STMicroelectronics

BC857B

BC857B Description

The BC857B from STMicroelectronics is a PNP bipolar junction transistor (BJT) housed in a compact SOT-23 surface-mount package. Designed for general-purpose amplification and switching applications, it offers a collector-emitter voltage (Vce) rating of 45V and a maximum collector current (Ic) of 100mA. With a transition frequency (fT) of 100MHz, it ensures efficient performance in high-frequency circuits. The transistor operates within a wide temperature range of up to 150°C (TJ), making it suitable for demanding environments.

BC857B Features

• High Voltage Tolerance: 45V Vce breakdown voltage for robust performance in low-to-medium voltage circuits.
• Low Saturation Voltage: 650mV @ 5mA, 100mA, minimizing power loss in switching applications.
• High DC Current Gain (hFE): Min 220 @ 2mA, 5V, ensuring reliable amplification.
• Low Leakage Current: 15nA (ICBO) cutoff current, enhancing efficiency in precision circuits.
• Compact & Reliable: SOT-23 package with MSL 1 (Unlimited) moisture sensitivity, ideal for automated assembly.
• Power Efficiency: 250mW max power dissipation, balancing performance and thermal management.

BC857B Applications

• Signal Amplification: Ideal for audio preamps, sensor interfaces, and other low-power analog circuits.
• Switching Circuits: Used in relay drivers, LED controllers, and load switches due to low Vce(sat).
• Portable Electronics: Suitable for battery-operated devices thanks to low leakage and compact form factor.
• Automotive & Industrial Systems: Withstands high-temperature environments, making it fit for automotive control modules and industrial automation.

Conclusion of BC857B

The BC857B is a versatile PNP BJT offering a balance of voltage tolerance, current gain, and switching efficiency. Its low saturation voltage and high transition frequency make it superior to generic PNP transistors in high-frequency and power-sensitive designs. While marked as obsolete, it remains a reliable choice for legacy systems or applications requiring proven performance in harsh conditions. Engineers favor it for space-constrained, high-reliability designs across consumer, automotive, and industrial sectors.