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Cadence Simulation Calculation Mismatch

nevinalex1234

Hi

i wanted to calculate the effect of load on the output of the inverter. Attached is the experiment setup. Vdd=1.1 and Vgs=0

So ideally when there is no load, the output is pulled closed to 1.1. (1.099V). That parts fine. Now when i put a load at the output 50 ohm as in Figure

But the output is only 293mV as by simulation.

Rup=1/gds=443.45

Rdown=1/gds=14.27K 

By calculation it should have been 50/(50+443.45) =101mV

But as shown in simulation (dc), the node rises to 293mV. Why this discrepancy?

How do i estimate the resistance of the inverter pair which is being loaded to 50 ohm ? 

  • Hi,

    I think gds is a small-signal parameter, which is obtained from DC Operation Point. However, when doing tran simulation, output is driven by a large signal. Besides, if inverter is directly connected to a 50Ohms load, the DC Operating Point is different with that when no load is present.

     

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