Hello,there is a circuit i saw which is supposed to amplify a DC signal input,in the circuit bellow its called Vdc as shown bellow.
I can see there is a feedback,In AC we have virtual ground and we do equation of currents.but here we have DC only input.
how do we calculate the Vout from Vdc and Va?
Thanks.

 

R2 and R3 are irrelevant with most modern Op-Amps having extremely high Input-Impedance,
especially when you don't know what the Source-Impedance of Vdc is in the first place.

R5 and R6 are also irrelevant because there is zero-Current flow.

As long as Vdc is above Ground, Va will be above Ground.

This leaves R1 and R4,
which are in the Inverting-Feedback-Loop of the Op-Amp.
They will determine the Gain of the Op-Amp,
which, in this configuration, will always be greater than "1".

Find the Formula for calculating the Gain of the Op-Amp.

Note: "In the real World",
most Op-Amps will not operate with either of their Inputs very close to Ground.
You must use the Data-Sheet to verify how far above
Ground the Inputs must be kept for predictable operation.
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.
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In AC we have virtual ground and we do equation of currents.

Please post your equation wo we can see if it is correct.

but here we have DC only input.
how do we calculate the Vout from Vdc and Va?

***Assuming*** that all of the operating parameters that you have *not* specified are such that the opamp is operating in its linear active region, then ...

The exact same way with the exact same equations.

Also - check your schematic to see if there are any drawing errors. What is the reason for R5 and R6 in the circuit?

ak