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Kmap
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how do i connect my bode plotter to a acitve lm741 band pass filter

I am trying to connect the Bode Plotter to my circuit which is a basic 2 stage  LM741 active band pass filter and cannot figure out where to connect the plotter.  This is for a project at school so if someone could help that would be great
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yyao
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

Hi,

 

Using the bode analyzer is pretty easy. Make sure that:

1 - You have an AC_POWER source on the input node of your filter

AC_POWER is in the Master Database under the Sources group and POWER_SOURCES family

2 - You are powering, biasing and loading your amplifier properly

3 - You connected the IN + terminal of the bode analyzer to the input node of your filter

4 - You connected the OUT + terminal of the bode analyzer to the output node of your amplifier

5 - You connect the IN - and OUT - terminal of the bode analyzer to ground (unless you want to measure differential voltages)

6 - You are using the interactive simulator (i.e. hitting the green play button or flipping the simulation switch or pressing F5)

7 - Double click on the bode analyzer to view results

 

Open the RFAmplifier example to see an example of the bode analyzer in action.

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Yi
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National Instruments - Electronics Workbench Group
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Kmap
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

is there a way i can send you a copy of my circuit and you can tell me what i am doing wrong because that is how i have it connected and i am not getting a bandpass reaction on the plotter
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yyao
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

When you click reply, there's a link underneath the message body labeled "Add Attachments". Click on that and the forum will allow you to attach your files.
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Yi
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National Instruments - Electronics Workbench Group
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Kmap
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

here is the circuit thank you for taking the time to look at this
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yyao
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

It looks like to me that you connected the analyzer correctly.

 

You may need to zoom out vertically in the bode plot. Try changing the upper limit of the vertical axis to 60dB instead of 0dB (what it is by default). See attached screen shot.

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Yi
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National Instruments - Electronics Workbench Group
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Kmap
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

yeah that made it so i could see the plot better i just don't understand why i am not getting a better bandpass response for this circuit it has that huge dB loss right in the center of the plot which is confusing me tremendously
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yyao
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

Unfortunately, I won't be able to help you with that as active filter design is outside my realm of expertise, but other people on this forum would probably know better and could help you.

 

I hope the bode analyzer is working fine for you. Let us know if you have any other questions.

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Yi
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National Instruments - Electronics Workbench Group
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NavyJay
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Re: how do i connect my bode plotter to a acitve lm741 band pass filter

I'm assuming you're trying to build a Sallen-Key 4-pole bandpass, right?  I didn't look too closely at the values in your circuit, but this app note should help you:  http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf

 

Without knowing your specifications, it's unclear what frequencies you're aiming for.  Dissect the circuit by stage until you get something reasonable.

 

BTW, the input on the bode plotter should be connected to the output of V5, not after R2.  That doesn't seem to be your problem though.  I also switched out the 741s with virtual opamps, but your cutoff frequencies seem to be off.  You're seeing some strange effects from device limitations <1Hz and >1GHz.