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fully differential amp without cmfb

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ljy4468

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hi everyone~

i've a question about amp without cmfb
I know that fully differential amp without cmfb 's output will go to vdd or gnd.
so it needs cmfb.

now i've a question.
why amp's output goes to vdd or gnd????(without cmfb)
 

Fully differential amplifier without feedback have very high differential gain and very high common mode gain.
Differential feedback provides certain differential gain and common mode feedback provides small common mode gain.
 

If CMFB is not present, the common mode gain will increase and deviates the basic idea of DIfferential Opamp. So inorder to control the common mode gain,we introduce as Voltage controlled voltage source circuit. This will maintain a constant DC level at the output. This value is actually half way between VDD and VSS. Common mode feedback will have little influence on differential gain,but affects the common mode rejection ratio. This also increases the power dissipation. We need to take care of the poles which may affect our unity gain frequency.
 

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