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transistor output impedance

Malolo

Hi,

I usually calculated transsitor output resistance by 1/gds, and gds is obtained from OP function in calcultor (based on DC sim). when I tried to double check it by using VF/IF of that node from AC simulation and check the value at very low frequency, the value is not close to 1/gds , what could the reason? 

 

thanks a lot,
Malolo

  • Hi Malolo

    You might want to contact your local Cadence support and send in a testcase. I think it would be easier for us to do the calculation and troubleshooting if there is a testcase. Perhaps you can also re-do the simulation using the latest version of spectre and check if the results are better.

    Best regards
    Quek

  • I have a problem very similar to yours. I tried measuring the output resistance of a two stage op-amp (a PMOS and an NMOS) using the method that you described. The parallel combination of 1/gds of the transistors is 24Kohms, but the value I obtained was 16.6Kohms. Did you ever find the reason of your discrepancy? Regards, Rodolfo

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