Hello,

I've designed a Notch Filter with cut off frequencies from 100Hz to 20kHz.
When I simulated the design using AC analysis, I found out that I've got a problem with high frequencies. The problem is that there is a decrease in the output. The following picture will explain my problem.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=50010&stc=1&d=1356210439

PS: I used active filters.

I'd appreciate some explanation of why do I have such problem.

 

There is no valid attachment.

 

I've designed a Notch Filter with cut off frequencies from 100Hz to 20kHz.
When I simulated the design using AC analysis, I found out that I've got a problem with high frequencies. The problem is that there is a decrease in the output. The following picture will explain my problem.

Besides the lack of a diagram of your circuit, can you tell me what you expect from a notch filter.

 

I'm sorry I thought it was already attached.

Here is my output:

About the design, I used LPF and HPF in parallel.

 

I expect it to reject everything in the range of 100Hz to 20kHz.

 

Two things ....

(1) Most designs have a unity gain mixer.
(2) Did you notice the gain of the amplifiers were different?

 

You can't connect the outputs of opamps together like that. What would happen if you connected two voltage sources in parallel, such as connecting a 12 volt source in parallel with a 10 volt source?

 

Google 'Summing Amplifier"

 

http://www.ti.com/lit/an/sloa093/sloa093.pdf 30 second filter design

 

Two things ....

(1) Most designs have a unity gain mixer.
(2) Did you notice the gain of the amplifiers were different?

Woops! I'm actually aiming for gain of 5.. do I make design both of them to give me gain of 5?

 

Google 'Summing Amplifier"

Summing amplifier! That's slipped out of my mind somehow! Thank you

 

You can't connect the outputs of opamps together like that. What would happen if you connected two voltage sources in parallel, such as connecting a 12 volt source in parallel with a 10 volt source?

Oh!! Yes that was a mistake.. summing amplifier slipped out of my mind somehow.
Thanks for pointing that out

 

LM741 does not have enough bandwidth or slew rate for your notch filter. LM741 is a dinosaur.
What is the highest frequency you want to pass?
What is the amplitude of your signal?
What are your supply voltages?

 

Your opamps are not powered so they will do NOTHING.
The lousy old 741 opamp has trouble above 9kHz.

Your filters are very simple with only one capacitor and only one resistor so their cutoff slopes are very gradual.

If you use a mixer then the lowpass filter cuts 100Hz slightly (-3dB), cuts 200Hz a little more (-6dB) and has reduced but still has plenty of output at 1600Hz.
The very simple highpass filter reduces 20kHz slightly (-3dB), cuts 10kHz a little more (-6dB) and has reduced but still has plenty of output at 1250Hz.

If the gains are the same then the phase shifts of the filters might produce a notch at about 1400Hz.

 

I simulated your extremely simple filter with good quality opamps.
10Hz and 100kHz have a gain of 5 (+14dB).
100Hz and 20kHz are down -3dB to +11dB.
200Hz and 10kHz are down a little more to +7dB.
Most frequencies will be at a lower level but are still very audible because the filters are so simple.

 

"I've designed a Notch Filter with cut off frequencies from 100Hz to 20kHz."

That is one hell of a wide "Notch"!

The term "Notch" filter is usually reserved for one which has a bandwidth appreciably smaller than its centre frequency.

Maybe a "Bandstop" filter would be a better name for what you are attempting.

As the others have said,it will have a very gentle slope.
It will,in fact be useless!
Maybe,you could use two separate channels with a high degree of isolation,one with a fairly savage LPF having a cutoff at 100Hz,& another,with a HPF cutting off below 20kHz,then add the two channels to get the required result.
I wish you all the best!