The purpose of this problem is to compare the value of tPLH obtained with a resistive load [see Fig.(a)] to that obtained with a current-source load [see Fig. (b)]. For a fair comparison, let the current source I = VDD/RD, which is the initial current available to charge the capacitor in the case of a resistive load. Find tPLH for each case, and hence the percentage reduction obtained when a current-source load is used.
