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Operational Amplifier- Virtual short

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kartheekRP

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Hi all,
As virtual short exists in opamp (with -ve feedback,open loop gain=infinite), Is output of this opamp is always zero?
And also why virtual short not valid in positive feedback?

Thanks.
 

LvW

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Hi all,
As virtual short exists in opamp (with -ve feedback,open loop gain=infinite), Is output of this opamp is always zero?
And also why virtual short not valid in positive feedback?
Thanks.

"Virtual short" means that the incoming signal (non-inv. input) is nearly identical to the feedback signal (at the inv. terminal); hence, it is not a real short with Vp=Vn.
How can such a signal being produced by an output voltage that is zero?
By the way - it is also valid for positive feedback - as long as the circuit is in its linear region. This is the case as long as the loop gain is smaller than unity.
However, there are only very few amplifier applications with positive feedback.
 

kartheekRP

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Can you please elaborate your explanation for virtual short in positive feedback case (by example).
 

D.A.(Tony)Stewart

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When an Op Amp in running in a linear mode, the negative feedback forces the differential input voltage to zero. Linear Mode means the output is not saturated.
When using positive feedback, such as for hysteresis, the output is saturated.
 

LvW

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When an Op Amp in running in a linear mode, the negative feedback forces the differential input voltage to zero. Linear Mode means the output is not saturated.
When using positive feedback, such as for hysteresis, the output is saturated.

However, as I have mentioned - positive feedback with a loop gain below unity is stable (still in linear mode)! Examples: Active filter circuits.
 

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