Please, If somebody can tell me about positive feedback I mean at Op Amps

 

It's simply feedback from the output to the non-inverting input, usually used in oscillators, although in much smaller quantities for schmitt triggers.

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I can give you a simple circuit, but it doesn't require an op-amp.

Just connect a small value capacitor between the emitter and collector pins of a BJT transistor, and you have designed the "positive feedback" circuit found in many FM tramsmitter schematics published on the net.

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hyedenny

once again, mstechca, - LOL!!

 

Positive feedback in opamp are used for two porpouses:
oscillators
Schmitt triggers, hysteresis comparators.

 

in op amps, pos feedback will cause the output to rail. Therefore you can use op aps wired up for pos feedback to create digital type high-low signals.