Question

A chain of four inverters whose sizes are scaled by a factor x is used to drive a load capacitance

equal to 1200Cin where Cin is the input capacitance of the first inverter.

a) Without increasing the number of inverters in the chain, find the optimum value of x that

results in minimizing the total propagation delay tp and find the corresponding value of tp in terms of Cin and the output resistance Ro of the first inverter. Let Ro=100Ω and Cin=50fF.

b) If you increase the number of inverters beyond what was used in part (a) in order to

minimize tp, how many additional inverters would be used and what value of the scale factor x would evolve? Calculate the resultant propagation delay for this set of inverters.

Answer 1