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# Need help to calculate cascaded bandpass filter

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#### siewfai2101

##### Newbie level 4
calculate bandpass filter

can anyone teach me how to calculate the centre frequency of this type filter??
i been searching all over for this kind of circuit but in vain..
i cant find a circuit that looks like this..
any kind people willing to show me the way?
thanks a lot![/img]

calculate bandpass filter response

It´s very easy to calculate. Try it by yourself.

Its the simplest BP you can imagine. Multiply the RC lowpass with the CR highpass function (it is allowed since both are decoupled). But consider that the time constant for the HP must be larger than for the LP.
As a result you have a bandpass function with an imaginary nominator.
At the mid frequency the phase must be 0 deg : That means the whole function real.
Thus you set the real part of the denumerator equal to zero.
This equation gives you the frequency for which the whole function is real with phase=0 deg. (Because numerator and denumerator, both are imaginary and the "j" cancels). OK ? The result is very simple !

so is it just using

1/(2*pi*R21*C21*R22*C22) ???

siewfai2101 said:
so is it just using
1/(2*pi*R21*C21*R22*C22) ???

Nearly ! You forgot the square root out of the last 4 factors.

should it be

1/sqrt(2*pi*R21*C21*R22*C22)??

or

1/(2*pi)sqrt(R21*C21*R22*C22)??

i still cant get their centre frequency though..

full problem should be like this :

i really hope u can help!!

By calculating according my hints you´´ll get

1/(2*pi)sqrt(R21*C21*R22*C22).

With this in mind the allocation of element values shouldn´t be a problem

but somehow, i still cant manage to get da value indicated such as 350Hz, 4kHz, and 9kHz..

anyway, would the resistor at the inverting pin influence my calculation?? or all i need to consider is the resistor and capacitor at non-inverting pin?

R12, R23, R33 are useless; they can remain as they are, but have absolutely no influence on the frequency dependence.

i still cant manage to get da value indicated such as 350Hz, 4kHz, and 9kHz..
As far as I see, it's not achieved exactly, actual values are 290 Hz, 3.42 kHz and 7.28 kHz.

Personally, I don't see much sense in using the term center frequency with a high-pass/low-pass combination. It's not more than an arithmetic mean, but a well defined property of course.

As far as I see, it's not achieved exactly, actual values are 290 Hz, 3.42 kHz and 7.28 kHz.

Personally, I don't see much sense in using the term center frequency with a high-pass/low-pass combination. It's not more than an arithmetic mean, but a well defined property of course.

ya.. i get that value too.. i not sure why is it using this kind of circuit at da first place.. most bandpass i found in the internet are using one op-amp instead of 2.. they just manipulation configuration with capasitor and resistor..

Can i ask for an opinion??
If I'm were to do an equalizer, it would be proper to use the circuit i have posted or i should go for the circuits i get in the internet??

this is one of the example :

[/img]

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