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polarization of a MOSFET !!!

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biolycans

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Hi all,

I upload a file. In this word there are figure related to my questions.

If I have a common source amplifier (figure 1), If I want to amplify a signal vgs, this signal should be greater than Vth for the conduction of the MOSFET, for that reason I use a constant value VGS for the polarization of the mosfet.

But for example if I have a differential amplifier like in figure 2, then I don´t need a constant voltage in the gate of M1 and M2, because if the voltage between gate and source is bigger than Vth, I can put a value for example negative in the gate of M1 or M2 so if the source has a potential smaller than the negative voltage in the gate, I will have a conduction in the mosfet. Am I correct ?

Regards,

JoaquinView attachment Questions.doc
 

In a mosfet what matters is the effective Vgs = Vg - Vs ... That means even if my mos is N-type, I can always turn it ON even with a negative Vg if Vs is sufficiently negative to have Vgs > Vth.
In Fig 1 the VSS=ground=0V and in Fig 2 VSS=-2.5V. This means even if my signal vgs is across 0V (VGS = 0V) the mosfets will turn ON. But consider a case where in Fig 2 VSS=0V then my signal vgs has to be given a positive dc shift by VGS > Vth > 0V.

I hope this will explain ... :)
 
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