Answering Amen to Your Own Bracha
The Gemara Brachot 45b says that one shouldn't answer amen to one's own bracha except for the bracha of Boneh Yerushalayim. Rashi (45b s.v. Ha) explains that Boneh Yerushalayim is only an example; in reality, the same would be true of any bracha that completed a series of brachot. Boneh Yerushalayim itself concludes the series of biblical brachot of Birkat HaMazon. For example, according to Rashi, after the brachot of Kriyat Shema one should answer amen since it completes a sequence of brachot. Tosfot (45b s.v. Ha), however, notes that the common minhag was only to answer amen to one's own bracha after Boneh Yerushalayim. The Rif cites a Yerushalmi that lists a number of instances where one should answer Amen to his own Beracha, including Birchat HaTorah, Sim Shalom, the ending Brachot of the Haftarah, and any Birchot haMitzvah. The Rosh explains that the scope encapsulates any series of Brachot, such as Birchot Kriyat Shema. The Rambam (Hilchot Brachot 1:16) limits it only to a series of ending Brachot, thereby excluding Yishtabach, Hallel, and Kriyat HaTorah, which only have one closing Bracha.
While the Ashkenazic minhag is simple and follows Tosfot, the Sephardic minhag seems not to follow Rashi, Tosfot, or the Rambam. As the Sephardic tradition is to answer amen after Yishtabach (OC 51:4) and Hallel, but not after Birchat HaTorah for Kriyat HaTorah or Bracha Achrona. According to Rashi, seemingly one should have answered amen to all of the above, while the Rambam would forbid answering Amen to Yishtabach and Hallel. To make things more puzzling, The Tur 215:1 comments that the minhag of his father, the Rosh, was to follow the Rambam.
In defense of the minhag, the Beit Yosef 51:3 suggests that really one should only answer amen to a concluding bracha if it concludes a sequence of brachot which were established to be said together, formally termed bracha ha'semucha lechaverta, i.e. a series of Brachot. However, when it comes to an opening and closing Bracha, since there's some act of eating or Mitzvah in between the Brachot, the two Brachot are each considered an individual Bracha. He adds that the brachot of Pesukei DeZimrah and Hallel are considered as if they were consecutive since they surround pesukim of praise and are themselves forms of praise.
What is the logic for such a distinction? Rav Moshe Feinstein (Igrot Moshe O.C. 5:9:1) explains that when one answers amen after a regular bracha it seems as though one is adding onto the institution of chazal. When one finishes a series of brachot, however, one may answer amen to one's own bracha because in such a case, amen is seen as a conclusion of a section and not an addition to the text of chazal. He compares this to the Gemara Brachot 34b which forbids one from bowing during Shemona Esrei at points where chazal didn't institute an obligation to bow since it appears as though one is adding onto chazal's establishment.
See further Yechave Daat 2:23, Yabia Omer vol. 8 OC 25:10 and vol 9 OC 108:102, Ohr LeTzion vol. 2 page 134,
- A number of rishonim agree with Rashi that Boneh Yerushalayim was only representative including the Rabbenu Chananel (cited by Tosfot 45b s.v. Ha), Bahag (cited by Tosfot 45b s.v. Ha), Rif (Brachot 33b), Rabbenu Yonah (Brachot 33b s.v. Ha DeAni), Rashba (Brachot 45b s.v. Lo Kasha), Rosh, and Shitah Mikubeset (Brachot 45b s.v. Ha BeShaar).
- The Mordechai (Brachot 162) and Maharik 2:31 (cited by the Bet Yosef 51:3) are in agreement with Tosfot.
- The Rama 215:1 writes that Ashkenazic minhag is in accordance with Tosfot.
- The Beit Yosef explains this understanding within the opinion of the Rambam.
He also cites the Mahari Ben Lev (Maharalnach) who offers alternative explanation via the opinion of the Rosh that any two Brachot which one may interrupt in between are not considered Semuchot Lechaverta/a series, so The Kaf HaChaim 51:6 writes that the Sephardic minhag follows the Shulchan Aruch to answer amen after Yishtabach and Hallel.