about Rail-to-Rail Opamp

Discussion in 'Homework Help' started by firstman, Apr 5, 2010.

  1. firstman

    Thread Starter New Member

    Mar 19, 2010
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    We know that output cascade of Rail-to-Rail opamp use common emitter.Common emitter has very high output resistance because two collectors connect in output. But when I measured output resistance of Rail-to-Rail opamp in micro-cap 9,I get low out put resistance . I want to know , why I get this result? Please explain me.The measurement result of output resistance of Rail-to-Rail ( AD8031 ) is shown in below.

    AD8031


    [​IMG]
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    I think Rout is low thanks to voltage negative feedback.
    Negative feedback decreases the output resistance by a factor ( 1 + β A_ol)
    A_ol is a open loop gain
    β - feedback factor ( feedback gain )
    So
    Rout = Rout_open_loop/(1+A_ol*β)
     
    Last edited: Apr 5, 2010
  3. firstman

    Thread Starter New Member

    Mar 19, 2010
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    thanks your answer. But it is without negative feedback. It is open loop output resistance measurement of rail-to-rail(AD8031).
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Post the schematic of your measurement technique.
     
  5. firstman

    Thread Starter New Member

    Mar 19, 2010
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    Output measurement of rail-to-rail (AD8031) in micro-cap 9 is shown in below.

    [​IMG]
     
  6. Ron H

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    Apr 14, 2005
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    How did you come up with 531μV for the input offset voltage?
     
  7. firstman

    Thread Starter New Member

    Mar 19, 2010
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    Input offset voltage effect the output resistance and other parameters.In this case, the offset voltage is so small and can neglect it. Output resistance measurement of rail-to-rail (AD8031) is shown in below, as we assume input offset voltage is zero.Clearly, I get same value of open loop output resistance.


    [​IMG][​IMG]
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think your op amp is saturated at one of the rails, so you are seeing the resistance of a saturated transistor.
    For the simulation, try running the op amp with unity DC feedback, but open loop for AC. You can do this by connecting a 10k resistor from output to -in, and a 1000 Farad cap from -in to GND. Connect +in to GND, and use the power supplies you show in your schematic.
    I have done this with many op amps. LTspice does not converge for this op amp. Perhaps your simulator will.
     
  9. firstman

    Thread Starter New Member

    Mar 19, 2010
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    I did simulation as you said , but result is same.If my simulation is wrong,please reply me.Thanks for your help.
    [​IMG][​IMG]
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Your sim circuit looks OK. How do you measure Rout? Is there an equation you use?
     
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