Why the common mode gain(Acm) in a real differential-amplifier is not zero?
we have that Acm is 0 in an ideal differential amplifier but its not in real. why this happens?

 

Because the devices in real circuits are not a perfect match and zero CM gain would require such a match.

 

yeah..but what happens then? i mean which current or voltages changes in comparison with ideal form?

 

Depends entirely on the design of the particular differential amplifier circuit.