Collectors or {HEAD ~ drains} of the first and fourth transistors are coupled together and {TAIL ~ collectors} or drains of the second and third transistors are coupled together 
Collectors or { HEAD ~ drains } of the first and fourth transistors are coupled together and { TAIL ~ collectors } or drains of the second and third transistors are coupled together
OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH