physical design engineer interview questions shared by candidates
CMOS Inverter , how to reduce the drive strength of Minimum size inverter
Drive strength is basically the current carrying capability of a circuit. more the Ids, the lesser the drive strength is. Increase the aspect ration (W/L) of inverter. Usually done in 2:1 ratio (PMOS is slower and sized approx. twice of NMOS)
"more the Ids, the lesser the drive strength is" This is exactly opposite of the actual fact...!
the first answer is definitely wrong, to decrease the driving capacity, please size down the ratio!
Draw a two-put NAND gate and size it, assuming the ratio of PMOS/NMOS is 2 in inverter. Then suppose two input are A and B for NMOS and PMOS. A is close to output and B is close to ground, input A change from 0 to 1 at t=t1, input B change from 0 to 1 at t =t1 (t1 > t0). Describe how the output change. Then input B changes from 0 to 1 first then input A changes from 0 to 1. Describe how the output changes. Are there any differences between these two scenarios?
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