The reference voltage Vref is chosen equal to half the supply voltage to maximize {HEAD ~ the AC voltage swing} of {TAIL ~ the output node} 
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The reference voltage Vref is chosen equal to half the supply voltage to maximize { HEAD ~ the AC voltage swing } of { TAIL ~ the output node }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL