Affects of feedback to noise and distortion?

[deflow]

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

 

[ronsimpson]

I will talk about two types of amplifiers.
1) Use the transistors in a low gain configuration. Each transistor has low gain and good distortion. This is usually a fast amplifier. (no feedback)
2) Use the transistors so they have maximum gain and we don't care about the distortion. The gain might be 10,000. times bigger than we need. Now use feedback to get low gain. (feedback) The gain is set by the ratio of two resistors. The non linear effects of the transistors almost disappears because the gain (with feedback) is set by resistors not semiconductors.

 

[crutschow]

Feedback has no effect on an op amp's input noise level. It does, of course affect how much this noise is amplified at the output. But the signal to noise ratio at the output is basically unaffected since signal and noise are equally amplified.

Feedback does reduce distortion.

 

[deflow]

It does, of course affect how much this noise is amplified at the output. But the signal to noise ratio at the output is basically unaffected since signal and noise are equally amplified.

Feedback does reduce distortion.

Can you please explain how can feedback affect how much noise is amplified at the output?
And how can feedback reduce the distortion?

 

[crutschow]

Can you please explain how can feedback affect how much noise is amplified at the output?
And how can feedback reduce the distortion?

As I stated previously, noise and signal are amplified equally by the amplifier gain so negative feedback has no effect on the signal to noise ratio, only the gain of the signal and noise.

Negative feedback, in effect, compares the output with the input. A portion of this difference between the output and the input, (such as from distortion) as determined by the feedback factor, is subtracted from the signal, thus reducing the distortion. The greater the feedback factor, the more the distortion is reduced. In a typical op amp circuit configuration, a very large amount of negative feedback is used (often essentially 100%) which gives a very low level of distortion at the output.

 

[RCinFLA]

For noise performance look here:
https://www.electro-tech-online.com/custompdfs/2012/01/DN355f.pdf