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what is a Gm stage?

Discussion in 'Electronic Design' started by [email protected], May 31, 2007.

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  1. Guest

    Hi,

    I'm trying to figure out how to implement a Gm stage. I understand the
    basic idea - voltage in, current out - and I've seen some figures like
    (a) and (b) shown in the following diagram, ie. a differential pair
    with no load representing a Gm stage:

    http://farm1.static.flickr.com/223/523614745_fc290139dd_b.jpg

    But I'm not sure what exactly to do with this differential pair with
    no load. I Can I just use the iout+/- from the differential pair
    directly as the inputs to the next stage? Or do I need a load for the
    differential pair? And what could I use as the load? I thought about a
    current mirror, as in (c), but that means it's just an voltage
    amplifier - the current never goes "out" of this stage, it just gets
    redistributed between the input transistors. This seems wrong (???).

    Perhaps some DC current sources (with common mode feedback for the
    biasing), as in (d). Then the iout current due to a change in Vin is
    forced out to the load? But isn't this just a differential amplifier?

    Thanks for your help,

    Ted
     
  2. Guest

    It is wrong. The point about a current mirror is that both of the
    transistors at the top of the diagram are sourcing the same current,
    so the difference between the currents being sunk by the two input
    transistors has to flow into, or out of, the output connection.

    If you used resistors as loads on each of the drains, then - and only
    then - you can run the stage as a source of voltage gain with very
    little current flowing in the output connection. Do pay attention.
    The Gm stage in a differential amplifier ....
     
  3. PeteS

    PeteS Guest

    A Gm stage is what you have stated but not realised; a forward
    transconductance stage. As you correctly surmise, it is an amplifier,
    but specified by it's Io vs. Vin characteristics rather than some other
    parameter.

    I didn't open your link, but maybe that helps.

    Cheers

    PeteS
     
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