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9th September 2006, 07:56 AM  #1 
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Join Date: Jul 2004
Location: Sweden

Formula for calculate feedback in Db
What is the formula to calculate feedback in Db? and what is the correct answer for the picture? Perhaps to complicated, and you need perhaps more. (see picture)

9th September 2006, 10:38 AM  #2 
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Gain(db) = 20(log A)
A = simple voltage gain. Your example = 21.6db 
9th September 2006, 11:08 AM  #3  
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Quote:
This is the open loop gain of A. I guess he wants the gain with the feedback loop closed, as shown? [Hint: it is NOT 20 log (47k+4.7K/ 4.7k)] Jan Didden
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9th September 2006, 11:27 AM  #4 
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9th September 2006, 11:40 AM  #5 
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This is an approximation, assuming that the gain of the amp block is (very) much higher than 1/beta. I understood from the first post that the amp block open loop gain was 12? Edit: Beta of course being the feedback network attenuation Jan Didden
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I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason  Daniel Dennett Check out Linear Audio Vol 7! 
9th September 2006, 11:58 AM  #6 
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OK!?
So what is the right formula to get the feedback Db? 
9th September 2006, 12:24 PM  #7  
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Quote:
Hi Progg70, Can you be a bit more specific? What exactly do you want? The gain of the amp? Closed loop, open loop? The gain of the feedback network itself? The gain of 12 you showed, what is that? The gain block, or the complete closed loop amp circuit? And if the latter, it's wrong. Please think a moment and then formulate your question. Jan Didden
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I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason  Daniel Dennett Check out Linear Audio Vol 7! 

9th September 2006, 12:32 PM  #8 
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I believe he's wanting you to assume that he's working with an opamp here (OLG very large).
It's what I assumed until I saw what you were getting at. 
9th September 2006, 01:10 PM  #9  
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Re: Formula for calculate feedback in Db
Quote:
= 20log(feedback fraction*fowardpath gain) Note that loopgain (and therefore stability or lack thereof) for the noninverting and the inverting arrangements is identical. 

9th September 2006, 02:22 PM  #10  
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Quote:
Yeah. It was the "gain = 12" that got me wondering what that was. I assumed it was the gain block gain. Now I realised he may have meant the total circuit gain, but then the 12 is wrong of course. Jan Didden
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I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason  Daniel Dennett Check out Linear Audio Vol 7! 

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