1) Although the general rule is that a person may not say Amen after a blessing which he himself has just made, there is an exception to this rule. The exception is when he has made two or more blessings, and the conclusion of these blessings is the end of a topic (as we shall see in the examples which will soon be mentioned), according to the Shulchan Aruch one does say Amen even after his own blessing (this is the custom of the Jews of Sephardic descent), and according to the ruling of the Rema one does not (this is the custom of the Jews of Ashkenazic descent). Even according to the opinion of the Rema, everyone says Amen after the blessing of Boneh Yrushalayim in the Grace after meals. (Shulchan Aruch and Mishne Berurah 51)

2) Examples of this category of blessing are: a) the blessing of Yishtabach at the end of the verses of praise which follows from Baruch She’amar at the beginning, b) Shomer amo Yirael at the end of the blessings of the Shema in the evening services, c) The blessing at the end of Hallel. In all of the above cases according to the Shulchan Aruch one says Amen after one’s own blessing. (ibid)