U.S. Patent No. 1,173,079 - 02/22/1916 - Ernst F.W. Alexanderson
Comments Concerning the Patent Gerald Stanley 03/03/2003: "First push-pull amplifier. Invented by E.F.W. Alexanderson, this most basic invention has shaped generations of audio amplifier design. This 1916 patent shows a cornucopia- of circuits with push-pull thrown in for good measure.  E.F.W. Alexanderson was one of America's most prolific inventors. The text Alexanderson - Pioneer in American Electrical Engineering by James E. Brittain on Johns Hopkins press 1992 is well worth the read. The appendix lists his some 300+ patents."