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cmfb - creation cm voltage

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akbarza

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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
 

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

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.
 

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
 

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