BC817-40 is an NPN external bipolar transistor widely used in general switching and amplification circuits. It features a relatively high current gain (hFE), typically in the range of 250 to 600, allowing a small base current to control a much larger collector current. The device uses an SMT surface-mount package, offering a compact size and stable thermal performance.
In terms of operating principle, the BC817-40 follows the current-control mechanism of a bipolar junction transistor (BJT). When a forward bias voltage is applied to the base, the emitter junction turns on, and electrons are injected from the emitter into the base region. Most of these electrons pass through the thin base region and reach the collector, forming a controlled collector current.
1. E — Emitter
The emitter is the terminal that releases charge carriers (electrons in an NPN transistor) into the base region. It is usually connected to ground or the lower-potential side of the circuit. In switching and amplifier applications, the emitter often serves as the reference node for current flow.
2. B — Base
The base is the control terminal of the transistor. A small current applied to the base controls a much larger current flowing between the collector and emitter. By adjusting the base current or base voltage, the BC817-40 can be turned on/off for switching or biased for signal amplification.
3. C — Collector
The collector is the terminal that receives the main current flowing through the transistor. It is typically connected to the load and the higher supply voltage through a resistor or other components. The collector current is controlled by the base current and flows to the emitter when the transistor is on.
| Parameter | Value |
| Transistor Type | NPN |
| Package | SOT-23-3 |
| Collector–Emitter Voltage (VCEO) | 45 V |
| Vce Saturation (Max) @ Ib, Ic | 700mV @ 50mA, 500mA |
| Collector–Base Voltage (VCBO) | 50 V |
| Emitter–Base Voltage (VEBO) | 5 V |
| Current - Collector Cutoff (Max) | 100nA (ICBO) |
| DC Current Gain (hFE) (Min) @ Ic, Vce | 250 @ 100mA, 1V |
| Continuous Collector Current (IC) | 500 mA |
| Power - Max | 250 mW |
| Operating Temperature | 150°C (TJ) |
| Collector–Emitter Saturation Voltage | 0.1 – 0.2 V |
| Transition Frequency (fT) | 100MHz |
| Base Product Number | BC817 |
| Feature | Description |
| Silicon Epitaxial Planar NPN | Ensures high stability and reliable high-frequency performance. |
| High Collector Current (Ic) | Supports up to 500mA, ideal for driving medium loads. |
| High Current Gain (hFE) | Features a high gain range of 250 to 600 for efficient amplification. |
| Low Saturation Voltage | Minimizes power loss during switching operations. |
| Complementary PNP Pair | Pairs perfectly with the BC807-40 for push-pull circuit designs. |
| SMT Package (SOT-23) | Small footprint designed for high-density PCB layouts. |
| Tape and Reel Packaging | Optimized for high-speed automated SMT assembly lines. |
| Application | Examples | Benefits |
| General switching circuits | LED switching, relay driving, load control | Fast switching and low saturation voltage reduce power loss |
| Signal amplification | Small-signal amplifiers, audio pre-stages | High current gain enables effective small-signal amplification |
| Driver stages | MCU/logic output drivers, buffer stages | Allows low-current logic signals to drive higher-current loads |
| Consumer electronics | Portable devices, smart home modules | Small SOT-23 package saves board space |
| Industrial control | Sensor interfaces, control boards | Reliable operation and stable gain performance |
| Communication circuits | RF control paths, signal conditioning | Good frequency response for moderate-speed signals |
| Power management support | Bias networks, regulator support stages | Helps control and stabilize current paths |
This circuit demonstrates a Darlington pair driver configuration based on BC817 NPN transistors, used to control a $$40\ \Omega$$ load. In this design, a GPIO signal drives the first-stage transistor Q1 through a $$10\ \text{k}\Omega$$ resistor. The resulting emitter current then drives the second-stage high-power transistor Q2, achieving very high current gain. This arrangement allows a tiny GPIO current (only about 240.9μA) to easily control a load current of up to 249.1mA.
According to the test parameters, the circuit operates very stably. When the input is 3.3V, the staged amplification delivers nearly a full 10 V drive at the load. At this point, Q2's saturation voltage is only 36.45mV (measured by VM1), indicating that the BC817 has very low power dissipation when fully turned on, making it highly suitable for driving small motors or LED arrays.
The BC817-40 comes in a compact SOT-23 surface-mount package, making it easy to fit onto circuit boards with limited space. The following are its detailed dimensions:
STMicroelectronics is one of the world's leading semiconductor manufacturers. As one of the three major semiconductor companies in Europe, ST focuses on delivering innovative chip solutions for smart mobility, power and energy management, as well as IoT and connectivity. Its product portfolio is extremely broad and provides core technology support for many industries, including automotive, industrial, and consumer electronics.
It is a silicon epitaxial planar NPN transistor designed for general-purpose switching and amplification.
It supports a high collector current of up to 500mA, making it suitable for driving medium-sized loads.
Yes, the complementary PNP type is the BC807-40, which is used for push-pull or complementary switching designs.
BC817-40 and other electronic parts are available from Heisener Electronics, a global distributor offering large in-stock inventory and fast delivery for OEMs, EMS providers, and electronics design companies.
