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By definition, the output voltage of an ideal OA is
Vout = g*(V+ - V-)
where g -> eternity
For real OAs, the formula looks like
Vout = g*(V+ - V- + Voffset)
The easiest way to measure Voffset (for unity stable OAs) is to connect the inverting (V-) input to output, and the non-inverting input to GND. The output voltage will be equal to Voffset.
For real OAs, Voffset can be in range from microvolts (for chopper stabilized OAs) to millivolts.
Offset consists of two parts, systematic offset and random offset. Systematic offset is due to mismatch in device sizing, currents,non symmetry,etc. Random offset is due to device property mismatch such as Vt,effective W,L.
For any opamp designed symmetrically, there will be no systematic offset. Random offset is a statistical parameter, depends on the statistical variation of the device properties.
Of course in an ideal opamp, there is no systematic or random offsets and hence no offset.
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