Dual pre-biased transistor in a 6-pin US6
The Toshiba RN4904,LF packs one NPN and one PNP pre-biased transistor into a single 6-pin US6 package (SC-88 / SOT-363 footprint). Each transistor has its own 47 kΩ base resistor and 47 kΩ base-emitter resistor, so you drop the external bias network — the part switches on with a logic-level signal directly. The 200 mW power limit and 50 V collector-emitter breakdown cover low-side and high-side drive in 5 V or 12 V rails. 100 mA continuous collector current handles relay coils, indicator LEDs, and small signal loads without a heatsink.
Bias resistor set — 47 kΩ / 47 kΩ
The RN4904 uses matched 47 kΩ resistors on both the base and base-emitter legs. That is a higher impedance than the 10 kΩ / 10 kΩ pair on the RN1702 or the 22 kΩ / 47 kΩ mix on the RN1908. The higher base resistance means lower drive current from the MCU pin, but it also raises the input threshold — the transistor turns on when the base voltage exceeds about 0.7 V plus the drop across the 47 kΩ base resistor. For a 3.3 V logic output the RN4904 switches cleanly; for 1.8 V logic the lower-resistance parts may be a better fit.
The LF suffix confirms lead-free / RoHS-compliant plating, which matches current assembly requirements.