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If u see the output curve between Vds and Id
u can easily see that in st region , for a very small change in i/p, there will be a large change in o/p(Id) ,,this is what we require in amps....So we bias it in saturation region
Cause In sat region your ids current is given by gm*Vgs where gm is the transconductance of the MOS. This is what gives gain to your circuit. In the triode region this doesn't happen!!!
In fact , Mos working in week inversion has bigger Gm/Id , but we use it in
saturation region for it's Id is just Vgs's function and change little with Vds.
So we get a good VCCS or transconductor .
From the Transfer Characterisitics of MOsfet, U obtain the Gm versus VGS curve .
so that u can observe that Maximum GM can be obtained inthe region of saturation.
thas the reason.
further details.
Refer to Razavi
Mos can be used as an amplifier in the saturation region only because mos characteristics show that gm value in the linear range is very less and very high in the saturation region
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