# LM324N active bandpass filter

Discussion in 'General Electronics Discussion' started by Lifeng, Dec 19, 2014.

1. ### Lifeng

1
0
Dec 19, 2014
Hello.
In ti datasheet of lm324 I found this active bandpass filter. How can I calculate components for another freq?
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Last edited by a moderator: Dec 19, 2014

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3. ### LvW

604
146
Apr 12, 2014
Lifeng, you should know that a second-order filter is described not only by its pole frequency but also by the corresponding pole Q (which, for example, detrmines the bandwidth of the bandpass output of this filter). Here is one set of possible design equations (can be found in each relevant filter design book) - with reference to the figure as shown in the given link
C1=C2=C; R5=R6
R2=R , R4=k4*R, R1=R/k1, R3=R/k3
wp=1/[RC*SQRT(k4)]
Qp=1/[k3*SQRT(k4)]
Lowpass gain Ao=k1
Ban pass gain (center) Ac=k1/k3

EDIT: Only now I have seen the updating in the first post with the real filter circuit (bandpass). Hence, the set of formulas as given above does not apply anymore. Give me some time - and I will write down an updated version for the formulas - applicable to the given bandpass which is a modified version of the multi-feedback topolgy.

Last edited: Dec 19, 2014
4. ### LvW

604
146
Apr 12, 2014
Lifeng, here is one simple set of design formulas for your circuit (figure 44):

C1=C2=C; |A|=R6/R5 .
R1=R3=R4=R; Rp=R1||R3||R4=R/3 ; R2=k2*R,
Center frequency wc=1/[RC*SQRT(3/k2)] ,
Q=SQRT(3*k2)/(2-|A|*k2) ,
Band pass gain (center) Ac=k2/(2-|A|*k2)
.

Hope it helps.
LvW