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  #1  
Old 05-02-2009, 04:26 PM
Lieta Lieta is offline
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Default Fliege Bandpass Filter transfer function's derivation

Hi, guys.
Could somebody help me out with the subject?
Link to circuit: http://www-k.ext.ti.com/SRVS/Data/ti...s/spflbphq.gif
Any information about it is welcome. I'll pay 10 $ for a full derivation.
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  #2  
Old 05-02-2009, 07:06 PM
The Electrician The Electrician is offline
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Lieta, don't offer money for help on this forum; it's very bad form. People here will be quite willing to help you, but you're supposed to show some effort to solve your own problem. What have you done so far to solve this problem?

This thread should give you some help:
http://forum.allaboutcircuits.com/sh...ad.php?t=15751
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  #3  
Old 05-02-2009, 07:31 PM
The Electrician The Electrician is offline
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Lieta, TI has an app note with a lot of opamp filters described:
focus.ti.com/lit/an/sloa058/sloa058.pdf

Also, have a look at this one:
http://www.ti.com/litv/pdf/slod006b

Are these problems you're working on actual class assignments? The reason I ask is because the twin T one was first posted back in January, and you didn't ask about it again until just a few days ago.

Are you going to be doing many of these problems where you must derive transfer functions? If so, it might be more efficient for you if you were to learn systematic ways to do it.

Do you know how to solve simultaneous linear equations? Have you studied any linear algebra (matrix algebra)?
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  #4  
Old 05-03-2009, 07:09 AM
Lieta Lieta is offline
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This is for my bachelor's work. The theme is "Voltage controlled bandpass filters". I'll have to analyse a few more, like Akerberg-Moosberg and BiQuad. It's not just deriving transfer function, but also considering how it can be controlled, etc., any information about the circuit.
I know how to solve simultaneous linear equations and have studied matrix algebra as well, but it takes a lot of time that I dont't have.
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  #5  
Old 05-03-2009, 09:05 AM
The Electrician The Electrician is offline
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Do you have access to mathematical software that can do symbolic linear algebra, like Maple, Mathematica or Mathcad?

It really doesn't take that long. Tomorrow I'll derive the transfer function for your Fliege filter and time myself.
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  #6  
Old 05-03-2009, 09:20 AM
Lieta Lieta is offline
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I have Mathcad 14 installed, have basic knowlede of it.
Thanks for Fliege, that would be great.
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  #7  
Old 05-04-2009, 07:09 AM
The Electrician The Electrician is offline
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Ok. It took less than 5 minutes to set up the admittance matrix for the Fliege filter, but about 1/2 hour to do the write-up.

This .pdf explains quite a bit about how and why to set up an admittance matrix:

http://users.ece.utexas.edu/~grady/E...07_Week_02.pdf

See the attachment for the worked out transfer function. If you're going to be doing a lot of them, you ought to learn how to do this.
Attached Images
File Type: gif Fliege.gif (22.1 KB, 56 views)
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  #8  
Old 05-04-2009, 10:34 AM
Lieta Lieta is offline
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Thanks. Still not clear how you got the elements of Y matrix containing A.
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  #9  
Old 05-04-2009, 12:10 PM
The Electrician The Electrician is offline
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For example, the row 2 elements are found like this:

Let Vbo be the voltage at the output of the bottom opamp. Then Vbo = A*(V4 - V3). Summing currents at node 2:

(V2 - V1)/R1 + V2 s C1 + (V2 - Vbo)/R2 = 0

substituting Vbo = A*(V4 - V3) we get:

(V2 - V1)/R1 + V2 s C1 + (V2 - A*(V4 - V3))/R2 = 0

collecting terms we have:

V1(-1/R1) + V2(1/R1 + 1/R2 + s C1) + V3(A/R2) + V4(-A/R2) = 0

The coefficients of V1, V2, V3 and V4 are the elements of row 2.

Similarly for the other rows.

This method is advantageous because it is systematic, and the individual elements of the matrix aren't very complicated. The various expressions become complicated when we solve the system (invert the matrix), but the computer does the algebra, and doesn't make any mistakes!
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  #10  
Old 05-04-2009, 02:04 PM
Lieta Lieta is offline
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Now it's clear. Thanks a lot.
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