MFBP Circuit for Narrow Band Pass Butterworth
Wideband Butterworth filters can be obtained by connecting
a low pass filter in series with a high pass filter. The 3dB
frequency of the high pass filter must be the lower 3dB
frequency of the band pass filter and the 3dB frequency of the
low pass filter must be the upper 3dB frequency of the
If the upper 3dB frequency of the band pass filter is less
than twice the lower 3dB frequency then narrow band pass filters must be used.
The Circuit below can be used to implement a second order
narrow bandpass filter and several sections can be put in series
to implement higher order filters.
The applet below can be used to calculate component values
for narrow band pass Butterworth filters with orders 2 to 5. The
function of the applet is determined by Fn which is set by scrollbar (0). Fn = 1 to 4 shows 2 to 5th order filters.
Scrollbars (3, 4) are used to set the center frequency of the
Scrollbar (5) sets the Q of the filter.
Scrollbar (6) sets the gain of the filter.
Scrollbars (7,8) are used to set the value of the capacitors.
Scrollbars (11 to 35) are used to change the component values by
Denormalized component values are shown in green on the applet.
The Image below shows a 5th order filter (Fn = 4) implemented
with 5 sections and 5 opamps(15 resistors and
10 capacitors). Denormalized
components are shown in green. In this case,
the center frequency is 1kHz and the bandwidth is 100Hz.
The capacitors are set to 0.1µF.
Pink and Cyan plots show the magnitude and phase error
introduced when the capacitor C2 in section 1 is 10%
larger than its correct value.
Numerical values of the errors can be seen at the bottom by using scrollbar 43 to move the white line. The blue/magenta plots are
the magnitude response of the different sections.
Return to main page
Return to page index
COPYRIGHT © 2010 Cuthbert Nyack.