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Impedence offered by cross coupled transistors

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khatri

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hi all,

I am implementing a VCO which had cross coupled NMOS transistors and i wish to find the impedence offered by the cross coupled pair.

as it is a oscillator, the impedence should be negative.

can anyone please tell me how to find its impedence in CADENCE.
 

khatri said:
... how to find its impedence in CADENCE.
Click to expand...
Connect a unit current source (analogLib/isin) with AC magnitude=1 and any Frequency (reasonable: your expected VCO output frequency) between the 2 points where you want to measure the impedance. Then run an ac analysis, sweeping the frequency between your points of interest. The voltage difference v between the 2 connected points (i.e. over the isin source) is your frequency-dependent impedance: z = v/i with i=1 , i.e. kVolts ≙ kOhms .
 

More exactly Real(Zin)=-2/gm..So for VCO investigation one should find Real(v/i)
 

i am using this MOSFET cross coupled pair for VCO implementation and i get the oscillations and desired tuning range. so i should get negative resistance for the cross coupled MOS pair but i m not getting it.

any idea why thats happening.
 

This example with BJTs but for MOS it should be the same

real((IF("/V6/PLUS") / (VF("/net033") - VF("/net0122"))))

67_1239977624.jpg


 
thanks for the detailed explanation Alex, i got my mistake and the negative resistance.
 

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