function [numa,dena]=tfapprox(num,den,dorder,wmin,wmax)
%Transfer approximation program
%[numa,dena]=tfapprox(num,den,dorder,wmin,wmax)
%
%Generates a transfer function approximation with numerator "numa" and 
%denominator "dena" of desired order "dorder"
%over the frequency range of wmin rad/sec to wmax rad/sec starting with
%a transfer function with numerator "num" and denominator "den".
%
%The approximation is achieved by curve fitting the original transfer
%function frequency response over the specified frequency range.
%
%file name "tfapprox.m"
%num=numerator polynomial of original transfer function
%den=denominator polynomial of original transfer function
%wmin=minimum frequency (rad/sec) for a good approximation
%wmax=maximum frequency (rad/sec) for a good approximation
%numa=numerator polynomial of the transfer function approximation
%dena=denominator polynomial of the transfer function approximation
%dorder=desired order of the transfer function approximation
%
norder=dorder-1;
Gorig=tf(num,den);
[mag,phase,w]=bode(Gorig,{wmin,wmax});
H=freqs(num,den,w);
[numa,dena]=invfreqs(H,w,norder,dorder);
Gapprox=tf(numa,dena)
wm=2*wmax;
bode(Gorig,Gapprox,{wmin,wm})