As the frequency response of the Butterworth filter is maximally flat (no ripples), you do not have an option to specify the passband ripple and stopband attenuation values in butter function. In designfilt, you can specify the passband ripple and stopband attenuation values, however, they are not accepted for butterworth filter. You can use designfilt to design a butterworth bandpass filter as shown below.
d = designfilt('bandpassiir', 'FilterOrder', 20, 'HalfPowerFrequency1', 0.4, 'HalfPowerFrequency2', 0.6, 'DesignMethod', 'butter');
fvtool(d)
For more information, refer the following link
