Source potential for MOSFET with grounded substrate

mosfetnmos

Considering the configuration below, the source's potential should be invariant with respect to Vg (as long as Vg>Vt). So Vs is equal to Vdd.

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But if the substrate is connected to the source, as shown here, when Vs reaches Vg-Vt, the transistor switches off, and Vs remains at Vg-Vt.

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Is this correct? I am asking because I read in a material that in the first situation Vs depends on Vg.

EDIT (quote from a lab guide):
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If the M1 transistor is in cut-off stage, for the case in figure 4.2b, the output voltage will depend on VGG and the threshold voltage, following a relation that contains also elements (parameters) specific to the intrinsic structure of the basis substrate.

Best Answer

In your first circuit, there exist "body effect". But if you can omit that effect

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$$ v_{GS} = v_{DS} = V_{dd}-V_{s}\\ v_{DS} > v_{GS} - V_{TN} $$

It should work in saturation region. Unless \$v_{GS} < V_{TN}\$, \$v_{DS}\$ can't be zero.

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