why oscillators should have postive feedback why not negative feedback?

even though negative feedback is more advantageous than postive.

Your Answer

For oscillations to exist, system should satisfy Barkhuasen’s Criterion ie Feedback Signal and Input Signal should have phase difference in integral multiple of 2π ie Phase difference between input and feedback signal should be 2πn..
Positive Feedback is preferred in oscillators as it provides positive Gain to the system, ie system is likely to Saturation Value depending on the Source Voltage provided (Generally 0.9 * Vcc).
Summarising, Positive Feedback system quickly attains Oscillations if Barkhausen Criterion is satisfies.
Positive feedback is advantageous than negative feedback. 

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