U.S. Patent No. 1,902,495 - 03/21/1933 - Alan S. Fitz Gerald
Comments Concerning the Patent Gerald Stanley 03/03/2003: "Power supply topology often said to be new in audio amps. The first electronic example of a totem-pole output stage or half-bridge as it is known in the power electronics literature was created by Alan Fitz Gerald of GE in 1929 (ed. 1,902,495). The full-bridge was created by him the next year in 1930 (ed. this patent). The series resonant converter was simultaneously invented, because the plasma tubes that he used required the current to cease (resonate to zero) to be able to turn off.  Many an audio amplifier of today uses this same basic circuit to power the product, but today the thyratrons have become IGBTs and MOSFETs."