What is the BC857 PNP Transistor and How Does it Work?

FREE-SKY (HK) ELECTRONICS CO.,LIMITED / 12-04 17:33

The BC857 is a widely used small-signal PNP bipolar transistor in a compact SOT-23 package, prized for its combination of low-power operation, high current gain, and −45 V collector-emitter rating. This article will discuss the BC857’s key specifications, typical circuit implementations, common alternatives and comparisons, and practical tips for using it effectively.

Catalog

1. BC857 PNP Transistors Overview

2. BC857 Transistors Pinout

3. BC857 Alternatives & Equivalents

4. BC857 Transistors Specifications

5. BC857 Transistors Features

6. BC857 Transistors Working in Circuit

7. BC857 Transistors Applications

8. Comparison: BC857 vs BC557 vs S8550

9. BC857 Mechanical Dimensions

10. Manufacturer

BC857 PNP Transistors Overview

The BC857 is a compact PNP bipolar junction transistor designed by Nexperia/NXP. Built in a small SOT-23 surface-mount package. It offers excellent performance for low-power electronics where space efficiency and signal control matter. With a –45 V collector-emitter voltage, 100 mA current rating, and high current gain (hFE up to 800 depending on grade), it performs well in precision control circuits and analog stages. Its complementary pairing with NPN devices (such as BC847) makes it ideal for push-pull stages and balanced amplifier designs.

If you are interested in purchasing the BC857, feel free to contact us for pricing and availability.

BC857 Transistors Pinout

BC857 PNP Transistors PINOUT

Pin Number	Pin Name	Description
1	Base (B)	Controls the transistor’s operation; receives the input signal to turn the transistor on/off.
2	Emitter (E)	Current flows out of the emitter for a PNP device; typically connected to the positive rail.
3	Collector (C)	Output side of the transistor; current flows into the collector when the device is active.

BC857 Alternatives & Equivalents

Part Number	Polarity	Package	VCE Max	IC Max	hFE Range	Notes
BC856	PNP	SOT-23	–65 V	100 mA	125–475	Higher-voltage alternative to BC857
BC860	PNP	SOT-23	–45 V	100 mA	Similar range	Very close electrical substitute
MMBTA56	PNP	SOT-23	–80 V	500 mA	100–300	Higher voltage and current; substitute if more power is needed
KST56	PNP	SOT-23	–80 V	500 mA	100–300	Similar to MMBTA56; robust substitute
FMMTA55	PNP	SOT-23	–60 V	500 mA	100–300	Higher voltage/current variant; workable alternative

BC857 Transistors Specifications

Specification	Value
Manufacturer	Nexperia USA Inc.
Transistor Type	PNP
Collector Current (Ic) Max	100 mA
Collector-Emitter Breakdown Voltage (VCEO)	45 V
VCE Saturation (Max) @ IB, IC	650 mV @ 5 mA, 100 mA
Collector Cutoff Current (ICBO)	15 nA
DC Current Gain (hFE) Min @ IC, VCE	125 @ 2 mA, 5 V
Power Dissipation (Max)	250 mW
Transition Frequency (fT)	100 MHz
Operating Temperature (Max)	150°C (TJ)
Mounting Type	Surface-mount
Package / Case	TO-236-3, SC-59, SOT-23-3
Supplier Device Package	TO-236AB
Part Status	Active
Packaging Options	Tape & Reel (TR), Cut Tape (CT), Digi-Reel

BC857 Transistors Features

-PNP Silicon Bipolar Junction Transistor

-Low-power, general-purpose switching and amplification

-Maximum Collector Current: 100 mA

-Collector-Emitter Voltage Rating: 45 V

-Low Saturation Voltage for efficient switching

-High DC Current Gain: hFE min 125 (varies by grade)

-High Transition Frequency: 100 MHz for small-signal applications

-Low Leakage Current (ICBO 15 nA)

-Power Dissipation up to 250 mW

-Operating Junction Temperature up to 150°C

-Compact SOT-23 (TO-236AB) surface-mount package

BC857 Transistors Working in Circuit

LED Indicator Circuit

In the LED circuit, the BC857 PNP transistor works as a high-side switch. Its emitter is tied to 3.3 V, and its collector feeds the LED and resistor down to ground. Because it is a PNP device, the transistor turns on only when its base is pulled low. When the control signal at R1750 goes low, the base–emitter junction becomes forward biased, the transistor conducts, and current flows from 3.3 V through the transistor and LED to ground, lighting the LED. When the base is high, the transistor stays off and the LED remains dark.

Relay Driver Circuit

In the relay driver, the BC857 again acts as a high-side switch, but its base is controlled through an optocoupler. When the optocoupler is off, the BC857’s base sits at the same voltage as its emitter, keeping the transistor off and the relay unpowered. When the optocoupler turns on, it pulls the base downward through the resistor network, switching the BC857 on. This allows current to flow from VCC through the relay coil to ground, activating the relay. The diode protects the transistor from the relay’s inductive kick.

BC857 Transistors Applications

-High-side switching of LEDs and small loads

-Relay driving on the high side (low-side base control)

-Level shifting from logic-low signals to high-side outputs

-Power control in battery-powered or low-voltage systems

-General PNP small-signal amplification

-Inversion of logic signals (LOW input → HIGH-side output)

-Simple current-sourcing stages for analog circuits

-Use in complementary push–pull amplifier stages with NPN partners

-Driving indicators from microcontrollers that cannot source enough current

-Providing isolation-friendly switching when used with optocouplers

Comparison: BC857 vs BC557 vs S8550

Parameter	BC857	BC557	S8550
Type	PNP	PNP	PNP
Package	SOT-23 (SMD)	TO-92 (Through-hole)	TO-92 (Through-hole)
Maximum Voltage (Vceo)	−45 V	−45 V	−25 V
Maximum Collector Current (Ic)	−100 mA	−100 mA	−1.5 A (much higher)
Power Dissipation	~250 mW	~500 mW	~1 W
Gain (hFE)	110–800 (A/B/C groups)	110–800 (A/B/C groups)	85–300
Typical Usage	Small-signal SMD switching, logic control, LED/relay driving	General small-signal amplification and switching	High-current switching, audio drivers, motors, loads
Speed / Switching	Fast, suitable for logic	Moderate	Lower than BC857/BC557
Thermal Behavior	Limited due to SOT-23 size	Better dissipation	Much higher dissipation
Notable Strength	Compact SMD for low-power circuits	Easy to use in breadboards/repairs	Very high current capability
Notable Weakness	Low power dissipation	Bulkier package	Lower voltage rating

BC857 Mechanical Dimensions

Manufacturer

NXP, the manufacturer of the BC857, is known for producing high-quality, reliable discrete semiconductors designed for modern compact electronics. Their BC857 series represents a robust family of small-signal PNP transistors built in a space-saving SOT-23 package, intended for general switching and amplification tasks in low-power circuits. NXP emphasizes tight parameter control, consistent gain groupings, and dependable thermal performance, making the device suitable for high-side switching, signal conditioning, and logic-level control in consumer, industrial, and automotive applications.