Question

Estimate tpd for a unit inverter driving m identical unit inverters.

Answer 1

Figure shows the equivalent circuit for the falling transition. Each load inverter presents 3C units of gate capacitance, for a total of 3mC. The output node also sees a capacitance of 3C from the drain diffusions of the driving inverter. This capacitance is called parasitic because it is an undesired side-effect of the need to make the drain large enough to contact. The parasitic capacitance is independent of the load that the inverter is driving. Hence the total capacitance is (3+3m)C. The resistance is R, so the Elmore delay is tpd= (3+3m)RC.