Cascaded OP-AMP Question (GBWP).

Discussion in 'Homework Help' started by Chuffed, Sep 9, 2015.

  1. Chuffed

    Thread Starter New Member

    Sep 9, 2015
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    "An op amp has a GBWP of 1MHz. Design a circuit that uses only two of these op amps, to yield a gain of 2, and a BW of 1MHz."

    Just started learning about this subject, but I cannot figure this out. With GBWP - the more you increase the gain, the more it cuts the BW right? Even with two of these amplifiers cascaded, is this possible? We were also taught that the "weakest link determines the BW for the circuit." So anything more than a gain of 1 drops the BW below that 1MHz. Can't find anything in the text, the instructor came up with this problem. I assume that this could be a typo, but I won't be able to ask him for a while.
    Few things I've tried
    -Single Amp
    Gain = 2
    BW = 1MHz/2 = 500kHz

    -2 Amps cascaded
    Gain = 2^(1/2)
    BW = 1MHz/1.414~ = ~707kHz

    Any thoughts?
     
  2. crutschow

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    Mar 14, 2008
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    You are correct.
    Any gain greater than a (plus or non-inverting) 1 will reduce the bandwidth below 1 MHz from an op amp with a GBWP of 1 MHz.
    Certainly adding op amps in series with increase the bandwidth for a given overall gain, but you can never reach a bandwidth equal to the op amp GBWP at a gain greater than 1.
     
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  3. Chuffed

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    Sep 9, 2015
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    I thought so. Thanks for the help.
     
  4. GopherT

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    The OP mentioned "cascade" but is question, in quotes, does not say "cascaded". It simply asks for a circuit with two op-amps to achieve gain of 2 at 1MHz. I haven't looked into this option but could the two op amps be configured as a bridge to make this happen?
     
  5. crutschow

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    Don't see how, but show me the circuit. ;)
     
  6. #12

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    1-(-1)=2
     
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  7. GopherT

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    Both at unity gain, one inverts. Doubles voltage to 1k load.


    upload_2015-9-9_22-33-28.png

    @Chuffed
     
    Last edited: Sep 9, 2015
  8. GopherT

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    Good thing I didn't see your answer. You saved me time and I didn't take advantage of it.

    Thanks.
     
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  9. crutschow

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    The inverting amp with a gain of -1 will have a bandwidth equivalent to a non-inverting gain of 2 or 500kHz, so that won't get you there either. :rolleyes:
     
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  10. #12

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    I didn't know that rabbit was in the hat.

    Could you point me to some education on this subject?
     
  11. crutschow

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    Here's an article that discusses op amp inverting/non-inverting bandwidth versus gain.
     
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  12. Chuffed

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    Sep 9, 2015
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    So I was able to talk to the professor about the question. He says it is not a typo, and that it is possible to do.
    So, back to square one. Seem kind of stuck again. Going to have to go read some more.
     
  13. crutschow

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    Well, I think he's blowing smoke. :rolleyes:
    There's no way to get a gain greater than 1 from an op amp without reducing the bandwidth.
     
  14. Russmax

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    Sep 3, 2015
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    There's a way, but it seems like it's cheating.

    GopherT and #12 both showed a circuit that almost works, except the inverting op amp only has 500 KHz BW.
    Take that circuit and put a C in parallel with the 10K input resistor (R2 in GopherT's circuit) to create a zero in the inverting transfer function at 1 MHz. Just to make it still somewhat your work, I'm not going to tell you what value of C to use. You have to calculate that.

    This will fight the 500 KHz pole of the inverting configuration and extend the frequency response of the inverting amp by 1 octave (doubling), when it bumps again against the unity-gain bandwidth. Then both the inverting and non-inverting op amps will have a BW of 1 MHz, and can add their gains together to get a gain of 2.

    At least Spectre thinks so.

    Regards
     
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  15. JoeJester

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    The simulations look like it's possible using 1V pk-pk. I'd have to see if I have enough parts to breadboard it.
     
  16. Chuffed

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    Know of any sites that I could visit to do some reading on this?
     
  17. crutschow

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    Good thinking out of the box (which I was stuck in). :oops:

    It's only cheating if it's not allowed by the rules and the professor didn't state any, so that's perhaps the type of circuit he intended to solve the problem,
     
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  18. Chuffed

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    Sep 9, 2015
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    Nevermind about my last post. Something just clicked while re-reading your post. Going to go play with some numbers.
     
  19. Russmax

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    Sep 3, 2015
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    Good luck, Chuffed.
    When I thought of it, I thought, "That will never work." Then I simulated it and it worked, so I learned something, which is cool.

    I don't have any particular website for you to visit. There are some good ones, I bet. This is just general amplifier feedback theory, aided by 30 years experience trying to finagle amplifiers to do things.

    Regards
     
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  20. MikeML

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    Oct 2, 2009
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    Here is an open loop response of an opamp that has a GBWP of 1e6:

    114a.gif

    Note Gain at 10Khz, 100Khz and 1MHz.

    Closing the loop two different ways, note that the BW (when gain is down by -3db) is 500kHz for the gain of -1, and 1MHz for the follower. The BW of the difference is actually 666kHz, so the problem goals have not been met...

    114b.gif
     
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