Formula for calculate feedback in Db
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Progg70
diyAudio Member

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)
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 9th September 2006, 10:38 AM #2 EchoWars   diyAudio Member     Join Date: Dec 2002 Location: Left of the Dial Gain(db) = 20(log A) A = simple voltage gain. Your example = 21.6db
jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
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)]

Jan Didden
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Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.

 9th September 2006, 11:27 AM #4 EchoWars   diyAudio Member     Join Date: Dec 2002 Location: Left of the Dial This is wrong?
jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally posted by EchoWars This is wrong?

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
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.

 9th September 2006, 11:58 AM #6 Progg70   diyAudio Member   Join Date: Jul 2004 Location: Sweden OK!? So what is the right formula to get the feedback Db?
jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
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.

Jan Didden
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.

 9th September 2006, 12:32 PM #8 EchoWars   diyAudio Member     Join Date: Dec 2002 Location: Left of the Dial 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.
mikeks
Account Disabled

Join Date: Jun 2004
Location: Animal farm
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.

jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
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.

Jan Didden
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.

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