Positive feedback op amp understanding

LvW

Joined Jun 13, 2013
1,016
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)
 
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LvW

Joined Jun 13, 2013
1,016
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.
 

Thread Starter

Zeeus

Joined Apr 17, 2019
537
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
 

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