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Old 27th February 2003, 05:24 AM   #1
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Default Negative feedback?

I've heard a lot of things about negative feedback but I don't understand how you can reduce noise more than you can reduce the signal. If you inject part of the output into the input again, you are reducing the noise, but at the same time, the signal. So when you crank the signal back up, isn't the noise there too? Where exactly does the noise come from?
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Old 27th February 2003, 05:36 AM   #2
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You will of course not reduce any noise that is fed to the input of your circuit.
What your feedback circuit actually does is reducing noise and distortion of the stages that are INSIDE of the loop.

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Charles
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Old 27th February 2003, 08:46 PM   #3
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Does NFB solve the problem of non linear output due to unregulated rails that sag when you draw a lot of power from them?
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Old 27th February 2003, 09:03 PM   #4
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Yes - to the extent that it can. If the voltage drops so low that linear operation cannot be maintained, then feedback will lose the battle.
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Old 27th February 2003, 09:27 PM   #5
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For this amplifier I tried to build, I had an NPN/PNP pair with emitters grounded (first stage). The first stage NPN's collector was tied to the secondary stage's PNP's base whose emitter was on the + rail. The opposite was true for the negative rail. In other words, both stages were common emitter (inverting). The first stage was common to ground, the second stage, one the NPN was common to the - rail and the PNP was common to the + rail. Both stages combined formed a non-inverting amplifier, so how could I use NFB in a situation like this? How could I get global NFB without using op-amps?
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Old 28th February 2003, 08:18 AM   #6
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Default NFB

You could put a NFB loop from the emitter of the last stage to the base of the input stage, but it is not strictly global.

To use global NFB you need another inverting stage, which is easy: base to collector of last stage, and give it equal emitter and collector resistors, so its gain is 1, but the collector has the inverted signal. That can be fed back to the input.

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Old 28th February 2003, 08:36 AM   #7
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Default Re: Negative feedback?

Quote:
Originally posted by Solid Snake
I've heard a lot of things about negative feedback but I don't understand how you can reduce noise more than you can reduce the signal. If you inject part of the output into the input again, you are reducing the noise, but at the same time, the signal. So when you crank the signal back up, isn't the noise there too? Where exactly does the noise come from?
You can't reduce any noise, therefore it's important to make the first stage low noise and it must amplify so much so the amplified noise is less than the noise created in the secnd stage.

Feedback can't handle noise, sorry.

Hasn't Texas got a large document, very wellwritten about feedback?

Can't find it right now but here are lot's of info.

http://dspvillage.ti.com/docs/catalo...applicationId=
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Old 28th February 2003, 08:37 AM   #8
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Snake,
Another way is to put a resistor from input emitters node to ground, then a feedback resistor for output to the emitters node.
Bam
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Old 28th February 2003, 08:43 AM   #9
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Default Re: Re: Negative feedback?

Quote:
Originally posted by peranders


You can't reduce any noise, therefore it's important to make the first stage low noise and it must amplify so much so the amplified noise is less than the noise created in the secnd stage.

Feedback can't handle noise, sorry.

Hasn't Texas got a large document, very wellwritten about feedback?

Can't find it right now but here are lot's of info.

http://dspvillage.ti.com/docs/catalo...applicationId=
Per, beg to differ, fb can handle noise as long as it is generated inside the loop. Anything inside the loop is reduced with the fb factor, noise, thd, temp drift, you name it.

Jan Didden
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Old 28th February 2003, 04:12 PM   #10
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You know, despite the >40dB (probably 60) NFB in my SS stereo (Onkyo TX-2500), it is still incredibly noisy (part of why I don't use it). Makes me wonder........

Tim
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