OTA Amplifier Design

[AMSA84]

Hello guys,

I'd like to know what are the main differences between a traditional OTA amplifier and the OTA amplifier on this figure:



(those circuits considering that the differential pair is designed using PMOS.

Kind regards.

 

[Dominik Przyborowski]

Refering to literature, ie Baker, Li, Boyce "CMOS circuit,etc" Your Circuit is a classic OTA - diff amp with only inputs and output, which are high impedance nodes.
A traditional diff-amp with current mirror as load is also consider as OTA (5 fets circuit), but from the usage point of view between this two circuits exists some differencies.For example if You will drive the OS stage with 5 fets OTA You have to care about dc matching of loading diff pair current mirror and next stage, while in 9 fets ota You don't care about it, because its output is symmetrical.
In some applications like gm-c filters is also prefereble to use symetrical OTAs.

 

[AMSA84]

So the difference is that the symmetrical we don't need to care for the DC voltage at the ouput and with the other type of OTA that I was referring to, that we can find in sedra's book, we need to take care with that DC voltage. Is that it?

 

[FvM]

A differential output amplifier must care for DC voltage level. Usually an auxilary common mode feedback path is implemented to handle it.

 

[AMSA84]

Well, my design will not consist on a differential output. I was thinking to implement my OPAMP using a symmetrical OTA or that well known Miller OTA amplifier, which can be found on Sedra's book.

The OPAMP is to be used on a error amplifier which will have the objective to derive the error between the input signal and the reference as well as to shape the frequency response of the overall system.

That said, which could be the best one, between those too? I can't find much information about the Symmetrical OTA.