Impedence offered by cross coupled transistors

[khatri]

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.

 

[oermens]

Zin=-2/gm...

 

[erikl]

... how to find its impedence in CADENCE.

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 .

 

[Alex_IC]

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

 

[khatri]

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.

 

[Alex_IC]

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

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

 

[khatri]

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