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Difference between rds and ro ?

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taofeng

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If you do ac analysis in spectre simulator, and then print the DC operating point of the interesting transistor, you will see gds and ro. Normally I treat 1/gds as the output impedance of the transistor, is that correct ?

However, I still see that there is another paramater : ro, is it the same one ?

which one should be used for analog design or small signal analysis ?
 

ro is 1/(Ids * lambda) where lambda is the channel modulation length factor. Ideally ro should be infinite. You can also get ro in terms of the early voltage va like at this website
**broken link removed**
 

ro related small signal use to calculate Gain,PSRR,
rds: related large signal=Vds/Ids.

I think that.
 

u have any paper or book reference.
 

taofeng said:
If you do ac analysis in spectre simulator, and then print the DC operating point of the interesting transistor, you will see gds and ro. Normally I treat 1/gds as the output impedance of the transistor, is that correct ?
Click to expand...

Yes, it is correct. gds - is trasconductance(g=1/r). Often, more simply calculate output resistance, using term transonductance, because most equivalent circuits has parallel resistance. And using term transconductance their equivalent resistance will be sum of transconductance, that more simply then (r1*r2/(r1+r2))
[/quote]
bicave said:
ro related small signal use to calculate Gain,PSRR,
rds: related large signal=Vds/Ids.
Click to expand...
equally
 

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