Negative feedback question

[GrahamnDodder]

The more I think about, the more confused I am getting about the NFB resistor.

In my IGC, it is running from 3 to 8, or, from the speaker out to the signal in.

Why is this "negative" feedback? On the surface it seems like it would be positive feedback and start a vicious cycle.

I searched, but there are so many references to NFB that I was unable to find this question.

In the hopes that I am not the absolute last person to understand this, could someone please explain to me and the others who may be suffering in silent ignorance?

What is the concept/purpose of the NFB resistor?

Thanks,

GnD

 

[JoeBob]

It wouldn't be positive feedback because the feedback is inverted (negative when compared to the original signal) when compared to the input.

 

[Bricolo]

It's negative feedback because your gainclone is an inverting amp (the output voltage is in the opposite phase compared to the input voltage. Example: if input=1V, out=-23V (because gain=23)

So, a part of the output goes thru the feedback resistor, and comes in the -in, in the opposite phase, and this way it cancels the input a little

 

[analog_sa]

It's negative feedback because your gainclone is an inverting amp

?????????


Can't you guys buy an electronic textbook?

 

[Bricolo]

?????????


Can't you guys buy an electronic textbook?

"It's negative feedback because your gainclone is an inverting amp and the feedback resistor goes from output to input"

are you happy now?

 

[analog_sa]

Whether the amp is inverting or not has nothing to do with the polarity of feedback

 

[Bricolo]

Whether the amp is inverting or not has nothing to do with the polarity of feedback

of course it has! At least if it is global feedback (from input to output).

Global feedback+invertimg amp=NFB

 

[analog_sa]

Global feedback+invertimg amp=NFB

Coming back to my question about the textbooks

 

[Bricolo]

Coming back to my question about the textbooks

just give me an example that proves I'm wrong

 

[Beggar]

dudes

Its called negative feedback because it is feedback from the output to the inverting input.

 

[peranders]

"Negative" feedback is when you subtract the feedback from the input. == The feedback input MUST be inverting with respect to the output.

This question is not something to debate about really. Negative = substraction, minus sign, invert, positive = addition, plus sign, non-invert

 

[GrahamnDodder]

... maybe I should have asked about abortion or the death penalty instead...?

 

[SY]

I'll invoke the memory of Mimi DiPietro in Grahams honor.

For an inverting amp, the feedback is taken from the output to the same input as the input signal, i.e., to the inverting input.

For a noninverting amp, the feedback is taken from the output to the other input than the input signal is fed to, i.e., to the inverting input.

Note the pattern.

 

[peranders]

What is the concept/purpose of the NFB resistor?

1 Reduce distortion
2 Reduce output impedance
3 Increase the frequency response
4 Adjust the gain without getting more noise
5 Reduce artifacts inside the amp

Have I forgot something?

 

[yeti]

feedback

To simplify it : positive feedback increases the amplification,this can lead to oscillation,negative feedback decreases amplification.
The amp doesn't need an inverting input necessarily, inverting the phase can be achieved by a phase shifter network.

 

[GrahamnDodder]

I am enlightened.

Thanks, guys.

 

[nobody special]

?????????


Can't you guys buy an electronic textbook?

THat's what I would call negative feedback!

Anyway... it's all about the phase of the input related to the output. If the amp inverts and you feed it back to the - input, you get positive feeback (restating the obvious)

 

[jh6you]

On the surface it seems like it would be positive feedback...

The “inverting” in the name of the inverting input terminal (– terminal) of op amps implies that, when the input signal drives the inverting input, we get the inverting voltage at the output. In comparison, the input signal to the “non-inverting” terminal means non-inverting voltage at the output. The op amps are typified to do this work.

 

[Joe Rasmussen]

I think you guys have missed the obvious:

In both inverted and non-inverted, the feedback resistors goes back to the inverted (-) input. May I repeat it, in both cases.

So what's the difference? It's where the input signal is inserted.

Think about it.

Joe R.

 

[DrG]

Perhaps analog_sa needs a good electronic textbook. Hehehe