Phase margin and ideal opamp

Discussion in 'General Electronics Chat' started by FerrariRace, Aug 7, 2015.

  1. FerrariRace

    Thread Starter New Member

    Aug 7, 2015
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    Hi guys,

    I see a lot of posts talking about the phase margin of an inverting opamp as beta*open loop gain. Here beta is the feedback factor. Lets say I have an ideal opamp in ltspice since I am using the universal opamp 2 model. A(open loop) is fixed and does not vary with frequency. Neither does the compensation network , assuming it is made of resistors only. How do I find the phase margin of such a circuit?
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,977
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    The LTspice opamp 2 component has no circuit and requires a subcircuit model to be added. It's the subcircuit characteristics that determine the gain and phase margin.

    An ideal op amp does not have a constant open-loop gain with frequency. It rolls off smoothly at 6dB per octave. If it did not roll off in this manner it would not be stable in a closed-loop configuration with feedback.
     
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  3. FerrariRace

    Thread Starter New Member

    Aug 7, 2015
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    I have attached a file . I did not add any circuit to the universal opamp. I still get a roll-off rate. Am I doing something wrong? How do I set the location of the first pole and second pole of the ideal opamp?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Okay, I was mistakenly referring to the opamp2 component, not the UniversalOpamp2, which does indeed have gain and internal poles.
    If you right-click on the op amp, you will see the parameters that can be adjusted. The can be set to whatever phase margin you desire.
    What is your definition of an "ideal op amp"? That will determine where the poles are set.
     
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  5. ScottWang

    Moderator

    Aug 23, 2012
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    Next time when you upload the *.asc, please also attach a circuit for *.gif file, it will let other members whom maybe not play the LTspice, but they still can help you.
     
  6. FerrariRace

    Thread Starter New Member

    Aug 7, 2015
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    OKay. I will do so. I was asking for the syntax in LTSPICE. I wanted to know how to set it in the circuit.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    gwb.gif
     
    • gbw.asc
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  8. FerrariRace

    Thread Starter New Member

    Aug 7, 2015
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    Ah okay. Thanks a lot for the help
     
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