075582814553
The BC557 is a widely used PNP small-signal bipolar junction transistor. This article will discuss the transistor’s key specifications and pinout, equivalents and substitutions, typical circuit roles, characteristic curves, and more.
The BC557 from Fairchild/onsemi is a versatile PNP bipolar junction transistor designed for low-power, low-noise applications. Housed in a compact TO-92 package, it offers reliable performance for signal amplification and general switching tasks. With ratings such as −45 V VCEO, −100 mA collector current, 500 mW power dissipation, and a 150 MHz transition frequency, it fits well in analog circuits, audio stages, and sensor interfaces that require stable and efficient operation. Compatibility with widely used complementary NPN devices like the BC547/BC548.
If you are interested in purchasing the BC557, feel free to contact us for pricing and availability.
| Pin Number | Name | Description |
| 1 | Collector | Output current terminal; connects to load. |
| 2 | Base | Controls transistor operation; receives input signal. |
| 3 | Emitter | Current flows out of this pin; typically connected to positive supply for PNP devices. |
| Spec | BC157 | BC558 | 2N3906 | S8550 |
| Type | Small-signal PNP | Small-signal PNP | Small-signal PNP | High-gain PNP |
| VCEO (Max Collector-Emitter Voltage) | −45 V | −30 V | −40 V | −25 V |
| IC (Max Collector Current) | −100 mA | −100 mA | −200 mA | −1500 mA (1.5 A) |
| Power Dissipation (Pd) | 500 mW | 500 mW | 625 mW | 1000 mW (1 W) |
| hFE (DC Gain) | 110–450 | 110–800 | 100–300 | 120–400 (varies by grade) |
| fT (Transition Frequency) | ~150 MHz | ~150 MHz | ~250 MHz | ~100 MHz |
| Package | TO-92 | TO-92 | TO-92 | TO-92 / TO-92-3 |
| Use Case | Close match to BC557 | Same family, slightly lower voltage | Good general substitute | Higher current circuits |
| Parameter | Value |
| Mounting Style | Through Hole |
| Package / Case | TO-92-3 |
| Transistor Polarity | PNP |
| Configuration | Single |
| Maximum DC Collector Current (IC) | −100 mA |
| Collector-Emitter Voltage (VCEO Max) | 45 V |
| Collector-Base Voltage (VCBO) | 50 V |
| Emitter-Base Voltage (VEBO) | 5 V |
| Collector-Emitter Saturation Voltage (VCE(sat)) | 250 mV |
| Power Dissipation (Pd) | 500 mW |
| Gain Bandwidth Product (fT) | 150 MHz |
| Minimum Operating Temperature | −65 °C |
| Maximum Operating Temperature | +150 °C |
| Series | BC557 |
| Packaging | Bulk |
| Brand | ONSEMI / Fairchild |
| Continuous Collector Current | −100 mA |
| Height | 5.33 mm |
| Length | 5.2 mm |
| Width | 4.19 mm |
| Unit Weight | 0.007055 oz |
| Factory Pack Quantity | 2000 |
| Subcategory | Transistors |
| Product Type | BJTs – Bipolar Transistors |
-PNP small-signal bipolar junction transistor
-Maximum Collector-Emitter Voltage: −45 V
-Maximum Collector Current: −100 mA
-Low Collector-Emitter saturation voltage
-High DC current gain (hFE 110–800, depending on grade)
-Transition frequency around 150 MHz
-Low noise performance
-Standard TO-92 package
-Suitable for general-purpose amplification and switching
-Wide operating temperature range (−65°C to +150°C)
-Excellent choice for low-power analog circuits
-Stable gain and performance across temperature variations
-Easily available and widely supported in existing designs
-Works well with complementary NPN transistors (e.g., BC547/BC548)
-Simple biasing and easy to interface with low-voltage systems
-Reliable for both signal amplification and low-current switching
-Cost-effective for large-volume production
In this circuit, the BC557 works as a PNP light-activated switch that turns the LED on when darkness increases. The LDR and variable resistor form a voltage divider controlling the base voltage of the BC557. When light falls on the LDR, its resistance drops, pulling the base closer to the positive supply, causing the BC557 to turn off, so the LED remains off. In darkness, the LDR resistance rises, pulling the base voltage lower. Once the base becomes sufficiently negative relative to the emitter, the BC557 turns on, allowing current to flow through R1 and powering the LED. This makes the circuit function as an automatic night-light.
In this configuration, two BC557 transistors operate together to maintain a stable regulated output. The Zener diode establishes a reference voltage, and Q1 senses the input voltage variations. When the input rises too high, Q1 conducts and biases Q2, causing Q2 to regulate the output voltage by limiting current flow. The LED provides indication of proper regulation. By responding to base voltage changes controlled by the Zener diode and resistor network, the BC557 pair acts as a compact regulator, ensuring that the output stays near 5 V despite input fluctuations.
Here, the BC557 acts as an amplifier for the signal coming from the TSOP1738 infrared receiver module. When the TSOP output goes low during IR detection, R1 and the capacitor C1 shape the input signal to the base of the BC557. A low signal at the base forward-biases the BC557, causing it to conduct and create a corresponding amplified output across R2. When no IR signal is present, the base stays near the supply voltage, keeping the BC557 off. This circuit allows the weak IR pulses from the sensor to be strengthened and used to drive additional stages or trigger logic circuits.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other