BC557 PNP General-Purpose Transistor – 45V, 100mA, TO-92

The BC557 is a general-purpose PNP bipolar junction transistor (BJT) designed for small-signal amplification, low-current switching, analog circuits, and complementary transistor applications.

With a 45V collector-emitter voltage rating and collector current up to 100mA, the BC557 is suitable for signal amplification, high-side switching, sensor circuits, audio electronics, driver stages, and general-purpose electronic projects.

Its compact TO-92 through-hole package makes it convenient for breadboards, educational projects, electronics repair, and custom PCB designs.

Technical Specifications

• Part Number: BC557
• Transistor Type: PNP Bipolar Junction Transistor (BJT)
• Polarity: PNP
• Collector-Emitter Voltage (VCEO): 45V Max
• Collector-Base Voltage (VCBO): 50V Max
• Emitter-Base Voltage (VEBO): 5–6V Max, Depending on Manufacturer
• Continuous Collector Current (IC): 100mA Max
• Peak Collector Current: Typically Up to 200mA
• DC Current Gain (hFE): Depends on Gain Grade
• Transition Frequency (fT): Typically Around 100MHz or Higher, Depending on Manufacturer
• Maximum Junction Temperature: +150°C
• Package: TO-92
• Number of Pins: 3
• Mounting Type: Through-Hole
• Complementary NPN Type: BC547

General-Purpose PNP Transistor

The BC557 can be used as both an electronic switch and a small-signal amplifier.

As a PNP transistor, it is particularly useful for high-side switching and complementary circuits where the load needs to be controlled from the positive supply side.

It can also be paired with an NPN transistor such as the BC547 in complementary analog and switching circuits.

BC557 Gain Groups

The BC557 family is commonly available in several gain-selected versions:

• BC557A: hFE 110–220
• BC557B: hFE 200–450
• BC557C: hFE 420–800

Actual current gain depends on the manufacturer, collector current, operating voltage, temperature, and test conditions.

If the transistor is marked simply BC557 without an A, B, or C suffix, avoid guaranteeing a specific gain class.

Switching Applications

The BC557 can be used for low-current switching applications such as:

• LED Control
• Buzzers
• Small Signal Loads
• High-Side Switching
• Logic Interfaces
• Sensor Circuits
• Driver Stages

When driven from a microcontroller or other logic circuit, an appropriate base resistor should be used.

When switching an inductive load such as a relay coil, suitable protection such as a flyback diode should be included where required.

Signal Amplification

The BC557 is also suitable for small-signal analog amplification and signal-processing circuits.

Common applications include:

• Audio Amplifier Stages
• Sensor Signal Conditioning
• Preamplifier Circuits
• Analog Driver Circuits
• Oscillators
• Complementary Amplifiers
• General Analog Electronics

BC557 vs BC556 vs BC558

The BC556, BC557, and BC558 are closely related PNP small-signal transistors. One of their main differences is their collector-emitter voltage rating:

BC556: 65V VCEO

BC557: 45V VCEO

BC558: 30V VCEO

All three are generally in the 100mA collector-current class.

The BC556 is preferable where a higher voltage rating is required, while the BC558 can be suitable for lower-voltage applications.

BC557 vs BC547

The BC557 and BC547 form a useful complementary PNP/NPN pair:

BC557: PNP, 45V, 100mA

BC547: NPN, 45V, 100mA

They can be used together in complementary amplifier stages, switching circuits, push-pull stages, and general analog electronics.

Microcontroller Applications

The BC557 can be used with controllers such as:

• Arduino
• ATmega
• ESP32
• STM32
• PIC
• Raspberry Pi GPIO

Because it is a PNP transistor, the BC557 is commonly used for high-side switching rather than the low-side switching arrangement typically used with an NPN transistor.

The supply voltage, GPIO voltage, base resistor, and required base current must all be considered when designing the interface.

Important Note

The 100mA collector-current rating is an absolute maximum, not a recommended continuous operating current under all conditions.

Safe operating current depends on collector-emitter voltage, base drive, ambient temperature, power dissipation, switching duty cycle, and thermal conditions.

For higher-current loads such as motors, high-power LEDs, large relays, heaters, or solenoids, use an appropriately rated transistor or MOSFET instead.

Electrical characteristics and Collector, Base, and Emitter pin configuration can vary between manufacturers, so always verify the datasheet for the exact component supplied before installation.

Applications

Ideal for small-signal amplification, high-side switching, complementary transistor circuits, audio electronics, sensor circuits, LED control, signal conditioning, oscillators, microcontroller interfaces, Arduino projects, educational electronics, electronics repair, breadboard prototyping, laboratory experiments, and DIY electronics.

Package Includes

1 × BC557 PNP General-Purpose Transistor – TO-92