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  1. #1
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    2 kinds of PMOS current sources

    1)Click image for larger version. 

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    2)Click image for larger version. 

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    Which one is more commonly used? option 1 doesn't need external bias voltage and option 2 has.
    option 2 can hooked up to a CMFB circuit to improve.

    •   Alt7th January 2017, 23:22

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  2. #2
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    Re: 2 kinds of PMOS current sources

    Since you seem to have a semi-infinite series of the
    most basic questions I think a textbook is right for
    you.

    But the textbook isn't going to teach you to
    recognize, for example, that "Vb" in the lower is the
    exact same thing as M3:M4 common gate node in
    the upper, only difference being whether or not the
    biasing is shown and how complicated it may be.

    The only way to develop this faculty is to think for
    yourself, not pose point-questions (that miss the
    point) and walk away with point-answers rather than
    developing a gut understanding of how circuits work.

    CMFB circuit is not to "improve" the current source.
    It's to get the right answer (of Vout swinging equally
    across Vin null) by setting the current source value.


    1 members found this post helpful.

    •   Alt8th January 2017, 00:21

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  3. #3
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    Re: 2 kinds of PMOS current sources

    Quote Originally Posted by dick_freebird View Post
    Since you seem to have a semi-infinite series of the
    most basic questions I think a textbook is right for
    you.

    But the textbook isn't going to teach you to
    recognize, for example, that "Vb" in the lower is the
    exact same thing as M3:M4 common gate node in
    the upper, only difference being whether or not the
    biasing is shown and how complicated it may be.

    The only way to develop this faculty is to think for
    yourself, not pose point-questions (that miss the
    point) and walk away with point-answers rather than
    developing a gut understanding of how circuits work.

    CMFB circuit is not to "improve" the current source.
    It's to get the right answer (of Vout swinging equally
    across Vin null) by setting the current source value.
    Option 1 doesn't need you to have another circuit to generate bias. It's self generated. Option 2 needs that.

    CMFB is a feedback, its an improvement in a sense that it keeps the pull up current and pull current the same.
    If it's not an improvement why bother to have extra circuitry to that?

    If no improvement between the two, option 1 is always preferred because it has less component.

    If you want engage into deeper, I can do the same.



    •   Alt8th January 2017, 00:40

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    Re: 2 kinds of PMOS current sources

    CMFB feedback isn't particularly related to current sources. It's a necessary element to bias amplifiers with differential output.

    M3:M4 of circuit 2 are a dual current source while M2 in circuit 1 is a single current source. M3 + Rbias form a bias circuit rather than a current source, as already explained.

    If your comparison possibly refers to a differential pair driving a current mirror (circuit 1 without Rbias), you get a different amplifier topology with singled ended output while circuit 2 is differential ended. In so far, both circuits can't compare.



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