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Buffer stage: CD vs CS

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exp

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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 ...
 

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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