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This is a simple two stage OTA ... with fully differential output at first stage and single ended at the second stage. Gain will be approx gm^2 (ro || ro)^2 .. assuming the all the ro and gms are same.
The MOS arm connected to VB2 is used for CMFB (Common Mode Feedback) as the first stage is fully differential.
Working of the CMFB Loop (as it seems to me): The purpose of CMFB is to hold the O/P of a fully differential amp at a steady value ... If say for some reason the output of the first stage increases then the VGS of the center lower NMOS increases. Therefore it draws more current.
As the current is fixed by a current source at the top (as in all differential amplifiers) the current through the input PMOS pair reduces and the output node is pushed down. Thereby nullifying the initial increase at that node. Here the O/P of the first stage is held at one VGS of the center lower NMOSs.
Ya compensation is needed if this amplifier is used in a closed loop configuration (like buffer may be). I don't know why it has not been shown here. Again the stability of the CMFB loop is also checked. If that is unstable we compensate that loop as well or else any disturbance at the output would not settle down rather it would grow and saturate at rails if the CMFB loop is unstable.
Note: An OTA can be used in open loop configuration but it will behave as a comparator...
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