned that in our school here in Philippines. I hope my memory not failing me. ;)
 

Harald Kapp

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Noise is a signal whose properties are only statistically defined (http://en.wikipedia.org/wiki/Noise_(electronics)). So there is no "time constant" for noise.
You can filter noise in the sense of unwanted signal components only if the frequency spectra of the signal and noise do not overlap. If they overlap, no filter or amplifier can separate noise from signal.

Do not confound this with noise shaping as it is used in digital signal processing. Here the noise spectrum is shifted out of the useful part of the spectrum into frequency ranges where it is either no longer disturbing (e.g. out of the range of audible frequencies) or where it can be filtered more easily (because it is now out of the frequency range of the signel).

Harald
 

Harald Kapp

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This works only if thge noise is in the high frequency part of the spectrum and the signal is in the low frequency part of the spectrum. That is, both spectra do not overlap.
If the signal has high frequency components, those will be attenuated, too.
It depends very much on the properties of the signal and the noise, whether you can treat them differently.

Dynamic Noise Limiting (DNL) in tape decks, for example, works by attenuating the high frequency components of the signal depending on the amplitude of the signal. This reduces high frequency noise in quiet passages of the music, but not in loud passages. This works because the tape noise is mainly in the high frequency part of the audio spectrum whereas the signal (music) is mainly in the middle frequency range.

But I think the question of deflow was not so much about how to filter noise but what effects the feedback loop in an ampplifier has on noise.
 

daGenie

Jan 23, 2012
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Well, yeah, but how do you pick out just the noise from the signal at the correct phase in order to feed back the noise?

what i feel happens is that part of the signal at the output of the amplifier is fed back after amplification of the input signal; d input signal contains noise and so does the op amp.......however, d noise is much smaller than the input signal (except in some drastic situations)............so what happens is that the feedback cancels out some of the input signal and some of the noise, but the cancellation of the noise is more noticeable bcos it is much smaller that d input signal.........hope dat help
 
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