Op Amp how does the feedback factor works ?

[Xenon02]

Hello !
I've got a problem because I don't understand the feedback factor in Op Amp.
All I know is that it is equal V-/Vout for negative feedback.
But what if V- is equal 0V ? Like here :

Also I heard that feedback factor is the reverse of the closed loop gain A = 1/B.
So is it -R1/R2 ? I've seen on some sites that it is R1/(R1+R2). I've even talked with some people and they say the same that feedback factor is equal -R1/R2.

And I'm confused but even if feedback factor is equal -R1/R2 or R1/(R1+R2). They how is it possible that it is equal 0V? I mean V-/Vout = B ("B" is for feedback factor). So 0V/Vout = B => 0V = B*Vout. It doesn't make any sense.

 

[bertus]

Hello,

You have inverting and non-inverting amplifiers.

More info in this TI application note:
www.ti.com/lit/an/sboa092b/sboa092b.pdf

Bertus

 

[Audioguru]

The biasing of the + input of an opamp depends on the type of its power supply:

 

[crutschow]

And I'm confused but even if feedback factor is equal -R1/R2 or R1/(R1+R2). They how is it possible that it is equal 0V?

You are confusing the feedback factor (gain) with the output voltage from that gain.
If the input is 0V, then the output will be the gain x 0V = 0V.
The voltage being amplified has no effect on the gain of the circuit.

Make sense?

 

[bertus]

Hello,

You could also have a look at this page of the text book:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/divided-feedback/
And the analogy:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/an-analogy-for-divided-feedback/

Bertus