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[SOLVED] a problrm in Ft doubler

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matin-kh

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hi every one,
this is a ft doubler which i want to use it in canceling the DC offset. i want to know what is Vcmr?? I read about it in Design of Integrated Circuits for Optical Communications dr razavi book it says:
"The bias voltage Vcmr, is chosen equal to the common-mode level of Vi+ and Vi-, allowing the differential pairs to operate with zero systematic offset."
I can't understand the concept of it could you please explain for me 8-O
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with best regards,
 

This looks similar to the concept in op amps.

Common-mode rejection allows the differential inputs to ignore equal DC voltage levels. (Example, ambient mains hum getting to the input wires.)

Then there is the DC offset, which lets you adjust things so your output is 0V when your input is 0V, instead of your output being something else.
 
Written as a formula, the text says you'll want to set Vcmr = (Vi+ + Vi-)/2.
 
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