Why we bias MOSFET amplifiers in the saturation

# Why we bias MOSFET amplifiers in the saturation - We can...

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Why we bias MOSFET amplifiers in the saturation region. D. W. Parent

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When designing an amplifier we usually want the magnitude of the gain to be at least 2. Operating in the saturation regime allows for a magnitude of the common source gain to be greater than 1. Why is this so?
Looking at the small signal equivalent of a CS amplifier (no Rs) one can see ro puts an upper bound on RD, and thus the gain. D A o D o D o m V I V r R r R r g A This equation for ro is only valid in saturation. DS D o dV dI r 1 This equation for valid everywhere.

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Unformatted text preview: We can see that for an ideal NMOSFET the slope of ID is zero and thus the ro is infinite In the linear regime the smallest ro is 1/0.036 Ω. Saturation ro goes to infinity. Even adding an Ro to the Model does not increase ro in the linear regime. D o D o m V R r R r g A For “Real” MOSFETs Ro in saturation is not fixed for all VGS! Design away effect of Ro, by setting RD<<Ro If RD<<ro, then the larger ro, the larger the common source gain. D A o D o D o m V I V r R r R r g A...
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## This note was uploaded on 10/23/2011 for the course SDASD 102 taught by Professor Dsfas during the Spring '11 term at Baptist Bible PA.

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Why we bias MOSFET amplifiers in the saturation - We can...

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