Biasing circuit with a MOSFET: am I missing something

[Granger]

Homework Statement

I have the following circuit with MOSFET (cascode amplifier).

[![enter image description here][1]][1] [1]: https://i.stack.imgur.com/BAqQh.png

Information given:
$V_t=1V$
$I_1=20\mu A$
$k=10 \mu AV^{-2}$

Knowing that $V_b$ is 5V determine the minimum value of the output voltage so that both transistors are saturated.

Homework Equations

3. The Attempt at a Solution [/B]

I'm kinda confused on how to proceed on this.
First of all, don't we need VDD? How to relate VB with this?
I started by assuming both transistors had the current I_1 (I assume this is correct).
Then I used the MOSFET equation for saturation $i_D=k(v_ {GS}-Vt)^2$ to determine $v_{GS}$ in both transistors. obtaining 2.4 V.
Then I applied the boundary condition for saturation $v_{DS}=v_{GS}-V_t$ and so obtained for both transistors $v_{DS}=1.4V$
So the minimum value of $v_O$ is 2.8V.

This is incorrect, the answer should be 4V. I'm really not understanding how to proceed... Can someone give me an hint on how to do this? Thanks!

 

[analogdesign]

What you're missing is that the drain of M1 cannot be at 1.4 V. This is because VB is fixed at 5V. The drain of M1 is instead at 5V - VGS2. Otherwise you're on the right track.