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.

  • 1
    \$\begingroup\$ Are you supposed to apply what you learned in your Circuit Theory class, or just find something on the internet? \$\endgroup\$
    – Tyler
    Apr 16, 2019 at 22:49
  • \$\begingroup\$ 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. \$\endgroup\$
    – Alexandros
    Apr 16, 2019 at 22:53
  • \$\begingroup\$ 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. \$\endgroup\$
    – Alexandros
    Apr 16, 2019 at 22:55
  • \$\begingroup\$ Use the Analog Devices part \$\endgroup\$ Apr 16, 2019 at 23:02
  • 2
    \$\begingroup\$ You may need a faster opamp. Rejection bands require lots of gain (and thus lots of bandwidth) to push the rejection far far down. \$\endgroup\$ Apr 17, 2019 at 0:21

3 Answers 3


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.

  • \$\begingroup\$ Okay then. I' ll go with that. Thanks. \$\endgroup\$
    – Alexandros
    Apr 16, 2019 at 23:37

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

So far you have;


  • 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


  • 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

enter image description here

  • \$\begingroup\$ Ignore the -2 trolls who have nothing to contribute \$\endgroup\$ Apr 17, 2019 at 15:00
  • \$\begingroup\$ I already commented use any tool IC but learn the above. Otherwise your Q may impact severely on the 741 which barely satisfies my example. See 2nd stage min GBW \$\endgroup\$ Apr 17, 2019 at 15:14

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.

  • 1
    \$\begingroup\$ Unless, of course, it's a school assignment that requires the LM741. I think that's the only reason it's still in production. \$\endgroup\$
    – TimWescott
    Apr 16, 2019 at 23:20
  • 2
    \$\begingroup\$ This isn't an answer to the question. It should fit nicely in a comment. \$\endgroup\$ Apr 16, 2019 at 23:35
  • \$\begingroup\$ I have said it before here that 741's should be banned. \$\endgroup\$ Apr 17, 2019 at 1:07
  • \$\begingroup\$ Still has 1MHZ GBW , not bad for a grandpa Op Amp but you are correct about full swing BW \$\endgroup\$ Apr 17, 2019 at 4:07

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