Bandpass filter help

Thread Starter

sjkim0329

Joined Oct 19, 2008
12
I've been having trouble trying to design a bandpass filter, six pole that is centered at 25khz w/ 5khz. I was trying to do 6-pole low filter and 6-pole high filter and combine them. But it hasnt worked out. Please help.
 

beenthere

Joined Apr 20, 2004
15,819
If you have placed your knees at the correct frequencies, it should. What hasn't worked? Can you post up your work so we can see what might be giving problems?
 

Thread Starter

sjkim0329

Joined Oct 19, 2008
12
I think all my analysis were wrong. So I just redesigned 6-pole low pass using the table(pole locations for each order, a+_ jb). Using multisim, i did ac sweep.
Could you tell me if I'm on right track here? Thank you guys.
 

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Audioguru

Joined Dec 20, 2007
11,249
The lousy old 741 opamp has trouble with frequencies above only 9kHz. The filter needs better opamps that work up to 90kHz. Many opamps work well up to 100kHz like the TL071 single, TL072 dual and TL074 quad opamps.
 

beenthere

Joined Apr 20, 2004
15,819
Yes, 741's are really obsolete and inadequate. Most manufacturers include the Spice parameters on their sites so you can see if another choice will work.
 

Thread Starter

sjkim0329

Joined Oct 19, 2008
12
After redesigning the circuit, it's not working correctly still.
So I have 6-pole low pass and 6-pole high pass connected(Vout).
Looking at graph of AC sweep, it looks like low pass isnt' working correctly.
I've looking at this circuit for a while and couldn't figure it out.
Please help, any suggestions..thank you guys.
 

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bertus

Joined Apr 5, 2008
20,653
Hello,

You are putting them parallel, this does not work.
Take a low-pass and a high passfilter and put them in series.
The low pass freq. = bandpass freq. + 1/2 bw
The high pass freq. = bandpass freq. - 1/2 bw

Succes,
Bertus
 

Thread Starter

sjkim0329

Joined Oct 19, 2008
12
I'm having trouble changing cuttoff frequency.
Currently, it's at 10kHz and I'm trying to get 30kHz.
After calculation, and redesign, I realized that at 30khz,
there is a big spike right before cutoff frequency.
Any suggestions please. Thanks
for my redesign, I just divided all the resistors by 30.
I worked when i changed cutoff frequency at 1k to 10k.
 

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Audioguru

Joined Dec 20, 2007
11,249
Most opamps can drive a minimum load resistance of 2k ohms. Your circuit has much lower resistances (200 ohms or less?).

A 30kHz filter needs to use opamps that work well at 300kHz or higher. The TL071 has an internal voltage gain of only 9 at 300kHz.
 
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