This article proposes a sliding digital-intermediate frequency (IF) quadrature digital transmitter (Q-DTX) for multimode multiband narrowband Internet of Things/Bluetooth low-energy (NB-IoT/BLE) applications. The digital upconverter (DUC) with a reconfigurable IF frequency between 0 and $f_{\mathrm {CLK}}$ is employed to replace the traditional fractional upsampling module in digital baseband processing and effectively avoid the fractional sampling spur issue. A quadrature digital power amplifier (DPA) with in-phase and quadrature (IQ)-sharing and transformer-based Doherty techniques are realized to enhance the efficiency at 0/3/6-dB power back-offs (PBOs) while supporting 0.7–2.5-GHz wide frequency range with a compact single-transformer footprint. Implemented in 28-nm CMOS, this Q-DTX occupies only 0.79-mm2 core area. Powered by 0.9- and 1.8-V supplies, this Q-DTX achieves 28.7-dBm peak output power with 47.7% system efficiency (SE) at 0.9 GHz. For 180-kHz NB-IoT signal, this Q-DTX achieves 25.4-dBm average output power ( $P_{\mathrm {avg}}$ ) and 40.9% average SE with an error vector magnitude (EVM) of-15.9 dB at 0.9 GHz and 21.8-dBm $P_{\mathrm {avg}}$ and 22.9% average SE with an EVM of-16.2 dB at 1.7 GHz. For 1-MHz BLE signal at 2.4 GHz, it obtains $P_{\mathrm {avg}}$ of 21.8 dBm and average SE of 16.6% with the Gaussian frequency shift keying (GFSK) error of 2.7%.