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BC639 General Purpose NPN Transistor

The BC639 is an NPN Transistor in a TO-92 package which has a VCE of 100V and a continuous collector current of 1A. With this characteristic and 1W total power dissipation, this transistor can be used in moderately high voltage switching applications.


BC639 Pinout Configuration 

Pin Number

Pin Name




Electrons emitted from the emitter into the first PN junction 



Electrons Emitted from Emitter Collected by the Collector



Controls the biasing of the transistor


Features & Specifications

  • Bi-Polar NPN, high voltage Transistor
  • DC Current Gain (hFE) is 160 maximum
  • Continuous Collector current (IC) is 1A
  • Emitter Base Voltage (VBE) is 5V
  • Base Current (IB) is 100mA maximum
  • Maximum Collector-Base Voltage |Vcb|: 50 V
  • Collector Dissipation: 1 W
  • Transition Frequency:100 MHz
  • Collector Capacitance 20pF
  • Operating Junction Temperature Max (Tj): 150 °C
  • Available in TO-92 Package


Note: Complete Technical Details can be found in the BC639 datasheet given at the end of this page.


BC639 Equivalent Transistor

BC547, BC548, BC549, BC488


General Description of BC639 Transistor

The BC639 transistor is a very versatile transistor that can be used in many different applications. This transistor can be used to switch moderately high voltage devices, like a small DC-DC converter, or a pre or mid amplifier stage of a power amplifier, generally, a power amplifier uses something like 60 to 120V DC this transistor can be useful for this kind of applications. The component is cheap and easy to work with so it is best suited when choosing a random switching device.  When this transistor is in a biased condition, then it can allow a maximum current of 1A across CE(Collector-Emitter) Junction, this state of the transistor is called the saturation state, and driving a load that consumes more current than 1A may damage the device permanently, also during the design process, you need to be aware that the maximum dissipation of this device is 1W, power more than that can damage the device.


As you already may know, a transistor is a current-controlled device so when the base current is removed the transistor becomes fully off, in this stage the transistor is working in its cut-off region/cut-off state and no current flows through the C-E junction. The BC639 transistor has a gain of 40 to 160, this value determines the amplification capacity of the transistor, the peak current that can be flown through this transistor is 1.5A which combined with the gain value makes this transistor a perfect choice for moderate-high voltage application.


Under normal circumstances and without external influence, the base of an NPN transistor will be 0V. As we all know, based on the working principle of an NPN transistor; having 0 voltage at the base puts an NPN transistor in a high resistance state. By characteristic a small amount of current needs to flow out of the base of the transistor to completely turn on the device, for this device you can see, in order to flow 1000mA of current through the collector to emitter, 50mA of current needed to flow through the base of the transistor.

BC639 Transistor Working


How to use BC639 Transistor

Transistors are current-controlled devices so to turn them on a little current is needed. For the BC639 Transistor, this base current is less than 50mA, as BC639 is an NPN transistor that means it will be on when the base is connected to the ground, and it will be off when a positive voltage is applied to the base of the transistor.


The simulated circuit below shows how this transistor behaves when the base of the basic circuit is connected to the ground and when it's connected to 5V of the power supply.

BC639 Transistor Circuit Diagram

When we turn on the transistor by connecting the base to the supply the transistor will remain on unless the voltage at the base of the transistor reaches less than the cut-off voltage of the transistor that is 0.7-0.9V. The base of the transistor cannot be left floating otherwise there could be false triggering, which may lead to issues in the circuit, to resolve the issue we need to add pulldown resistors as shown in the example, a 10K resistor is used to pull up the base of the transistor to VCC.



  • Simple switching applications
  • Microphone preamplifiers
  • Lighting systems
  • Relay drivers
  • Audio Amplifiers
  • Signal Amplifiers


2D Model and Dimensions

If you are designing a PCB or Perf board with this component then the following picture from the Datasheet will be useful to know its package type and dimensions.

BC639 Transistor Dimension

Component Datasheet

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