Where precisely in the synagogue must the Bimah (the table where the Chazzan stands and the Torah is read) be situated?
The Chatam Sofer (Rabbi Moshe Sofer, Germany-Hungary, 1762-1839) addresses this issue in one of his responsa (vol. 1, O.C. 28), in response to a question regarding a synagogue that wished to add more seats to the sanctuary, which necessitated moving the Bimah from the center of the synagogue to the front. The Chatam Sofer cites a ruling of the Rambam (Hilchot Tefila 11:3), which is mentioned as well by the Rama (Rabbi Moshe Isserles, Cracow, 1525-1572) in his glosses to the Shulchan Aruch (O.C. 150), requiring that the Bima be situated at the center of the synagogue, so as to ensure that everybody could hear the Chazzan. The Kesef Mishneh (commentary to the Rambam's code of law by Rabbi Yosef Karo, author of the Shulchan Aruch) claims that this requirement would not apply in a small synagogue, where everyone can easily hear the Chazzan regardless of where he stands. The Chatam Sofer, however, held that even in small synagogues the Bima must be placed at the center, because its status in the synagogue corresponds to that of the altar in the Temple. On Sukkot, for example, we encircle the altar with the four species (Hakafot) to commemorate the encircling of the altar with the Araba in the Temple. Likewise, we recite the Korbanot ("sacrifices") section of the prayer service from the Bima, as a substitute for the actual offering of sacrifices on the altar in the Temple. The Gemara in Masechet Shekalim tells that the altar was situated directly at the center of the courtyard of the Temple, and therefore the Bima must likewise stand in the middle of the synagogue, and not towards the front near the ark.
Many synagogues today do not abide by this ruling of the Chatam Sofer, and place the Bima near the front of the sanctuary. They perhaps rely on the Kesef Mishneh's ruling, restricting this requirement to situations where some congregants may find it difficult to hear the Chazzan if he would not stand in the center. The acoustics in contemporary synagogues allow for the Chazzan's voice to reach everyone in the synagogue regardless of where he stands, and for this reason, perhaps, it has become customary to allow situating the Bima even towards the front of the synagogue.
Summary: Although the Chatam Sofer held that the Bima must be situated specifically in the center of the synagogue, many communities allow situating the Bima towards the front of the synagogue, and they have legitimate sources upon which to rely in this regard.
