% first order of IIR Peak filter
% copyright by Etual

clear;
Ac=3;
Gb2=10^(-Ac/10);
w0=0.35*pi;
deltaw=0.1*pi;
b=1/(1+tan(deltaw/2)*(sqrt(Gb2)/sqrt(1-Gb2)));
B=[1 0 -1].*(1-b);
A=[1 -2*b*cos(w0) (2*b-1)];
w=0:pi/500:pi;
H=freqz(B,A,w);
plot(w/pi,abs(H));
grid;