Multisim Help

Discussion in 'Homework Help' started by Energetic, Jan 3, 2011.

  1. Energetic

    Thread Starter Member

    Jan 3, 2011
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    Hi All,

    I am doing an assignment with multisim and am having trouble. Can anyone identify this circiut and tell me what is supposed to come out on the scope please? I have a feeling it isn't doing what it's supposed to so any guidence would be appreciated.

    Cheers!:)
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    At a glance, a bandpass filter. When you set the function generator to sweep, the bode plotter on the far right should show the frequency response as a curve.
     
  3. Energetic

    Thread Starter Member

    Jan 3, 2011
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    Thank you, i had assumed it was a filter or amplifier of some kind as that's the kind of thing we've been looking at in class but no idea really.

    How do i set my generator to sweep? My bode plotter is showing nothing at this point. I am unfortunately completely out of my depth...:eek:
     
  4. hobbyist

    Distinguished Member

    Aug 10, 2008
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    If I remember correctly, the bode plotter does the sweeping of the frequencies, you have to assign certain values, the start and stop frequencies.

    It's been a long time since I used it in a circuit.

    Try switching the generator, with a AC volt. source.

    Here is what it says in the help menu.

    The Bode plotter generates a range of frequencies over a specified spectrum. The frequency of any AC sources in the circuit does not affect the Bode plotter. However, an AC source must be included somewhere in the circuit.
    The initial and final values of the vertical and horizontal scales are preset to their maximum value. These values can be changed to see the plot on a different scale. If the scale is expanded or the base changed after simulation is complete, you may need to activate the circuit again to get more detail in the plot. Unlike most test instruments, if the Bode plotter’s probes are moved to different nodes, it is best to re-activate the circuit to ensure accurate results.
     
    Last edited: Jan 3, 2011
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I had similar laboratory assignment. It is bandpass filter. The op-amp on the left is high pass filter. The op-amp on the right is low pass filter.

    You don't have to have the bode plot to see what is going on. Set sinusoidal ac power supply to some voltage, we did 2 volts peak to peak. Put oscillascope on the output. Start changing the frequency of the sinusoidal ac power supply, in my experiment we did 10 Hz, 5 kHz, 15 kHz and 100 kHz.

    The result of the my experiment was that:
    at 10 Hz, 2 volts went in, 100 mV came out
    at 5 kHz, 2 volts went in, 480 mV came out
    at 15 kHz, 2 volts went in, 1.34 V came out
    at 100 kHz, 2 volts went in, 70 mV came out
    Do you see the correlation? At around 15 kHz, most of the input showed up at the output, the signal passed, this is frequency that lies in the passband. At lower frequencies, tiny amount of input showed up at output, most of the input was blocked, in other words, filtered out. Again, at much higher frequency, same thing happened, most of the input was blocked, filtered out.
     
  6. Energetic

    Thread Starter Member

    Jan 3, 2011
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    Thanks everyone, i have been playing around with the frequency and i'm not seeing any change in amplitude. I think there is something really wrong.

    From the pic below - the red sinusoid is the input i think and the purple line is constant and i think is showing DC output regardless of the input freq.

    I'm confused. Can anyone see anything obviously wrong from the first pic, i may have to start again...
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    The supplies that power the op-amps are hooked up wrong.
     
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I have run your circuit. I also get a dc output. I am not sure what is going on there. The circuit that I did for my class did not have resistors R2, R3, R7, R8 and R9.

    The bode plot does look bandpass. There is -2.912 dB at 758.578 Hz and -2.909 dB at 9.441 kHz. I think that is your passband, from 758 Hz to 9 kHz.
     
  9. Energetic

    Thread Starter Member

    Jan 3, 2011
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    0
    I've sussed it, Swap the polarity on the bottom two batteries and it will work!!! Definately a filter as you've mentioned... Thanks for your effort!!!
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    R9 is useless and is not needed. It is the circuit's load.

    R2 and R3 set the highpass filter to have a sharp Butterworth response. R7 and R8 set the lowpass filter to have a sharp Butterworth response. Without these resistors increasing the gain so that the positive feedback is increased then the responses are a droopy Bessel response.
     
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