MATLAB code for IIR Butterworth Filter using Impulse Invarient Transform

clc;
clear all;
close all;
Ap=2;
As=40;
fp=100;
fs=900;
fst=2000;
wp=2*pi*fp;
ws=2*pi*fs;
[n,wn]=buttord(wp,ws,Ap,As,'s');
[b,a]=butter(n,wn,'s');
[bz,az]=impinvar(b,a,fst);
freqz(bz,az,1000);