# Cascode Current Mirror 2Vt + 2Von

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? 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.

• Probably is a dumb question but I still dont see why the InputVoltage = Vgs3 + Vgs4?
– EMPV
Oct 4, 2015 at 1:50
• The drain of M3 connects directly to the source of M4. Think of each of M3 and M4 as just being a voltage source (a battery for example). They are just connected in series so the total voltage is the sum of the two. Oct 4, 2015 at 1:51
• Yes. Be careful about mixing up the relative voltage terms and the voltages referenced to ground. Oct 4, 2015 at 1:55
• So that means I can see each M3 and M4 as a diode connection and the voltage of each one is Vt + Von and the sum is 2Vt + 2Von?
– EMPV
Oct 4, 2015 at 1:59
• That's right, you've got it. Oct 4, 2015 at 2:19