sallen key filter

Discussion in 'The Projects Forum' started by franky_acc, Apr 22, 2014.

  1. franky_acc

    Thread Starter New Member

    Jan 23, 2014
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  2. franky_acc

    Thread Starter New Member

    Jan 23, 2014
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    Another comment. Looking at figure 4, of http://en.wikipedia.org/wiki/Sallen–Key_topology
    show a Sallen Key unity gain. I build that circuit on a bread board with signal that has a 2Vp-p. The output is the same frequency but the amplitude is less than half. I would say it was .25 of 2V, .5Vp-p. I assume what I am seeing is incorrect. I was expecting since it is unity gain that it would be the same frequency, which it is, and the amplitude.
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    It is a filter. It supposed to filter some stuff out. Seems to me it is working fine.
     
  4. shteii01

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    Feb 19, 2010
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    From wiki: A Sallen–Key filter is a variation on a VCVS filter that uses a unity-gain amplifier (i.e., a pure buffer amplifier with 0 dB gain).

    So. If you want gain, add a second stage whose sole job is to provide gain.
     
  5. franky_acc

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    Jan 23, 2014
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    Hi Shteii01,
    Thanks for the reply. Okay, maybe I don't understand unity gain. But I thought that the signal will keep the same amplitude it has and not attenuate it (of course very little attenuation expected)? But in what I see a 75% attenuation on amplitude.

    thanks
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Unity gain means: Gain=1. I think you knew that.

    The attenuation that you see... Well, could be a few reasons.

    1. Is your o-scope setup right? Maybe you are looking at input signal on Channel 1 and you are looking at the output signal on Channel 2. Are both channels setup for the same? Do you have same vertical scales on both channels?

    2. You have built a filter. Filter's job is to pass signals of certain frequency, block signals of other frequency by attenuating them. So. Maybe your input signal has frequency that the filter is designed to attenuate. If so, then what you see is what is supposed to happen. Since you have not provided any info about the filter, there is no way for the rest of us to know.
     
  7. franky_acc

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    Jan 23, 2014
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    Hi Shteii01,
    The answer to 1 is yes.
    The cutoff frequency I designed the high pass sallen filter for is half of the input frequency. For example if my input frequency is 500kHz, my fc is 250kHz.

    thanks again.
     
  8. shteii01

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    Feb 19, 2010
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    Well. At this point there is nothing the rest of us can do for you. You double check your calculations, double check your parts, double check your circuit connections.
     
  9. franky_acc

    Thread Starter New Member

    Jan 23, 2014
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    Okay,
    so I tried creating the LP sallen and the bandpass sallen just incase I missed something. Both of these, I get the expected output. I can't seem to get the HP to work right?
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Looks like you have a LP filter, not a HP filter. What is the output at 10kHz?

    Edit: Post the schematic.
     
    Last edited: Apr 23, 2014
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What are you using for the amplifier? A typical opamp will struggle to make unity gain at 500 KHz with any gain margin left for feedback.
     
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