Combining Negative and Positive Feedback in an OpAmp: What Are the Effects?

[khotsofalang]

What happens if you combine both negative and positive feedback in one opAmp?

assuming you only have unity gain in the negative configuration and no capacitors? I reckon
adding capacitors would add some oscillatory behavior but would that be the case if both configurations are made only of resistors? why?

 

[NascentOxygen]

What happens if you combine both negative and positive feedback in one opAmp?

assuming you only have unity gain in the negative configuration and no capacitors? I reckon
adding capacitors would add some oscillatory behavior but would that be the case if both configurations are made only of resistors? why?

The positive feedback will cancel out some of the negative feedback. Usually you would make the positive feedback frequency-dependent to tailor the frequency response of the overall system. If the positive feedback comes close to cancelling the negative feedback at some frequency, then the system will be lightly damped and show an oscillatory response to a step input.

If the op-amp were ideal, then both purely resistive networks would be equivalent to just less negative feedback.