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I was wondering why do we so often want out transistor to work in saturation mode? Ie. For a MOS Current Steering Circuit we want our transistors to operate in the saturation region....

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FETs in saturation are typically used to obtain a constant current. If you look at the I-V plot for a FET, the lines are relatively flat in the saturation region. For a given \$V_{GS}\$ curve, the current will be basically constant over a large range of \$V_{DS}\$. This is what we want from a current source; the flatter the slope, the larger the output impedance.

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Transistors of various types are often used a "switch" you want it turned on or off. This mean saturation to turn it on, and cut off to turn it of.

The obvious exception is when transistors are used as part of an amplifier.

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