I'm solving a problem in my textbook and I got stuck, and I have the solution manual of the textbook, so I went to see through the solution and I found a thing that I suspect to be wrong, due to my understanding, but It obviously could be wrong, so I wanted to get help, as I'm self-studying electronics.

At first, the book says that it will use this model for the MOSFET: enter image description here

To my knowledge, and due to calculations that is previously in the book using the (switch-current source) model the boundaries of the saturation region are: $$ V_T\le v_{in} \le V_T + \frac{-1 + \sqrt{1 + 2V_sRK}}{RK}$$

in this problem, it's about two cascaded MOSFETs: enter image description here

now comes what I don't understand. I will quote the solution manual. enter image description here

this condition on the saturation region is what I don't get. It's essentially the same as the upper bound of the condition that I provided above, but it has the boundaries on \$ V_{s} \$ How are both of them related as I can't see what I'm having wrong?

Also, I think that the details I provided are sufficient for my question, I can provide the whole question and answer from the textbook if needed.

  • \$\begingroup\$ May I know the name of the book you are self-studying please? \$\endgroup\$
    – Drake
    Commented Jan 28, 2020 at 21:45
  • \$\begingroup\$ @Drake Yes sure, It's Foundations of analog and digital electronics by Anant Agarwal and Jeffrey Lang \$\endgroup\$ Commented Feb 10, 2020 at 7:56

1 Answer 1


The first sentence you quoted from the solution manual has a typo. It should say,

First of all, if \$V_{in}\le V_T\$ then \$V_{MID}=V_S\$, so the second FET ...

That is, in this condition, the "\$V_{in}\$" for the second FET is \$V_S\$.

  • \$\begingroup\$ Thanks so much, I realized that also, that typo in the sentence, but I thought that It was getting conditions for the saturation region for the first MOSFET, so it didn't seem relavent. \$\endgroup\$ Commented Jan 27, 2020 at 3:23

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