OpAmp low pass filter

Discussion in 'Homework Help' started by linuxxon, Sep 15, 2015.

  1. linuxxon

    Thread Starter New Member

    Sep 15, 2015
    1
    0
    Hello!

    I got an assignment in class to design a pre-amplifier, based on op-amps, for a mic->speaker circuit but the simulation doesn't confirm my calculations and I can't figure out what is causing my problem.
    The amplifier should have a voltage gain of 26dB (20x times), lower cut off frequency of 100Hz and a upper cut off frequency of 10kHz. I blocked the lower frequencies with a high pass filter on the signal which works perfectly. The higher frequencies are blocked by a capacitor parallell with a resistor in the feedback loop

    I calculated the cut off frequency to be about 10kHz (should not be less) by choosing R and C to both match 10kHz = 1/(2*pi*R*C)
    I chose R=180k Ohm and C=82pF
    These are then connected to ground through a 10k resistor giving the circuit 20 times amplification.

    Everything but the higher cut off frequency is looking well, it's 1.4kHz off. Why? I think it might be because of the capacitor in the loop being to small.
    Any ideas?

    Screens of the simulation [​IMG]
    [​IMG]
    // Linuxxon
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    I think you need to research op-amp filter topology. Did you get this circuit from some source or did you just imagine that this is the way a filter should be constructed? You also don't say what attenuation is required in the stopband which dictates the number of poles in the filter. A good reference is Van Valkenburg, M.E., Analog Filter Design, or an online description and calculator for the Sallen-Key topology.

    http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

    Some of the behavior you see may be due to the limitations of the opamp. Without any component considerations, the gain starts to degrade above at about 9 kHz. To plot the response of a 741 at 1 MHz. is just plain ludicrous.
     
    Last edited: Sep 15, 2015
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,387
    497
    Are you required to use just one op amp?

    Can you use 3 op amps? One for high pass, one for lower pass, one for gain?
     
    absf likes this.
  4. Russmax

    Member

    Sep 3, 2015
    81
    12
    linuxxon,
    You've made a common mistake. You wanted to make the -3 dB frequency for your HP filter 10 kHz, which you appear to have managed. What you did not realize is that at 200 kHz and above, your C2 is effectively a short. An amplifier with its output shorted to its inverting input is in the unity gain configuration. That's why your response levels off at 0 dB above 200 kHz, until the amplifier reaches its unity gain bandwidth at around 1 MHz, and then begins dropping again.

    You could do several things. You could put an R in series with your source and create a HP filter on the non-inverting input of the opamp. That pole could occure at 200 kHz to cancel the zero you made with your feedback cap. You could put a C in parallel with R7 of just the right value, and the response will continue it's high-pass roll-off without the shelf at 0 dB. You could add a 2nd op amp to do the filtering with a Sallen-Key or Multiple-Feedback filter topology, or even design an S-K or MFB filter around your 26 dB amplifier.

    Regards
     
  5. RBR1317

    Active Member

    Nov 13, 2010
    232
    48
    Plotted the magnitude of the transfer function for your circuit using an ideal op-amp. Deviations in your simulation are likely due to the choice of op-amp, i.e. 741.
    bandpass-amp.png
     
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