I've been spending a lot of time trying to know why the gate voltage of M2 is 2Vt + 2Von. I know that if all transistors have the same characteristics then all the Vgs = Vt + Von. Can somebody explain me why it is 2Vt + 2Von?
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The gate voltage of M2 is the same as M4, equals the voltage at Iref input.
M4 sits on top of M3 that has a gate voltage of Vt+Von.
So the total a the Iref input = M4(Vgs) + M3(Vgs)
As you stated the Vgs of all devices is Vt+Von as they are all passing the same current so substitute in the above
Ref input = Vt+Von + Vt+Von = 2Vt + 2Von.