The S8550 transistor is a compact and efficient PNP bipolar junction transistor (BJT) commonly used in low-power amplification and switching applications. With its SOT-23 surface-mount package, it offers high current gain, low saturation voltage, and stable operation across a wide temperature range. This article will discuss the S8550 transistor’s specifications, features, applications, characteristics, and key manufacturer information.

Catalog

What is S8550?

The S8550 is a PNP bipolar junction transistor (BJT) housed in a compact SOT-23 surface-mount package, designed for general-purpose amplification and switching. It operates with a collector-emitter breakdown voltage of about –25 V, collector current up to 500 mA, and a power dissipation of around 300 mW. With a transition frequency near 150 MHz, it efficiently handles low-to-medium signal frequencies while maintaining stable performance across a temperature range of –55 °C to +150 °C.

This SMD plastic-encapsulated transistor is widely used in signal amplification, LED driving, relay switching, and low-power control circuits. Its small footprint makes it ideal for compact PCB designs, while the SOT-23 encapsulation provides reliable protection and heat dissipation for surface-mount assembly. The S8550 pairs well with the NPN S8050 transistor for complementary push-pull amplifier designs or general switching applications.

If you are interested in purchasing the S8550, feel free to contact us for pricing and availability.

S8550 Alternatives and Equivalents

Pin Configuration of S8550 Transistor

S8550 Specifications

S8550 Maximum ratings

S8550 Electrical Characteristics

S8550 Features

• Type: PNP bipolar junction transistor for low-voltage applications.

• Collector–Emitter Voltage (Vce): –25 V maximum, suitable for low-power circuits.

• Collector–Base Voltage (Vcb): –40 V for stable operation under moderate voltage.

• Collector Current (Ic): Up to 1.5 A, allowing use in medium-load switching.

• DC Gain (hFE): 85–300 depending on current, providing good amplification.

• Transition Frequency (fT): Around 100 MHz, ideal for signal amplification and fast switching.

• Saturation Voltage (Vce sat): Low, typically –0.5 V, minimizing power loss.

• Package: Available in TO-92 (through-hole) and SOT-23 (surface-mount) formats.

• Thermal Resistance: Efficient heat dissipation for reliable operation.

S8550 Applications

• Audio Amplifiers: Used in low-power audio stages to amplify weak signals.

• LED Drivers: Controls and powers LEDs in indicator and display circuits.

• Relay Drivers: Switches relays and small loads in automation circuits.

• Signal Amplification: Boosts analog or digital signals in various devices.

• Switching Circuits: Acts as an electronic switch in low-voltage systems.

• Voltage Regulation: Works with regulators for biasing and feedback control.

• Battery-Powered Devices: Ideal for small gadgets due to low power consumption.

• Motor Drivers: Controls mini DC motors in hobby and low-current applications.

• Logic Circuits: Interfaces between different logic levels in digital systems.

• Sensor Circuits: Amplifies sensor outputs for microcontroller inputs.

S8550 Ratings And Characteristic Curves

Figure 1 Collector Current (Ic) vs. Collector-Emitter Voltage (Vce). This graph shows how the collector current (Ic) changes with the collector-emitter voltage (Vce) at different base currents (Ib) when the transistor operates in a common-emitter configuration at 25°C.

Each curve represents a specific base current ranging from –40 µA to –400 µA. As the base current increases, the collector current also rises, indicating stronger transistor conduction. The curves flatten at higher voltages, showing the transistor entering its active region where Ic becomes nearly constant. This behavior demonstrates how the S8550 amplifies current - a small change in base current produces a much larger change in collector current.

The figure 2 Collector Power Dissipation (Pc) vs. Ambient Temperature (Ta). This curve shows how much power the transistor can safely dissipate depending on the surrounding temperature. At 25°C, the S8550 can dissipate around 400 mW, but as the ambient temperature increases, the power dissipation capacity decreases linearly. By about 150°C, it drops to nearly zero.

This means the transistor must be operated within its thermal limits - higher temperatures require lower power operation to prevent overheating and damage.

Figure 3 Capacitance vs. Reverse Bias Voltage. This graph shows how the junction capacitance of the transistor changes with reverse bias voltage. It includes two curves - Cob (collector-base capacitance) and Cib (input capacitance) - measured at 1 MHz and 25°C. As the reverse voltage increases, both capacitances decrease. This happens because a higher reverse bias widens the depletion region, reducing capacitance. The low capacitance values (in the picofarad range) indicate that the S8550 can switch signals quickly, making it suitable for high-frequency and amplification applications.

The Figure 4 Collector-Emitter Saturation Voltage (VCEsat) vs. Collector Current (Ic). This curve shows the relationship between collector-emitter saturation voltage and collector current at 25°C. As the collector current increases, the saturation voltage also rises gradually. A low VCEsat value means the transistor can conduct efficiently with minimal voltage loss when fully ON. This characteristic demonstrates that the S8550 offers good saturation performance, ensuring low power loss and better switching efficiency in amplifier and switching circuits.

S8550 Mechanical Drawing

Recommend Pad Layout

The diagram illustrates the suggested PCB pad dimensions for mounting the S8550 transistor in a SOT-23 package. The pad layout ensures proper soldering, electrical contact, and mechanical stability on the circuit board. The spacing between pads is precisely defined - with a horizontal distance of 1.9 mm between the lower pads and a vertical distance of 2.02 mm to the upper pad. Each pad measures approximately 0.6 mm × 0.8 mm, allowing for reliable heat dissipation and solder joint formation.

The layout dimensions are measured in millimeters, with a general tolerance of ±0.05 mm. This ensures accurate PCB design and compatibility with automated assembly processes.

S8550 LED Driver Circuit

simple example circuit using the S8550 PNP transistor as an LED driver controlled by a Raspberry Pi I/O pin. The circuit allows the LED to turn on or off based on the digital signal from the Pi while safely managing the required current.

The R1 (1.2 kΩ) resistor limits the base current from the Pi, protecting the transistor and the I/O pin. R2 (150 Ω) provides bias stabilization, and R4 (1.8 kΩ) pulls the base to ground, keeping the transistor off when no signal is present. R3 (22 Ω) limits the current through the LED (D1) to prevent damage and ensure proper brightness.

When the Raspberry Pi outputs a LOW signal, the base-emitter junction becomes forward-biased, turning the transistor ON and allowing current to flow through the LED, lighting it up. When the Pi outputs HIGH, the transistor turns OFF, and the LED goes out.

This circuit demonstrates how the S8550 transistor can act as a reliable switch or driver in low-voltage circuits.

S8550 Advantages and Limitations

S8550 Advantages

• High Current Gain (hFE): It provides strong amplification, allowing small base currents to control larger collector currents efficiently.

• Low Saturation Voltage (VCE(sat)): Ensures minimal power loss during switching, improving energy efficiency.

• Compact SOT-23 Package: Ideal for modern compact PCBs and high-density circuit designs.

• Good Frequency Response: Works well in audio frequency and small-signal amplifier circuits.

• Thermal Stability: Performs reliably under normal temperature ranges with consistent characteristics.

• Easy Biasing and Control: The PNP configuration simplifies circuit integration with NPN counterparts in push-pull or complementary stages.

S8550 Limitations

• Low Power Handling: With a maximum power dissipation around 400 mW, it’s not suitable for high-power applications.

• Limited Voltage Rating: The device typically supports up to –25 V, restricting its use in higher-voltage circuits.

• Moderate Switching Speed: Although sufficient for general use, it’s not optimized for high-frequency or fast-switching digital applications.

• Thermal Sensitivity: Performance may degrade at high ambient temperatures if proper heat dissipation is not maintained.

Manufacturers

The S8550 transistor is manufactured by several reputable companies, including UMW (UMW IC), Shenzhen Slkormicro Semicon Co., Ltd., YANGJIE Technology, JCET Group Co., Ltd., Unisonic Technologies, and Wing Shing Computer Components. It is also distributed by firms such as NextGen Components, Inc., which supply various S8550 versions - typically PNP transistors in SOT-23 packages rated for 25 V and 0.5 A. These manufacturers ensure broad availability and consistent performance across applications in consumer electronics, amplifiers, and switching circuits.

Conclusion

The S8550 is widely used PNP transistors for low-power electronic designs. Its reliable gain characteristics, compact SOT-23 form factor, and compatibility with modern PCB layouts make it suitable for audio, signal, and switching circuits. While it has limitations in power handling and high-frequency performance, its advantages - such as low voltage drop, thermal stability, and ease of integration - make it a preferred choice for everyday circuit applications.