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  1. #1
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    Buffer stage: CD vs CS

    Hi,

    I need to add a buffer stage to my OTA design to make it an op amp. My design will be in 28nm VDDnom=1V (but I increase to 1.2V).

    Until now, when I heard buffer I always thought about CD but I realized that a CS stage with unity gain can also be a buffer. Can someone give the advantages/disadvantages or provide a reference?

    I played around with both but I am not sure which one to pick.

    The CD stage seems much more linear, has lower Rout and better bandwidth for a smaller current. Am I observing that right? However, this comes at the expense of the common mode voltage shift of VGS, which means my OTA must provide a common mode voltage much smaller than VDD/2 which reduces VDS and hence gain of the OTA.

    In terms on linearity, the total linearity seems to be dominated by the high gain OTA anyway:
    OTA (simple diff pair): -15 dBm
    The CS buffer: +10 dBm
    The CD buffer: +30 dBm (!)

    So I think in terms of linearity both would cut it.

    So if anone could provide the advantages/disadvantages for each topology this would be great. Just wondering why I have never heard about CS stage as buffer ...

    •   Alt9th December 2016, 06:17

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  2. #2
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    Re: Buffer stage: CD vs CS

    A common source stage will only be a unity gain amp
    either under very specific bias, load and process
    conditions, or when driven in a feedback loop. It will
    be hard to stabilize across all. The common {source,
    emitter} stage is a voltage amplifier and to reduce
    its gain to 1, needs a (compensating) fractional
    gain. And because gain variability is high, feedback.

    A common drain stage will never be truly unity gain
    unless also feedback controlled, but may be "close
    enough" without feedback, and can be easier to
    stabilize - sometimes a very simple diff amp can
    do the job without compensation, provided you
    don't need rail-rail input range. Any plain common-
    {drain, collector} stage is sub-unity voltage gain
    on its own, and to get unity gain you need to have
    a preceding gain stage (not much, but some).



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