# Discrete MOSFET Amplifier question: Trouble finding the Overdrive Voltage

I am working on an exercise problem (5.18) regarding Discrete MOSFET Amplifiers. I have taken pictures of the question material (Disregard the section under the question) and my latest attempt at a solution:  Alright. Keep in mind that the circuit above is from a different question, they are just re-using it in this one, but they changed the value of VDD from +15V to +5V. Also, they remove RL, but I don't think that matters for my specific issue.

Now, I decided to begin with a DC Analysis to find VOV (Overdrive voltage). I know that I can replace the capacitors with open circuits and then use the MOSFET Saturation current equation and KVL for another current equation, equate them, then solve the resulting quadratic.

However, this method yields an answer different from the book. The book provides VOV = 0.319V, whereas I get VOV = 0.83V.

Is my approach correct? I'm pretty sure the math is sound, so I'm not really sure how to proceed. My solution:  • I love those "infinite" capacitors. Obviously, they denote "arbitrarily large" and are meant just for DC blocking, but in fact, an infinite capacitor is indistinguishable from a short circuit! Mar 14 '14 at 0:48
• @DaveTweed, to be sure, an "infinite" capacitance capacitor is indistinguishable from an ideal voltage source (and a 0V ideal voltage source is indeed indistinguishable from an ideal short circuit). In fact, once one has the DC solution, replacing a coupling cap with an ideal voltage source of voltage equal to the DC voltage across the cap gives the same result as having infinite capacitance. Mar 14 '14 at 1:17
• I want a font made from your handwriting. Mar 14 '14 at 3:44
• Haha! Are you kidding it looks horrible! :P Mar 14 '14 at 5:03
• Thanks though :) Getting a good pen really helps! Mar 14 '14 at 5:04