fs = 100e3;
fc =  fs/4;
Q  = 10;
B2 = 0.5*fc/Q;

[b,a] = butter(1, [ fc-B2 fc+B2 ] / fs);
H = tf(b, a, 1/fs);

axis([ 1e3 pi*fs -80 5 ]);

H

f = linspace(0, 2*fs, 3000);
m = freqresp(H, 2*pi*f);
m = m(1,:);

plot(f, 20*log10(abs(m)));
axis([ min(f) max(f) -70 5]);
xlabel('Frequency  [Hz]');
ylabel('Magnitude  [dB]');
fixfig;