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Old 9th September 2006, 07:56 AM   #1
Progg70 is offline Progg70  Sweden
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Cool 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)
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Old 9th September 2006, 10:38 AM   #2
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Gain(db) = 20(log A)

A = simple voltage gain.

Your example = 21.6db
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Old 9th September 2006, 11:08 AM   #3
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Quote:
Originally posted by EchoWars
Gain(db) = 20(log A)

A = simple voltage gain.

Your example = 21.6db

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)]

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Old 9th September 2006, 11:27 AM   #4
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This is wrong?

Click the image to open in full size.
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Old 9th September 2006, 11:40 AM   #5
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Quote:
Originally posted by EchoWars
This is wrong?

Click the image to open in full size.

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


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Old 9th September 2006, 11:58 AM   #6
Progg70 is offline Progg70  Sweden
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OK!?
So what is the right formula to get the feedback Db?
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Old 9th September 2006, 12:24 PM   #7
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Quote:
Originally posted by Progg70
OK!?
So what is the right formula to get the feedback Db?

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.

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Old 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.
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Old 9th September 2006, 01:10 PM   #9
mikeks is offline mikeks  United Kingdom
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Default Re: Formula for calculate feedback in Db

Quote:
Originally posted by Progg70
What is the formula to calculate feedback in Db?
20Log (loop-gain)
=
20log(feedback fraction*foward-path gain)

Note that loop-gain (and therefore stability or lack thereof) for the non-inverting and the inverting arrangements is identical.
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Old 9th September 2006, 02:22 PM   #10
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Quote:
Originally posted by EchoWars
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.

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.

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