exp
Full Member level 1
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 ...
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 ...