Approximation of GaAs FET two-port network Z-parameters by rational functions

In this paper, a least squares approximation procedure for the measured values of transistor Z parameters by rational functions is described. The orders of polynomials in the denominator and numerator are determined by an equivalent circuit or obtained empirically. Some rational functions of Z parameters have one zero and pole which could be cancelled so that the functions having a lower order polynomial in the denominator and numerator are obtained. All Z parameters have one common pole in the denominator. This procedure does not include computation of derivates. The computation of the polynomials in the numerator and denominator of a rational function is fully described for the high frequency GaAs MESFET.