OpAmp low pass filter

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

  1. linuxxon

    Thread Starter New Member

    Sep 15, 2015

    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]
    // Linuxxon
  2. Papabravo


    Feb 24, 2006
    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.


    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
    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


    Sep 3, 2015
    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.

  5. RBR1317

    Active Member

    Nov 13, 2010
    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.