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My assignment for a school project on Circuit Theory, is to construct a Band Pass Filter at least 3rd order with a stop band around 20kHz.

First things first, I need to design the circuit. I have to use a 741 op amp but after I searched a ton of stuff on the internet, I didn't find any implementation that qualifies these meets. Could a combination, of a 3rd order Low Pass Filter and a 3rd order High Pass Filter, give me a 6th order Band Pass Filter?

I also checked Analog Filter Wizard, and after I inserted the desirable settings, it gave me the circuit with all the components but 741 op amp isn't available. What direction should I follow?

Thanks in advance.

Alexandros
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    Are you supposed to apply what you learned in your Circuit Theory class, or just find something on the internet? – Tyler Apr 16 '19 at 22:49
  • Of course I am supposed to apply the things that I learned, but the assignment is about the lab lessons that we take on this course. The lab lessons and the theory lessons are very different. I know that this isn't the proper way, but even the teachers told us that this assignment has to be done with the help of the internet. So, here I am. – Alexandros Apr 16 '19 at 22:53
  • My main consider at this time is to design the circuit. I'm very close to finding it but I'm not exactly sure, so that's why I asked here. – Alexandros Apr 16 '19 at 22:55
  • Use the Analog Devices part – Tony Stewart EE75 Apr 16 '19 at 23:02
  • So I suppose, I will go different with that. About the design, though, should I go with a combination of a low-high pass filters or use the Analog's generated design? – Alexandros Apr 16 '19 at 23:04
  • Are you supposed to use **one** op-amp in your circuit, or are you allowed to use multiple ones? – TimWescott Apr 16 '19 at 23:22
  • @TimWescott There are no limitations. The only thing that I should take care of is the order of the filter that it should be at least 3rd. – Alexandros Apr 16 '19 at 23:32
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    You may need a faster opamp. Rejection bands require lots of gain (and thus lots of bandwidth) to push the rejection far far down. – analogsystemsrf Apr 17 '19 at 00:21
  • @analogsystemsrf I will try to figure it out. The main problem that I face is how to deisgn the circuit. – Alexandros Apr 17 '19 at 00:27
  • @analogsystemsrf has a valid concern. Since you have no limitations, consider using inductors & capacitors (and resistors)...rather than the more limited R,C filters of Analog's generated design. Doing so can ease requirements on the opamps. Understand that engineers often avoid inductors - they are less than ideal components - a design using them might be considered less elegant. – glen_geek Apr 17 '19 at 00:54
  • I can't think of a way to lead you to this with questions: design three 1st-order bandpass sections (assuming that when you say "3rd-order" you mean three resonant pole pairs), and just cascade them. Section A feeds section B feeds section C. – TimWescott Apr 17 '19 at 00:54
  • @TimWescott Okay. I am close to that, although I implemented it with 2nd order bandpass sections. – Alexandros Apr 17 '19 at 00:58
  • "...with a stop band around 20kHz" - what exactly does this mean? – Bruce Abbott Apr 17 '19 at 03:33
  • [Things to keep in mind when you **must** use the ancient 741.](https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp/304522) – JRE Apr 17 '19 at 07:44

3 Answers3

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So long as your frequency range is reasonable, and you're using large enough plus and minus power rails, you should be able to swap in the 741 for other op amps the design tools give you. Just match up the functions to the right pin numbers.

Scott Seidman
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In order to design any filter, there must be realizable specifications and tolerances before designing on an implementation.

So far you have;

Specs

  • Filter Order N >= 3
  • Stop Band = 20 kHz
    But you need to include more.
  • Gain = ____ dB, inv. or non inv. optional)
  • fo = ______ Hz, center frequency
  • Rp = _____ dB, Allowable Passband Ripple e.g. 0, 0.1 to 3 dB **
  • BWp= ____ Hz, Passband ** ( which affects Q of each stage )

  • BWs= _____ Hz Stopband **

    • Asb= _____ dB, stopband **

  • Component Tolerances =____, R=5%,1%,0.5%,0.1%,0.01% C=10%,5%,2%,1%,0.5%

  • ___ GBW min of Op Amp or __ TBD depends on Q², fo, Gain
  • other ? DC Vref ? Single or split PS? tuneable?

Response options;

Bessel, Chebyshev 0.5dB, 1dB , Linear phase 0.05 deg, 0.5 deg, Butterworth, Raised Cosine, etc..... ** options above ** are more important for these

Configurations

  • single ended, differential, Multiple feedback
  • Sallen & Key

e.g. Gain =1 , fo=1kHz, Rp=0, BW=100Hz 4th order. GBW=1MHz, 1% parts, Sallen & Key

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Tony Stewart EE75
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The 741 opamp design is 52 years old and has trouble with high output levels above 11kHz. You need a modern opamp that works well to at least 110kHz.

Audioguru
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