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Feedback MOSFET

by roeb
Tags: feedback, mosfet
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roeb
#1
Jan19-10, 09:20 PM
P: 107
1. The problem statement, all variables and given/known data


An n-channel enhancement MOSFET for which Vt = 2 V and K = 100 uA/V2 is operated from a 9 V supply using a drain resistor RD of 60kohms and the feedback bias scheme depicted. What drain current results? What is VD What is the largest sine wave output signal for which operation in the saturation region is maintained?


2. Relevant equations



3. The attempt at a solution

I'm able to the first two questions but I'm having difficulty with the last sine wave part.

To find Vd and Id, I simply solved (9 - Vd)/60k = (100uA/V^2)(Vd - 2)^2, where I get Vd = 3 V and Id = .1 mA.

To maintain saturation in a n-channel MOSFET this condition: vds >= vgs - vt has to be satisfied. vds >= 3 - 2 = 1. Since Vd = 3 V at DC, then I would think that the sine wave could be +/-2 volts. Unfortunately, the book states the answer is 1.85 volts peak. I think this has to do with the feedback that I may not be correctly taking into account. Does anyone see what I am doing incorrectly?
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