Here is a definition of noise gain that I read:
Noise gainis the gain seen by a noise source (input voltage noise) or voltage source (input offset voltage) connected in series with an opamp input.

If so in case noise source is series with opamp input then it seems to me that in all case the noise gain and signal gain are always equal. But as in the picture above, in B and C noise gain and signal gain are completely different.
Can you tell me the way to figure out these noise gain formulas?

 

The easiest way is to view the input noise as a noise voltage generator in series with the (+) input (you could also place it in series directly at the (-) input but the calculations aren't as obvious). And remember that an input with a voltage source is an AC ground for the purposes of the noise gain calculation.

Make sense?

 

Thanks, I can calculate it all now.
It seems to be that noise voltage is treated as output offset in opamp. The noise voltage can be refered to each input.

 

Thanks, I can calculate it all now.
It seems to be that noise voltage is treated as output offset in opamp. The noise voltage can be refered to each input.

Yes. Since the input is differential and noise has no polarity, the input noise can be viewed as a noise source in series with either input, as you desire.