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PSPICE vs Hand calculations



Jan 1, 1970
Hello All,

I am in need of your experience in clarifying a question that is
confusing me a lot. I have done my research and studied information
from various text books and stuff.

I am designing a Schmitt trigger (CMOS) and this design follows the
basic design in Digital Integrated Circuits (RABAEY) - Pg 367 (my
book). I will draw an ASCII art if anyone wants to look at it..
However, firstly I used ABN 1.5u process to simulate the circuit using
PSPICE. The switching threshold voltages Vm+ and Vm- follow my hand
calculations. The W/L of the inverters were 13/2 for PMOS and 6/2 for
NMOS. The W/L of the feedback transistors also came out to be the same.

I migrated to new feature size and calculated the Vm+ and Vm- value.
Becuase the Kp/Kn ratio is same, I found out that W/L values can still
stay the same. For the assigned W/L value, I hand calculated the Vm+
and Vm- value, and it comes out to be the what I want (2 V and -2 V).
But, PSPICE simulation gives me a big error.

To achieve the Vm+ and Vm- values of 2 and -2 respectively, keeping the
transistors of the inverter configuration same as of previous config
(13/2 and 6/2), my calculated W/L ratios for the feedback transistors
do not work. I played around until I got the required hysterisis.

Am I doing anything stupid? I have tried my best to pose the question
as clearly as possible.

Is there anything specific I have to consider when comparing my hand
calculations with PSPICE simulations? (for 0.5u tech).

Any help would be greatly appreciated.

Thanks a lot,