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OPAMP virtual short in feedback

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ljy4468

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Hi
I've a question about feedback opamp .
In feedback, inp & inn is same (If open loop gain is infinity)
But Why??
this question may seems to be ridiculous...but I want to know mathmatically....clearly.

anyone gives me help~
Thanks in advance.
 

some of the book about opamp also introduced
 

If we have infinite gain ... by

Vo = A ( Vinp-Vinn) then

Vinp- Vinn = Vo / A ..

So, we have

Vin - Vinn ~~ 0 ..

If this is not enough .. let me know
 

if the voltage gain is finite, virtual short is not corrent.
in fact, if the opamp with feedback work normally, the two input terminal is approximtely virtual short. But if the opamp acts as a comparator, the two terminal is not virtual ground. in short, the phenomenon of vitual short is based on the negative feedback.
 

hi qutang!
thanks for your answer.
But, In Open loop configuration,
[Vo = A ( Vinp-Vinn) then
Vinp- Vinn = Vo / A ..
Vin - Vinn ~~ 0 .. ]
is also right, isn't it?

what's the difference?
 

if the voltage gain is not infinit, the two terminal is not virtual ground.
 

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