Built-in resistor network for base drive
The RN1107ACT(TPL3) is an NPN pre-biased transistor — a digital transistor that integrates the bias resistors into the same die. The 10 kΩ base resistor (R1) and 47 kΩ emitter-base resistor (R2) set the turn-on threshold and limit base current, so the part switches directly from a microcontroller I/O without an external resistor pair.
Saturation voltage and gain at the operating point
Vce(sat) is 150 mV maximum at 250 µA base drive and 5 mA collector current — the on-state voltage is low enough that self-heating stays minimal in a 100 mW power budget. The minimum DC current gain is 80 at 10 mA collector current and 5 Vce, confirming the transistor has enough headroom to saturate a 10 mA load with the built-in bias network. Collector cutoff current is 500 nA maximum — leakage is low enough that the output stays high-impedance when the base is pulled to ground, which matters in battery-powered or high-impedance load circuits where off-state current drains the supply.
SOT-883 (CST3) package and board integration
Housed in the SC-101 / SOT-883 package, also designated CST3 by Toshiba. It is a 3-pin surface-mount package with a 0.65 mm pitch and a body size of roughly 1.0 × 0.6 mm — the footprint fits into dense PCB layouts where a discrete resistor-pair plus a standard SOT-23 transistor would take up more board area. Supplied in Tape & Reel and Cut Tape options, so it works for both prototype assembly and volume reflow. The part is RoHS compliant with no exemptions listed.