I also read that a long-tailed pair as an op-amp (this might be the wrong part). So according to the buffer circuit above, I believe the circuit using long-tailed pair will be like this (edited: this is my own idea only of how the long-tailed pair can be turned into a unity gain buffer which I hope to be confirmed or corrected):
However, the problem is that this configuration implies that the non-inverting input terminal will always remains at ground (0V) while the inverting input will be at G*Vin (G: gain). Hence I can't understand how negative feedback is implemented in this case since the Vout is supposed to be G(Vin - Vout). Please correct me if there are any mistakes and demonstrate how long-tailed pair can be used as unity gain buffer (and explained with circuit schematic will be very appreciated).