Did not understand how this Rf resistor below A1 is providing positive feedback

Please how so?

Thanks

 

The shown circuit is an oscillator. In order to meet Barkhausen´s oscillation condition we need a loop gain of unity at one single frequency only (unity magnitude; and 360 deg phast shift). For the closed-loop, this condition is identical with the reqirement that the overall negative feedback is compensated (outbalanced) by a positive feedback loop. Hence, both feedback factors must be identical.
That is the background for the positive feedback loop

Simple Example - Wien oscillator: The resistive voltage divider realizes negative feedback (k=1/3) and the frequency-dependent RC feedback path realizes positive feedback (k=1/3 at the oscillation frequency only)

 

Try to analyze this circuit:

Do you see how R2 adds positive feedback?

https://forum.allaboutcircuits.com/...s-but-im-troubled-by-them.64696/#post-444315b

 

Try to analyze this circuit:
View attachment 199432
Do you see how R2 adds positive feedback?

https://forum.allaboutcircuits.com/...s-but-im-troubled-by-them.64696/#post-444315b

Sorry late reply.. Yes I see

in the image in #1..The feedback signal from Rf and V0ut2 are not equal.. shouldn't they be for positive feedback?

 

The feedback signal from Rf and V0ut2 are not equal.. shouldn't they be for positive feedback?

Can you elaborate a bit more, because I don't understand what you are trying to tell me?

 

Zeeus...If you like, you can remove Rf .......and the circuit will oscillate !
The only purpose of the RF path is to allow a safe start of oscillation.
However, this is - sometimes (!!) - only necessary during simulation when there is no other "kick-off" puls for starting the oscillation.
In real life (hardware) such a kick-off is created always by the switch-on transient of the power supplies.

 

Can you elaborate a bit more, because I don't understand what you are trying to tell me?

The question you asked, my understanding is that there is positive feedback because Vx and Vout will have the same phase
Correct?

But in #1 circuit. The 2 signals adding are not in phase

Zeeus...If you like, you can remove Rf .......and the circuit will oscillate !
The only purpose of the RF path is to allow a safe start of oscillation.
However, this is - sometimes (!!) - only necessary during simulation when there is no other "kick-off" puls for starting the oscillation.
In real life (hardware) such a kick-off is created always by the switch-on transient of the power supplies.

Yes, it works without Rf

Although i remember in the past doing this same circuit in real life and it did not work. Please look at image