BC549 Transistor: A Comprehensive and Detailed Guide


About BC549 Transistor, NPN, and PNP transistors are families of transistors falling under BJT. BJT (bipolar junction transistors) use electron holes and electrons as their source of charge carriers.  

In today’s article, we will discuss a type of NPN transistor known as BC549. We’ll give you a piece of detailed information on its features, operating principle, pinout, applications, and alternatives.  

What is a BC549 Transistor?

BC549 transistor is a general-purpose BJT falling in the NPN transistor family. It has three layers which include one P-doped layer and two N-doped layers. Additionally, it comprises two PN junctions. One is in a reverse-biased state, while the other is in a state of forward-biasing.  

A PN junction

BC549 Transistor Pinout and Pin Configuration

BC549 has three pins, the emitter, base, and collector pins. Despite being different in doping concentration and operation, the three pins connect externally to other circuits.

The base voltage is smaller than the collector voltage. Also, compared to other terminals, the emitter terminal has high doping.  

BC549 electric symbol and terminals 

BC549 electric symbol and terminals 


  • Emitter terminal/pin1 – It often drains the current.
  • Base terminal/pin2 controls the current between the emitter and collector.
  • Collector pin/pin3 – It allows current flow.

Furthermore, you can use the BC549 transistor for three main configurations, namely;

  • Common collector configuration,
  • The standard base configuration, and
  • Common emitter configuration.

The common emitter type is applicable mainly since it has exact voltage and current rating values for the amplification process.

Beta (β) is an amplification factor that defines the amplification’s nature. Practically, it’s the ratio between base current and collector current. Often, we have an amplification factor ranging from 420 to 800 in a BC549 transistor.

Conversely, we have alpha (α) as the current gain (ratio between emitter current and collector current). The value ranges from 0.95 to 0.99, but you can also use it as a unity.  

BC549 Transistor Features

BC549 transistor has the specifications and features;

  • First, it has a maximum collector-emitter voltage (VCE) of 30V. Similarly, the maximum collector-base voltage (VCB) is also 30V.
  • Then, the maximum emitter-base voltage is 5V.
  • It’s a low voltage and low current NPN transistor.
  • The collector capacitance is 4.5 pF, whereas the transition frequency is 200MHz.
  • The minimum and maximum DC gain also range from 110 to 800.
  • The maximum collector current is 100mA, while the maximum collector dissipation is 500mW.
  • In addition, its maximum operating temperature and storage should range from -65°C to +150°C.
  • Lastly, its package type is SOT54 and TO-92.

BC549 Working Principle

Under normal circumstances, the emitter voltage in the BC549 transistor is 5V. That means when you apply the 5V at its base terminal, it’ll receive biasing and then trigger an electron reaction.

The base terminal in the BC549 transistor works like a control valve that regulates electrons’ numbers.

Later, the collector terminal collects the emitted electrons from the base terminal. However, since the collector current is 100mA, the transistor won’t favor a large load.

Moreover, because you only need 5V to begin the electron reaction process, microprocessors and microcontrollers can benefit from it.  

BC549 transistor as an amplifier 

If it functions as an amplifier, you’ll only use a small current as an input signal at the base pin. The process results in a larger electric current at emitter and collector terminals.  

BC549 as a switch 

BC549 switches a small current in one transistor part so that other transistor parts have larger currents.

Furthermore, it’ll hold two different value states in one and zero forms. You’ll mostly encounter the scenario in a computer’s memory chip where you can turn a transistor off and on individually.  

What can I use Instead of a BC549 Transistor?

Under circumstances where you lack a BC549 transistor, you can use its equivalents like 2N2222, BC547, BC550, or BC548 NPN.  

BC549 Transistor:

BC548 transistor 

Source; Wikipedia   

Often, the pinouts of the replacements can vary from the ones in BC549. Therefore, it is advisable to check the pin configurations before use.

Additionally, you can opt for complementary PNP transistors like BC557, BC560, and BC559.  


Some of the applications of the BC549 transistors include the following;

BC549 Transistor: A basic BJT astable multivibrator 

A basic BJT astable multivibrator 

Source; Wikipedia  

  • Current mirror circuits,
  • Oscillator and comparator circuits,
  • Small signal amplifiers and audio amplification,
BC549 Transistor: guitar amplifier

(guitar amplifier)


  • Darlington pair,
  • The LED array sequence controller
  • Small current load switching of under 100mA.


The BC549 transistor is an NPN that allows the current to flow from the collector to the emitter terminal. You’ll often find it in switching applications or for signal amplification purposes that require current loads less than 100mA.

For more information on the BC549 transistor, kindly reach out to us.    

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