#### jwcc

##### Newbie level 2

I have a question regarding the input common mode voltage on the gain of a simple differential amplifier with a tail current source.

From what I have read so far, the differential Gm of the circuit is a function of (W/L) of the input transistor and the tail current. This equation implies that if I am to increase the common mode input level, the differential gain would stay the same provided that I keep everything in saturation. Is this correct?

If I increase the common mode input level, Vgs will increase, correct? Does that not increase the gm of the input transistor? If the gm of the input transistor pair increases.... doesn't that increase the gain of the circuit? Why does increasing input common mode voltage not have the same effect as increasing (W/L) of the input pair?