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  #1  
Old 01-18-2009, 10:09 AM
Lieta Lieta is offline
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Default Twin T Bandpass Filter transfer function's derivation

Hi, guys.
Could somebody help me out with the subject? I've searched the net, but didn't find anything useful.
Link to circuit: http://www-k.ext.ti.com/SRVS/Data/ti...s/spttbphq.gif
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Old 01-18-2009, 10:15 AM
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Hello,

Take a look at the PDF in the link.
http://www.drp.fmph.uniba.sk/ESM/twin.pdf

Greetings,
Bertus
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Old 01-18-2009, 10:18 AM
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Assume an ideal op amp so you can say that the voltage on the non-inverting terminal (V2) equals the voltage on the inverting terminal (V1). The voltage are measured with respect to ground. First find the transfer function (relationship) between Vin and V2. Then find the transfer function between Vo and V1. Finally, because V1=V2, substitute V1 with the equation relating V2 with Vin to get the transfer function between Vo and Vin.
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Old 01-18-2009, 10:38 AM
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Quote:
Originally Posted by Lieta View Post
Hi, guys.
Could somebody help me out with the subject? I've searched the net, but didn't find anything useful.
Link to circuit: http://www-k.ext.ti.com/SRVS/Data/ti...s/spttbphq.gif
What do you need to know about this circuit topology?

hgmjr
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  #5  
Old 01-18-2009, 02:53 PM
The Electrician The Electrician is offline
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Quote:
Originally Posted by hgmjr View Post
What do you need to know about this circuit topology?

hgmjr
Judging from the subject line, I think he wants you to produce one of those lovely detailed transfer function derivations that you do so well. :-)

But, he hasn't shown any of his own work yet.
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  #6  
Old 05-01-2009, 05:49 PM
Lieta Lieta is offline
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Thanks, bertus, for the document. The twin-T filter without an op-amp is a band-stop filter (see fig. 4).
Denote
Vout - output of whle circuit.
Vin_pas - input of passive twin-T filter
V(-) - input of inverting input of op-amp.
V(+) - input of non-inverting input of op-amp
Vin - input of the whole circuit
Vin_pas = V(-) = V (+) = Vin*k, where k = R5/(R4+R5).
Vout/Vin_pas - band-stop filter transfer funct. and because Vin_pas = Vin*k,
Vout/Vin also band-stop filter transfer funct., but actually the circuit if band-pass filter. How to take an op-amp into account? Where am I wrong?
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  #7  
Old 05-02-2009, 01:44 AM
The Electrician The Electrician is offline
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Are you still interested in deriving the overall transfer function for the opamp plus twin-T feedback network?

Are you required to use any particular method to derive it, or can you use any method you like?

This thread is several months old, so I thought I would ask if your interest level is still high.
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  #8  
Old 05-02-2009, 09:18 AM
Lieta Lieta is offline
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I'm still very interested in deriving the overall transfer function for the opamp plus twin-T feedback network. Any method is fine.
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  #9  
Old 05-02-2009, 11:43 AM
The Electrician The Electrician is offline
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You said "How to take an op-amp into account?". Does this mean that you don't want to make the usual assumption that the opamp is ideal, with infinite gain?
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Old 05-02-2009, 11:48 AM
Lieta Lieta is offline
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No, assume op-amp as ideal. I ment how to derive transfer function for whole circuit, including op-amp, because without op-amp it's a notch filter, but with it it's band pass filter.
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