The correcting transistor serves to maintain mirrored currents between {HEAD ~ each} of the source transistors and each of the sink transistors to eliminate the implications of inequivalent voltage drop across corresponding series of transistors in the amplifier so that the range of output voltage swing, even in low supply voltage applications, closely approaches the value of {TAIL ~ the power supply voltage} 
The correcting transistor serves to maintain mirrored currents between { HEAD ~ each } of the source transistors and each of the sink transistors to eliminate the implications of inequivalent voltage drop across corresponding series of transistors in the amplifier so that the range of output voltage swing , even in low supply voltage applications , closely approaches the value of { TAIL ~ the power supply voltage }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL