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Buffered Opamp

Discussion in 'Electronic Design' started by [email protected], Sep 17, 2008.

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  1. Guest

    I'm studying buffered opamp and i've found this configuration
    http://img156.imageshack.us/img156/5640/afsdafen7.png
    i don't understand second statement
    "reduces output load at the drains of M6 and M7"
    I don't understand why a large resistance at the drains of M6 and M7
    is a bad thing...if i consider voltage gain of second stage, it is
    similar to
    (gm6+gm7)*output resistance
    so, if output resistence is high, the voltage gain of the second stage
    is high and the opamp votage gain is high....i think this is a good
    thing....
    What's the problem?
    Thanks in advance
     
  2. Phil Allison

    Phil Allison Guest


    ** When the " load " on some source is " reduced " - it means the load
    impedance seen by that source is increased.

    The logic is simple - if you add load to the back of a horse, the horse is
    not so happy about it.




    ...... Phil
     
  3. Eeyore

    Eeyore Guest

    Then you need to read up on what a source follower does. Or emitter
    follower in the case of bipolar. It's very important.

    Graham
     
  4. Eeyore

    Eeyore Guest

    It would be a bad thing if M6 and M7 weren't buffered.

    Graham
     

  5. "reduces output load at the drains of M6 and M7"
    I don't understand why a large resistance at the drains of M6 and M7
    is a bad thing...

    A large resistance here is a good thing!! That is what the output
    stage looks like to the FET's driving it. Perhaps you are confused by
    what "reduces output load" means. This means an increase in the
    impedance.
    George
     
  6. Guest

    M8 and M9 form a simple low impedance circuit that both source and
    sink base current in Q19 to reduce the miller effect, It's not rocket
    science
     
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