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Bandpass Filter Design
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Rigolon:
Hello,

I've been working on a project that has 2 low pass filter (LPF) and 2 high pass filter (HPF) to make a band pass filter (BPF). I am using Sallen-key topology.
My question is: If sallen-key is a second order filter, when using 2 low pass it's considered a 4th order low pass filter and the same for the high pass. So my band pass filter, which has a 4th order LPF and a 4th order HPF, is a 4th order BPF or an 8th order?

If it's an 8th order the transfer function is the product of each filter transfer function correct? [Hlp1(s)*Hlp2(s)*Hhp1(s)*Hhp2(s)]
But what is the transfer function if the filter is considered to be 4th order?

And for last, should I use buffer stages between each filter?
Benta:
First, don't use Sallen-Key, there are better filters, eg, MFB.

To your second question: if you have two identical second-order filters cascaded, it's a 4th order filter with a second-order squared response. This is probably not what you want.
A true 4th order filter will have two different sets of complex conjugate poles. These poles will depend on your desired response, eg, Butterworth, Bessel etc.
The low-pass/high-pass combo you are describing would normally not be called an 8th order filter, as the cutoff frequencies are different.

No reason to use buffers if your opamps are good.

Wimberleytech:
Go here: http://www.analog.com/designtools/en/filterwizard/

When I was first introduced to filter design, it was all tables.  Active filters of anything other than 2nd order were implemented with gyrators or leap frog.  Now, somebody has done the work for you.
schmitt trigger:
Either that, or solve polynomials in the s-plane.  >:D
Rigolon:
Thanks for everyone's answer.

I will try using MFB, made some research and saw that it's better than sallen-key for LPF and BPF. Thx for the tip.
So a true 4th order filter is a filter with same cutoff frequency but different poles (i.e. different capacitors and resistors combination for each stage)?
Since I want to go for linear phase response I guess my best choice should Bessel right?

@Wimberleytech
Thx for the link I've been using some similar tools, but my goal is to also learn the math, the problem is that I haven't done this "more advanced" type of math in a few years, so I'm a little rusty.

@Schimitt Trigger
As said previously solving the polynomials is what I want to do, but I'm making a lot of mistakes since I have to remember a lot from my classes  ;D
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