Introduction to BC549

Introduction to BC549

• BC549 is an NPN general-purpose bipolar junction transistor. It carries three pins named emitter, base, and collector and is available in TO-92 and SOT54 package.
• BC549 transistor contains three layers i.e. two N-doped layers and one P-doped layer. The P-doped layer stands between two N-doped layers. Plus, it contains two PN junctions where one is forward biased and the other is reverse biased.

• When no voltage is applied at the base terminal it is considered as grounded. In this case, the transistor acts like an open switch where both emitter and collector remain open.
• When voltage is applied at the base terminals it gets biased and draws current which is used to connect other terminals and current starts flowing from collector to emitter terminal.
• As this is an NPN transistor, here current flows from collector to emitter when voltage is applied, unlike PNP transistor where current flows from emitter to collector.
• Plus, both electrons and holes play a vital role in conductivity. In NPN transistor electrons are majority charge carriers and in PNP transistor holes are majority carriers.
• The movement of electrons, however, is faster than the movement of holes, the reason NPN transistors are better and faster than PNP transistors.

BC549 Pinout

• 1: Emitter
• 2: Base
• 3: Collector

• All three pins are used for external connections with other circuits. These pins are different in terms of operation and doping concentration.
• The collector voltage is larger than the base voltage and emitter terminal is highly doped compared to other terminals.

BC549 Pin Configuration

• Common emitter configuration
• Common base configuration
• Common collector configuration

• The amplification factor is called beta and is denoted by ß. It is mainly used to define the nature of amplification. It’s a ratio between collector current and base current. In BC549, the amplification factor ranges from 420 to 800.
• Another important factor is the current gain which is a ratio between collector current and emitter current. It is known as alpha and is denoted by a. The alpha value stands from 0.95 to 0.99 but more often than not its value is taken as unity.

BC549 Working Principle

• In BC549, emitter-base voltage is 5V which means when 5V is applied at the base terminal it gets biased and triggers the electron reaction.
• In the NPN transistor, the base terminal behaves like a control valve that controls the number of electrons.
• When voltage is applied at the base terminal, emitter starts emitting the electrons into the base terminal which controls the number of electrons. These emitted electrons passing through the base are collected by the collector terminal of the transistor.
• Moreover, the collector current is a mere 100mA i.e. you cannot drive heave loads using this transistor.
• As the only 5V is required to start the electron reaction at the base terminal, it can be easily employed across microcontrollers and microprocessors.
• When BC549 operates as an amplifier, small current as an input signal at the base terminal is used to induce a much bigger electric current at the other terminals.
• And when it works like a switch, it switches the small current at one part of the transistor into a much larger current across the other parts of the transistor.
• When transistor acts as a switch, it carriers two distinct states i.e. values are stored in the form of zero and one. Memory chip used in a computer is a common example of an electronic device that contains billions of transistors which can be turned on and off individually.

BC549 Power Ratings

• Emitter-Base voltage and Collector-Emitter voltages are 5V and 30V respectively.
• And Collector-Base voltage is 30V with collector current 100mA i.e. it cannot drive loads carrying ratings more than 100mA.
• The current gain ranges from 420 to 800 and transition frequency is 100MHz.
• These are stress ratings. Before you employ this transistor into your project and execute, make sure the ratings don’t exceed from absolute maximum ratings, else they will damage the product, and worse, your entire project, eventually.
• Also, if these ratings are applied for more than the required time, they can affect device reliability.

BC549 Alternatives

• BC547
• BC548
• BC550

• BC559
• BC560

BC549 Applications

• Used in liner audio amplifiers.
• Employed in Darlington pairs and sensor circuits.
• Used in oscillator and comparator circuits.
• Finds application in current mirror circuits.
• Used in Astable and Bistable multivibrators.
• Used for impedance buffering and switching applications.
• Finds application in low noise stages in audio frequency equipment.

BC549 Physical dimensions