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How to design Multiple-feedback band-pass filter?

 There are couples of methods to create a band-pass filter such as cascaded band-pass filter, multiple-feedback band-pass filter, state variable band-pass filter, biquad band-pass filter etc. The process of designing a cascaded band-pass filter was explained in the earlier tutorial How to Design Active Band Pass Filter - Cascaded HPF and LPF Band Pass Filter. Here the process of designing a multiple-feedback band-pass filter is explained with online calculator to calculate the component values.

The multiple-feedback band-pass filter circuit diagram is shown below.

multiple-feedback band-pass filter circuit diagram

The above multiple feedback band-pass filter circuit has two feedback, the path from output via R2 back to input and the path from output via C1 back to the input. That is why it is referred as multiple-feedback. The circuit has both low pass filter and high pass filter. The capacitor C1 and resistor R1 forms the LPF while the capacitor C2 and R2 forms the HPF.

The frequency response graph of a band-pass filter is shown below.

multiple-feedback band-pass filter frequency response

The center frequency f0 of the band-pass filter can be expressed as,

fc=12π(R1||R3)R2C1C2

because as viewed from capacitor C1 feedback R1 is parallel with R3.

Lets take C=C1=C2 then,

fc=12π(R1R3)R1C2
Resolving we can write expression for center frequency as,

fc=12πCR1+R3R1R2R3 

The Quality factor or Q-factor is,

 Q=f0BW
or,
 Q=f0fc2fc1

 Without derivation, the value of the three resistors R1,R2 and R3 are as follows,

R1=Q2πf0CA0

R2=Qπf0C

R3=Q2πf0C(2Q2A0)

Solving for gain A0 at center frequency using relation for R1 and R2 we get,

A0=R22R1

But from the expression for R3 above, the denominator has to be positive. For denominator to be positive we have the condition,
 A0<2Q2
It should be noted that the maximum gain A0 occurs at the center frequency f0.

 Online Calculator for Multiple-Feedback Band-Pass Filter

Inputs:


(A0<2Q2)


 
Theoretical Results:

Multi-Feedback Band-Pass Filter Design Example

Consider the design of a multi-feedback BPF with center frequency at 1KHz and bandwidth of 1KHz. Let the maximum gain of the filter which occurs at the center frequency be 1.5. Let us select value of the capacitor C1 and C2 of 0.01uF. Then using the above online multi-feedback band pass filter calculator we get,

Q = 1, R1 = 10.62KOhm, R2 =  31.85KOhm, R3 = 31.85KOhm

Let us use LM324 operational amplifier. The circuit diagram of multiple feedback BPF designed with calculated values is shown below,

Multi-Feedback Band-Pass Filter Circuit Diagram
The frequency response of the above multiple feedback band pass filter is shown below.
frequency response of multiple feedback band pass filter
Following are related tutorials and online filter calculators.

- How to design Active Filters- Low Pass Filter & High Pass Filter

- How to design Twin T Active Notch Filter?

7 Comments

  1. Anonymous

    Hello what would be the transfer function of this amplifier?

    Reply Delete May 18, 2023 at 9:42 PM
    1. ee-diary

      i have not calculated the transfer function of this filter. but, transfer function is basically Vout/Vin and to calculate this ratio, current and voltages at the different junction and branches have to calculated in term of Vin and Vout.

      Reply Delete May 18, 2023 at 10:21 PM
  2. Anonymous

    if c1 != c2, how to calculate this..?

    Reply Delete December 4, 2023 at 5:50 PM
    1. ee-diary

      what do you want to calculate? the cutoff frequency when c1!=c2 is provided you can use that to determine the component values.

      Reply Delete December 12, 2023 at 7:23 AM
  3. Anonymous

    What will be the formula for low and high cutoff frequencies?

    Reply Delete December 4, 2024 at 4:30 PM
  4. Anonymous
    Reply Delete December 4, 2024 at 4:31 PM
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