simple current sink question

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luckyvictor

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for a BJT/MOSFET in common emitter/source mode,

what is the advantage adding a resistor between E/S and Vss (negative voltage supply)?

and

why the output resistance now is this resistance multiply by the output resistance of the transistor and the transcondutance? e.g Rout = gm*rds*R

Can it be explained without using small signal circuit? just intuitively please?

Many thanks
 

Ok, if we have to get an intuitive understanding about the result:Rout = gm*rds*R
we can do it like this:
the output resistance of current sink without degeneration Rs or Re is rds, with the current variation of ΔI, the Vgs remain unchanged but the output voltage will rise ΔI*rds.
the case with degeneration resistor will be different. With the current variation of ΔI, the current caused by ΔVgs can be expressed as : gm*R*ΔI which can be considered flow into rds to drive the Vo.
so,the Rout can be gm*rds*R
The book of Razavi includes the intuitive description which is more reliable, I think.
 

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