# 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

// Linuxxon

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
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
3,518
515
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
270
54
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.