I was simulating a cmos inverter circuit on Ngspice and while plotting the drain current, it came to my attention that there are sudden spikes in drain current during switching. While I understand that momentarily both nmos and pmos are closed during transition, wouldn’t that mean that current would just flow from vDD to ground for both high to low and vice versa? However here I see that there is a spike in the negative direction as well?
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\$\begingroup\$ Which drain current have you plotted? PMOS or NMOS? \$\endgroup\$– saiCommented Sep 2 at 10:41
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\$\begingroup\$ Its the current from NMOS drain to ground \$\endgroup\$– Ahan ShettyCommented Sep 2 at 13:45
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\$\begingroup\$ Something is amiss. On a rising edge of the input, I would have expected a positive current spike because there will be current from the NMOS drain to ground because the NMOS is actually discharging the out node to 0V. \$\endgroup\$– saiCommented Sep 2 at 15:25
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here I see that there is a spike in the negative direction as well
What you are observing is a current surge due to charging and discharging the drain source capacitances of the MOSFETs inside your inverter. Hence current flows in both directions.
