The output resistance of the MOSFET ?

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Hi,

As is well known from the textbook that the small signal output resistance (rout) of the MOSFET (common source) is approximately equal to r0 which is in the order of tens of KΩ. However, when I simulated the output resistance by S-Parmeter, it turned out to be only about 100Ω!! I tried several different models, the results are almost the same!! This is interesting, but why??


Thank you.
 

You should take into account Drain-to-source parasitic capacitor, it's about ~2-20pF. At high frequency, This parasitic cap is dominant , instead of r0 output resistance.
 

happy2005 said:
You should take into account Drain-to-source parasitic capacitor, it's about ~2-20pF. At high frequency, This parasitic cap is dominant , instead of r0 output resistance.
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Hi,

Thanks for your reply. But the truth is even when simulated at a 1KHz frequency, the output resistance is still less than 2KΩ. werid ...
 

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