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Most extensions of the original finite-difference time-domain (FDTD) method to incorporate lumped components are based on equivalent-circuit models. However, for active components, most manufacturers provide only the measured S-parameters of the device. This letter proposes the combination of the two-port lumped-network FDTD method and rational-fitting techniques to incorporate linear active devices characterized by measured 5-parameters into FDTD simulators. To this end, first the Y-parameters are obtained, which are then approximated by rational functions of the frequency over the band of interest. The polynomial coefficients resulting from the rational fitting are directly used to feed the FDTD simulation. The approach proposed here is applied to the calculation of the S-parameters of a microstrip amplifier. The results obtained are compared with those provided by the commercial simulator advanced design system and with measurements.