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Making a filter that remains the same when loaded

  1. May 15, 2015 #1
    To make a filter out of a bjt where the poles don't shift when you load it, do you need to have a buffer stage that follows the bjt such as a unity gain mosfet and then put the load after that buffer stage?
     
  2. jcsd
  3. May 15, 2015 #2

    NascentOxygen

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    Basically, yes, though it may be useful to have some gain in the buffer stage.
     
  4. May 15, 2015 #3
    Yes, this is usually the case.
     
  5. May 15, 2015 #4

    Baluncore

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    The form that the output buffer takes will be determined by the signal power, impedance and the characteristics of the load.
    You may be able to use an emitter follower that is already part of the filter.
     
  6. May 15, 2015 #5
    Can't you just use something really simple like this under all circumstances?



    2000px-N-channel_JFET_common_source.svg.png
     
  7. May 16, 2015 #6

    meBigGuy

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    That will not be linear. Look up "emitter follower" or "source follower". That would be simplest buffer (with no voltage gain). If you need voltage gain, then you essentially need to build an amplifier. The characteristics of the amplifier will depend on what you need to accomplish.
     
  8. May 16, 2015 #7
    Your circuit seems to be an amplifier rather than a filter. In addition it will give 180º (ish) phase shift. And as meBigGuy pointed out it's non-linear. (The phase shift is 180º for most of its bandwidth, but drifts near the knee frequency.)

    What is it you want this stage to do? If it's a filter, what characteristics do you want it to have (bandwidth, etc.). Are you operating at a really high frequency? Is that why you want a BJT?

    There are lots of nice op-amp filters online, but they tend to be frequency limited. Push-pull amplifiers are more linear, but have higher output impedances. Current mirrors can provide impedance matching (like an emitter follower) but tend to be the frequency limiting part of the op-amp.

    Filter design is an arcane art. There are lots of considerations that mostly don't matter -- except when they do. We need more information about your project.
     
  9. May 16, 2015 #8

    Baluncore

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    Every circumstance is different. There is no one solution to all problems.

    Please post the schematic of your prototype BJT filter with your required specifications. We can then give you some good solutions to the output impedance problem.
     
  10. May 16, 2015 #9
    Alright, thanks. Between C1 and R1 is where the output will be coming from.

    7Xb6r5Q.jpg
     
  11. May 16, 2015 #10
    The circuit I posted above is a bandpass filter with a pretty reasonable gain in the midband. There's 40db/decade rolloff either side of the midband and the cut off frequencies are at about 100hz and 10khz.
     
  12. May 16, 2015 #11

    NascentOxygen

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    Is this your design for a particular real-world purpose, or is it a circuit you were provided with and asked to investigate as a lab exercise?
     
  13. May 16, 2015 #12
    real world
     
  14. May 16, 2015 #13

    Baluncore

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    A real world problem.
    The Q1 amplifier with primitive bias will be a disaster.
    Gain will be signal amplitude dependent.
    There will be harmonic generation due to the non-linearity of the Vbe junction.

    Attached is the .asc file for those with LTspice.
     

    Attached Files:

  15. May 16, 2015 #14
    You don't need to worry about that. I've ensured that the BJT is staying within the active region throughout the complete cycle of the AC signal by doing some fairly drawn out small signal analysis.
     
  16. May 16, 2015 #15
    I graphed the voltage drop across the Vce junction in the circuit you uploaded and you can see that it never falls into saturation, so what are your concerns exactly?

    foTUKKd.jpg
     
  17. May 16, 2015 #16
    Also, I can't see anything wrong with this bode plot.
    WigiMcD.jpg
     
  18. May 16, 2015 #17

    NascentOxygen

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    There are a number of problems with such rudimentary biassing. Okay, you have tweeked it so it works just fine right now, but ....
    things will change as temperature drifts, but worse is when that transistor burns out (for some mysterious reason that I cannot always pin down) the transistor you replace it with will be quite different in gain and the circuit will need to be checked and tweeked all over again. That's why a more stable arrangement is always used.

    Do you need this filter to continue to work reliably for any length of time?
     
  19. May 16, 2015 #18
    This is what real world op-amps consist of, you just aren't accustomed to seeing this type of thing because everyone just buys IC op-amps these days.
     
  20. May 16, 2015 #19

    Baluncore

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    That is incorrect. You are getting advice here from experienced professional electronics design engineers.

    What is the vertical scale dB/div ?
     
  21. May 16, 2015 #20
    fixed the graph xNbydf2.jpg
     
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