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Filters

Low and
High Pass Active Filter Calculator

Bandpass
Filter Calculator

LC Filter
Calcualtor (LP, HP, BP, BS)

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Electronics
Calculators

Bandpass Filter Calculator

These are parts value calculators I wrote to help design analog active bandpass
filters. They are op-amp based filters and are most useful in the audio
frequency range.  These parts calculators are based on formulas and tables
from the book "Electronic Filter Design Handbook" by Arthur B.
Williams.

Bandpass filters pass a contiguous range of frequencies
while attenuating those above and below the passband.  The calculators compute parts values for 2,
4 and 6 pole multiple-feeback bandpass (MFBP) filters.  Click here for a tutorial on Bandpass filters.

Paul Falsted has a great online interactive Java program to help visualize
filter responses with different poles and response types.  Use it to see
how a particular filter will perform.

Usage

Select the desired filter type from the drop down menu. Butterworth is optimized for flat frequency response in the
pass band. Chebyshev
sacrifices flatness for a steeper roll off in the stop band. This version has
0.1 dB of passband ripple.   Bessel filters sacrifice both flatness and roll off for linear
phase in the pass band.

Select the desired value for the capacitors.  All the capacitors in the
circuit are identical and for best results should be 1% tolerance.  For
midrange audio frequencies 0.01 uF is a good starting point.   Note:
Narrow bandwidth filters require tighter component tolerances than wide band
filters.

Enter the center frequency of the desired passband and the bandwidth.

Enter the desired voltage gain.  This the the ratio of the output to
input voltage, not dB.  If you select a gain higher than the circuit can
deliver you will see "ERROR" in the output data values.  Maximum gain will
vary depending on bandwidth and number of sections.  Maximum gain per
section is 2 times Q squared.  gmax = 2*Q^2 .

Click COMPUTE and read the resistor values.  If the values are not
optimum try a different capacitor value and try again.  For best results
use 1% tolerance resistors.

Also computed are the Q and center frequency of each section of the
filter.  If the Q is too high parts values will be very critical and the
op-amp will require higher performance.  Q values up to about 20 are
reasonable.  Above that may result in an unstable circuit.  Note: The
center frequency of each section can be adjusted by varying R2 (R5 or R8).
Frequency goes up as resistance goes down.

More information about MFBP circuits can be found with the help of Google. The source code is here.  Report bugs via my contact page.

Op-Amps

The circuits below assume classic dual supply op-amps that use plus and minus
power such as the LM348.
If you use a single supply op-amp (eg: LM324) you will need to connect the grounds (R2,R5,+op-amp
inputs)  to a virtual ground, usually half way between real ground and
Vcc.  One way to do this is connect two 1K resistors in series between Vcc
and ground.  Connect a 10uF capacitor from the junction of the two
resistors to ground.  The junction of the two resistors is a virtual ground
at Vcc/2 .  Click
here for an example.  It's for a high pass filter but the principle is
the same.

I like to use single supply quad cmos op-amps such as LMC660 or LMC6484 because of their rail to rail output swing and wide
bandwidth.  The dual version is LMC6032.

4 Pole Bandpass Active Filter

Input

Computed Values

Filter Type

Butterworth Chebyshev 0.1
dB Bessel

Capacitors (uF)

Center Freq (Hz)

3dB Bandwidth (Hz)

Voltage Gain

C1,C2,C3,C4 (uF)

R1 (K Ohms)

R2 (K Ohms)

R3 (K Ohms)

R4 (K Ohms)

R5 (K Ohms)

R6 (K Ohms)

Section

1

2

Q

Freq

6 Pole Bandpass Active Filter

Input

Computed Values

Filter Type

Butterworth Chebyshev 0.1
dB Bessel

Capacitors (uF)

Center Freq (Hz)

3dB Bandwidth (Hz)

Voltage Gain

C1,C2,C3,C4,C5,C6 (uF)

R1 (K Ohms)

R2 (K Ohms)

R3 (K Ohms)

R4 (K Ohms)

R5 (K Ohms)

R6 (K Ohms)

R7 (K Ohms)

R8 (K Ohms)

R9 (K Ohms)

Section

1

2

3

Q

Freq

2 Pole Bandpass Active Filter

Input

Computed Values

Capacitors (uF)

Center Freq (Hz)

3dB Bandwidth (Hz)

Voltage Gain

C1,C2 (uF)

R1 (K Ohms)

R2 (K Ohms)

R3 (K Ohms)

Filter Q

Hit
Counter = 64396

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© Copyright 2004-2016 - Dale A. Heatherington

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