cmfb - creation cm voltage

[akbarza]

hi
how can we create in cmfb amplifier circuit a CM voltage?
explenation that CM voltage is ideal( or desired) common mode output voltage.
i am reading Gray & Mayer book and in chapter 12 (about cmfb) has been used CM voltage , but in this chapter does not have explained how to create this voltage
i think with an inverter that its output tied to its input ,maybe can be create desired voltage but the mismatch and thermal effect will change this value.
thanks

 

[Dominik Przyborowski]

Before chapter 12 is a chapter 4 in this book. Read it, there is an answer for your question.

 

[ashrafsazid]

The CMFB compares between your "output common mode" and "desired common mode" and feeds back a negative signal for regulation. The desired CM voltage you can generate by using a resistive divider from rail-to-rail, assuming your rails are already robust.

 

[akbarza]

hi ashrafsazid
itis in first glance, but then below problem are raised:
large area
power consumtion
matching thermal effect

i think can create with inverter that output connected to input, but then thermal effect and mismatch are leaved