How does positive feedback works in opamp circuits?

[esmeco]

I was wondering,how does positive feedback works in opamp circuits?Like is there any formula for calculating positive feedback?I know that with negative feedback we have Vp=Vn but I don't know how it works with positive feedback!Thanks for the reply!

 

[berkeman]

The only times that I know of that you use positive feedback would be for oscillators and to add hysteresis to a comparator circuit. And with the oscillator, it's still feedback to the - terminal, but with a 180 degree phase shift.

Here is a good explanation of positive feedback for comparator hysteresis:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/3616

As for your specific question, just remember that the key feature of the opamp is its high gain from input to output. For the situation of negative feedback, that forces Vn = Vp. But for positive feedback, it just locks the + input at some voltage, determined by which way the output pins, and what the resistor divider looks like from Vo back to the + input.

 

[abdo375]

Go to http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-002Circuits-and-ElectronicsFall2000/VideoLectures/index.htm"
and watch the postive feedback lecture.