Formula for calculate feedback in Db

Progg70 · 9th September 2006, 07:56 AM

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)

EchoWars · 9th September 2006, 10:38 AM

Gain(db) = 20(log A)

A = simple voltage gain.

Your example = 21.6db

janneman · 9th September 2006, 11:08 AM

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

EchoWars · 9th September 2006, 11:27 AM

This is wrong?

janneman · 9th September 2006, 11:40 AM

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

Progg70 · 9th September 2006, 11:58 AM

OK!?
So what is the right formula to get the feedback Db?

janneman · 9th September 2006, 12:24 PM

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.

Jan Didden

EchoWars · 9th September 2006, 12:32 PM

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 · 9th September 2006, 01:10 PM

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.

janneman · 9th September 2006, 02:22 PM

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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Formula to calculate capacitor value?	klitgt	Parts	9	9th January 2006 03:29 PM
how to calculate feedback resistor	Lico	Tubes / Valves	7	9th June 2004 05:41 AM
Formula to calculate speaker impedance	Perry Babin	Multi-Way	5	21st November 2003 06:19 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

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

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

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?

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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)