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fully differential opamp implementation

Discussion in 'Electronic Design' started by tiger66, Mar 10, 2007.

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  1. tiger66

    tiger66 Guest

    Hi All
    I need a fully differential opamp for my circuit design.
    Unfortunately, the simulator I use, multisim, does not have a build in
    fully differential opamp.

    So, I was wondering if any of you have experience implementing a fully
    differential opamp, can you please help me to implement one?
    Any resources (i.e. netlist, circuit diagram) will be really

  2. John  Larkin

    John Larkin Guest

    google "three opamp differential amplifier". Or cheat and use a Spice
    vcvs, which is a perfect diffamp.

  3. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    But the output is not diff.

    Or cheat and use a Spice
    But the output isn't referenced WRT to common mode. You'll have to use 2
    VCVS for this, or 3 if you don't want to reflect the output current to
    the CM ref input.
  4. Jim Thompson

    Jim Thompson Guest

    I'll knock one out for you for $25K, payable upfront ;-)

    ...Jim Thompson
  5. tiger66

    tiger66 Guest

    Thanks very much John for the response.

    I was designing my fully differential opamp using "three opamp
    differential amplifier" concept initially because it only requires two
    basic opamps and 4 resistors which my simulator has all the

    Unfortunately, after I connect the input of the three opamp
    differential amplifier to the output of my modulator, I get "timestep
    too small" error message when simulate. Do you know how to debug this

    Currently, my circuit can simulate fine using vcvs as you suggested.

    Do you know if there are any disadvantage for using vcvs instead of
    normal fully differential opamp? Would it affect my output signal
    spectral analysis significantly?

    Thanks again for your assistant
  6. tiger66

    tiger66 Guest

    Hi Fred
    Are you talking about "common mode feedback circuit"?

    I was reading about that when I was researching about fully
    differential opamp. Unfortunately, I can't find a detailed explanation
    about the CMFC(i.e. the articles only show the schematics but not the
    values I should use for MOSFET, capacitor, resistor and reference
    voltage). So I really don't know how to integrate CMFC with opamp.

    Do you have experience implementing the fully differential opamp? Can
    you send me a netlist of that, so I know what values to put for each

  7. tiger66

    tiger66 Guest

    Thanks for the offer Jim, but I am a poor engineering student. So I
    can't afford that kind of money

    But I think I can offer you $1 if you can help me design a fully
    differental opamp. $2 if you can help me design a chopper stabilized
  8. Jim Thompson

    Jim Thompson Guest

    "Perfect" VCVS has no delay or phase shift, infinite bandwidth... all
    of the constituents of instability.

    Back up and study how to model an OpAmp, with at least one pole.

    ...Jim Thompson
  9. Eeyore

    Eeyore Guest

    I suspect you don't mean 'op-amp' you probably mean differential amplifier.

  10. John  Larkin

    John Larkin Guest

    Is that what he meant by "fully differential"?

  11. John  Larkin

    John Larkin Guest

    If you are trying to evaluate the spectral properties of a diff-output
    mixer, the vcvs is literally ideal. An opamp thing will just add its
    own complications and abberations.

    If you eventually want to build the circuit, you will need a diffamp,
    I suppose. Or a transformer. There are some nice, integrated, very
    fast diffamps around these days.

  12. Hi,
    If he search the spice models he should look by the factories...

    may be a good start.

  13. tiger66

    tiger66 Guest

    Thanks, I guess I need to learn how to use Pspice now.

    Can Pspice generate the Power Spectral Density of a signal? I need to
    do some spectral analysis in my circuit

  14. Ban

    Ban Guest

    With the 3 opamp IA circuit you can just add an identical output
    differential amp stage with the inputs inverted to obtain the other output
  15. Jim Thompson

    Jim Thompson Guest

    Yes. You can do a spectral analysis in Probe, or can define
    distortion analysis in the setups to get a table.

    ...Jim Thompson
  16. Hi tiger,
    You do not need to switch the program. Multisim can use (p)Spice models as
    well as LTSpice for example. The changes that may have to be made are very
    little and almost known in the support forums.


    .... who prefers to use LTSpice
  17. vasile

    vasile Guest

    Even it can, and even if you do it right on simulation, on the real
    world will be different
    (could be very different at high frequencies)
    Focus on fully differential OA datasheet available at Analog Devices,
    Texas Instruments and Linear Technology.
    There are a lot of articles about those. Some clues: AD8132, THS4121,
  18. John  Larkin

    John Larkin Guest

    The cold end of the resistor going into the + input of the 3rd amp is
    the output reference; it need not be ground.

  19. tiger66

    tiger66 Guest

    Hi All
    Does anyone have experience using multisim?

    I keeps getting a "timestep too small" error message when I simulate.
    I tried all the debugging method I found online, but non of them
    solves my problem

    Any input/assistant on this would be really appreciated

  20. Eeyore

    Eeyore Guest

    I told you before.

    It's crap.

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