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I've got a question about feedback factor. Because I don't know how to calculate it. I've been talking with some people and they say that feedback factor here is equal -R1/R2. Which is the reverse of closed loop gain = -R2/R1.

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Also I read that Feedback factor for inverting is V-/Vout. But V- is 0 V so how is it posible to have Vout*B("B" stand for feedback factor). And from it I have 0 V? Vout isn't 0 V also B is also a certain value not 0. So how does it work?

winny
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user331990
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1 Answers1

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The feedback factor is the fraction of the output that appears at the inverting input and, for your circuit it's solved by looking at the potential divider of R1 and R2 with the input grounded.

Hence it's simply \$\dfrac{R_1}{R_1+R_2}\$.

If you did the same analysis on a non-inverting amplifier, the answer would be the same.

enter image description here

Andy aka
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  • Ok but what does it says ? I know that feedback factor is V-/Vout = B. But V- is equal 0V. So this equation doesn't make sense. 0V = Vout*B – user331990 Aug 06 '22 at 12:23
  • @user331990 Your feedback factor is Vin/Vout, regardless of your opamp configuration. Ignore V- and calculate the gain according to Andy’s formula above. – winny Aug 06 '22 at 12:33
  • The closed loop gain is Vin/Vout. But for the feedback factor it is V-/Vout. – user331990 Aug 06 '22 at 12:36
  • The closed loop gain is -Vout/Vin. By definition the feedback factor is what I've said in my answer. – Andy aka Aug 06 '22 at 12:37
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    In the answer there is only R1/R1+R2. I am talking about feedback factor V-/Vout. Many sites tell the same. I'll ask again the equation also for feedback factor is V-/Vout. So 0V = Vout*B ??? and it will be still 0V ?How? – user331990 Aug 06 '22 at 12:39
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    So, who are you going to trust? And try and be more mathematically correct in your equations for instance R1/R1+R2 = 1 + R2 and I'm sure you didn't mean that. Stop and think and don't keep blasting comments because, at the end of the day, my answer is correct and anyone in doubt will just have to take that on-board. – Andy aka Aug 06 '22 at 12:41
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    I don't know to be honest because many people tells me many versions. Like closed loop gain = Vin/Vout https://www.electronics-tutorials.ws/opamp/opamp_2.html – user331990 Aug 06 '22 at 12:43
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    I'm not going to try and explain why other sites may say this. If you want to know why other sites say different things then you should raise that problem as a new question. YOU asked what feedback factor is and I have supplied an answer. End of. – Andy aka Aug 06 '22 at 12:45
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    No, I've made my answer to your question and that's it. Ask a new question if you have other problems. This site is not a progressive learning experience, a talking shop or a forum; it's a Q and A site where you supply the Q and someone like me supplies the A. – Andy aka Aug 06 '22 at 12:50
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    @user331990 if we're done here now you should consider accepting my answer. – Andy aka Aug 06 '22 at 14:20