http://en.wikipedia.org/wiki/Differential_amplifier

http://www.ece.mtu.edu/faculty/goel/EE-4232/Diff-Amps.pdf

http://www.ecircuitcenter.com/circuits/bjt_diffamp1/bjt_diffamp1.htm

http://www-personal.engin.umd.umich.edu/~fmeral/ELECTRONICS%20I/0070%20DifAmp/0070A%20Dif%20Amp%20Prob%20Answ.pdf