Can the Gain-Bandwidth Product be increased by combining multiple Op Amps?

Discussion in 'General Electronics Chat' started by Matjelo Naleli, May 19, 2016.

  1. Matjelo Naleli

    Thread Starter Member

    Oct 7, 2015
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    I am trying to build a high Q resonator at 15MHz using op amps. The trouble that I run into is that when the Gain-Bandwidth Product of the op amps is low the resonance shift dramatically and the Q-factor drops. In the attached picture I have shown a circuit diaram, the intended transfer function and the Bode diagrams for a case where GBP is 50MHz and where it's 20GHz. At GBP of 20GHz the frequency response is very close to what I expected/intended it to do (which is resonance at 15MHz).

    So what I am really seeking help with is how one can improve GBP perhaps by combining several op amps (if it's something possible) or some other way. I would also appreciate any suggestions on some op amps with high GBP. By high in this case I mean hundreds MHz to a few GHz or higher.

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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  3. Papabravo

    Expert

    Feb 24, 2006
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    Sorry, but GBW doesn't work that way.
     
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  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    You can use multiple stages, each stage providing small amplification. This way you keep a lot more of the bandwidth in each stage because each stage has only small gain. The downside is that you use a lot more parts and there are more inductances and capacitances due to all the wires and components.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    So back of the envelope how many stages would be required for say 25 dB gain and a bandwidth of 15 MHz.?
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    That's gonna depend on the opamp selected. 25 dB is a signal gain of about 18, so at 15 MHz that's a GBP of 266 MHz.

    Back of the envelope, if you want to do it with five stages then each stage needs a gain of 5 dB, or about 1.8. So opamps with a GBP of about 30 MHz would put you in the ballpark with very little margin. I don't have a good feel for how much margin you want in practice.

    The TI THS3001 has a GBP of 420 MHz, so if everything else falls in line a single amp might be sufficient. I have very little experience (and none of it within the last couple decades) with current-feedback opamps, so I don't know what kind of gremlins are lurking in an application of this type.

    Since this is well outside my normal playground, I'm probably missing something major.
     
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    If you are trying to help the OP, you are way off from his request of 20GHz GBP. If you're carrying on a side conversation to win a pissing contests, knock it off or start your own thread.
     
  8. Marley

    Member

    Apr 4, 2016
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    Op-amp, high-Q filter, 15Mhz - can't be done! Think discrete inductors and capacitors together with simple gain stages.

    Or even crystal or piezoelectric filters.
     
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  9. WBahn

    Moderator

    Mar 31, 2012
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    More trolling, huh?

    I just answered a question (one that you apparently didn't have a problem with).

    I guess, according to you, providing a choice of op-amp that has a GBP of 420 MHz is "way off" his request of, "I would also appreciate any suggestions on some op amps with high GBP. By high in this case I mean hundreds MHz to a few GHz or higher."
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    No, I think #12 was heading in the right direction. We need the OP back to see what suits him/her. Until then, keep proposing as many sub-standard solutions as you want and creating more side-bar conversations - it doesn't muddy the conversation at all.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    I don't quite see it that way. A statement was made that you could achieve a result if you strung enough lower capability devices together. I wanted to put a number on it to see if it was practical. Your mileage may vary. We all want to help the TS/OP, but the best way to help him with his problem remains unclear. I do think he needs to weigh in. It is this kind of thinking out loud that leads to amazing results. Point is nobody has to be perfect 100% of the time and have all the answers. Going down blind alleys can be a useful exercise, so lighten up will 'ya.
     
  12. The Electrician

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    Oct 9, 2007
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