We demonstrate thermo-optically tunable ${\mathrm{Er}}^{3+}$ -doped fiber amplifiers covering S and $C+L$ bands $(1490–1610\mathrm{nm})$ using fundamental-mode cutoff filters discretely located in a $17.5\mathrm{m}$ long standard ${\mathrm{Er}}^{3+}$ -doped fiber. The material dispersion and waveguide structure of the tapered fibers are locally modified to produce high-cutoff-efficiency filters that make the unwanted amplified spontaneous emission highly attenuated at the longer wavelengths, and the optical gain is thus moved toward the shorter wavelengths. The maximal signal gains are measured to be 18.92, 37.18, and $15.19\mathrm{dB}$ with $980\mathrm{nm}$ pump power of $135\mathrm{mW}$ in the S, C, and L bands, respectively.