Berachot 50a: Who is Raba Tosfa'ah?

Sorry for the gap. I'm so tired and busy.

From Berachot 50a:

Raba Tosfa'ah said: If three persons had a meal together and one said grace for himself before the others, his zimmun is effective for them but theirs is not effective for him,¹⁹  since zimmun cannot be said out of its place.²⁰

This Rava Tosfaah is someone to call attention to. His students are the first of the Savoraim, so he is a bridge figure.

To cite Jewish Encyclopedia:

Babylonian amora of the seventh generation. He was a pupil of Rabina I. (Suk. 32a; comp. Halevy, "Dorot ha-Rishonim," iii. 96) and a contemporary of Rabina II., with whom, sometimes, he is mentioned in the Talmud (Shab. 95a; M. Ḳ. 4a). A few independent decisions of Rabbah have been preserved (Ber. 50a; Yeb. 80b). One of them (Yeb. 80b) assumes that the pregnancy of a woman may extend from nine to twelve months. The chief work of Rabbah was to complete, by additions and amplifications, the compilation of the Talmud begun by R. Ashi. These additions consisted for the most part of short, explanatory remarks, indispensable for an understanding of Talmudic themes or for deciding between the conflicting opinions of older authorities (Halevy, l.c. p. 20). From these additions and amplifications (tosafot) to the Talmud he is said to have derived his name of Tosefa'ah (= "the completer"; Halevy, l.c. iii. 19; Brüll's "Jahrb." ii. 19). It is more probable, however, that he was so named after his birthplace—Tusfah = Thospia (Brüll, l.c.). Rabbah Tosefa'ah is seldom mentioned by name in the Talmud—only in nine places. However, all sayings in the Babylonian Talmud introduced by "Yesh omerim" (some say) are ascribed to him (Heilprin, "Seder ha-Dorot," iii. 337; Brüll, l.c. ii. 13). Rabbah Tosefa'ah succeeded Mar b. R. Ashi (Tabyomi) as head of the Academy of Sura, which position he held for six years. He died in 494 (Sherira, in Neubauer, "M. J. C." i. 34; Abraham ibn Daud, "Sefer ha-Ḳabbalah," ib. i. 59).

Wikipedia lists him as eighth generation, and there is a list of each generation.

Rabbah Tosafa'ah (Hebrew: רבה תוספאה‎) was a jewish Amora sage of Babylon of the eighth generation of the Amora era, during the latter part of that era. According to the Iggeret Rav Sherira Gaon, he headed the academy of Sura, following Mar b. Rav Ashi, for six years, between 467-474 (ד'רכ"ח-ד'רל"ד Hebrew calendar). He was one of the Talmud's arrangers. According to the Historian Yitzhak Isaac Halevy Rabinowitz, who wrote in his work "Dorot Harishonim", that since the Talmud was in the process of its completion during his times, and Rabbah Tosafa'ah, who was one of the arrangers, had added-on it, this is how he earned the name Tosafa'ah (תוספאה), which literally means an Add-on or a supplement. Other argue that he was an erudite in the Baraitas and Tosefta, which the latter also literally means an 'add on' or a 'supplement', and both are an 'add on' works to the classic texts, and in their view, this is the way he earned his known name.

Hebrew Wikipedia has a lot more.