CMOS VTC – Believe it or not – You use it ‘daily’ – Part 3


If you have read through my previous posts on the above topic, things have seriously become interesting over here. I mean, isn’t that exciting, we take along the MOSFET curves, play around with them and come up with fresh set of curves that is (eventually) being used behind all timing analysis, though its “black box” for users of any STA timing tool.

Now “black box” is what I am trying to open up for you. Let’s continue

The below is called the load curve of PMOS and its a function of only Vin and Vout (no internal voltages anymore)


Step 2:In this step, we would take the above NMOS Ids-Vds curves, and convert them as a function of ‘Vin’ and ‘Vout’. We would use a similar approach, take the equations on top left box, and plot.

For NMOS, things are pretty simple, as the NMOS is present at bottom of CMOS inverter, and hence, it’s as good as analyzing NMOS as an independent transistor.

For eg. ‘Vgsn’ becomes ‘Vin’ and ‘Vdsn’ becomes ‘Vout’. Its just a matter of replacing the above voltage names and there you go !!

So, now that you have the load curves for both transistors, it’s just a matter of just connecting the below 3 objects

1) Load curve for PMOS transistor

2) Load curve for NMOS transistor

3) CMOS inverter

Creativity is just connecting things – Steve Jobs

Lets do that in my next post 🙂


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