why oscillators should have postive feedback why not negative feedback?

even though negative feedback is more advantageous than postive.

8 Answers
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8 Answers
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  • TO maintain the output amplitude of the oscillator stable it is important to use positive feedback so that the closed loop gain will become unity or you can understand that the gain within the loop as it is closed (provided by amplifier) will be equal to the losses within the loop caused by the feedback circuit 
    however you can also use negative feedback their is no such rule but the point is that in negative feedback the gain will be less in comparision to that in positive feedback so thats why we use positive feedback in case of oscillators
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  • Dear ramya sree, this is tejeshwar naidu and checkout this below link ofr your clarification upon above doubt. Thank you.
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  • Positive feedback is used to have more gain . For oscillator the gain should be at maximum to have sustained oscillations .
    On other hand negative feedback decreases the gain but gives stability to the system . Usually negative feedback is used in amplifiers .
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  • In oscillators using positive feedback it is important that amplitude of the oscillator output remains stable. Therefore the closed loop gain must be 1 (unity). In other words, the gain within the loop (provided by the amplifier) should exactly match the losses (caused by the feedback circuit) within the loop.
     
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  • 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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  • An Oscillator is a device which produces signals by itself without any signal being fed into it.
    To initiate the process it uses the transients.
    So to continue this and maintain stability it needs feedback signal which should be a positive one.
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  • Oscillators require large gain(ideally infinity), to produce sinusoidal output. This can be achieved through connecting positive feedback only. Negative feedback decreases gain, and decreases oscillations. So i hope i have answered your question
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