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