Introduction to BC548

Introduction to BC548

• BC548 is an NPN general-purpose transistor, comes with three pins named emitter, base and collector.
• A small current at the base side is used to control large current at the collector and emitter terminals. The reason, it’s normally called a current controlled device.

• Decent current gain and power dissipation make it a suitable pick for amplification circuits and pre-amplification stages in other electronic applications. Moreover, it is also used as a switch to carry the load below 500mA.
• This BC548 transistor belongs to the NPN family where it contains three layers i.e. two N-doped layers and one P-doped layer. The P-doped layer is sandwiched between two N-doped layers.
• Also, it contains two PN junctions where one is reverse biased and the other is forward biased.
• The NPN transistor is like an electron valve where the base terminal controls the movement of electrons. It is widely used for amplification purposes in many electrical and electronics projects.
• Plus, electrons behave like majority carriers in NPN transistors whereas in PNP transistor holes behave like majority carriers. In conclusion, the mobility of electrons is better than the mobility of holes, thus NPN transistors are better and faster than PNP transistors.

1. BC548 Pinout

• 1: Emitter
• 2: Base
• 3: Collector

• All these terminals are different in terms of operation and doping concentration.
• The base terminal controls the current, emitter is used to emit the number of electrons when voltage is applied at the base terminal which is then collected by the collector.
• The small increase in input voltage applied at the base terminal produces a large change in output voltage across both collector and emitter terminals. This practice is used for amplification purposes.
• The emitter terminal is highly doped as compared to collector and base terminals. Plus, the collector voltage is larger than the base voltage.

2. BC548 Pin Configuration

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

• The amplification factor is used to define the nature of amplification. It is called beta and is denoted by ß. It is a ratio between collector current and base current. As it’s a ratio between the same factor i.e. current, it contains no unit. In this NPN transistor, the amplification factor ranges from 110 to 800.
• Similarly, the ratio between collector current and emitter current is called current gain and is mainly known as alpha, denoted by a. The alpha value lies between 0.95 to 0.99 but most of the time its value is taken as unity.

3. BC548 Working Principle

• When the base terminal is grounded, both emitter and collector terminals remain disconnected.
• When the voltage is applied at the base terminal, it gets biased, forming a bridge between collector and emitter.
• As this is an NPN transistor, the current will flow from collector to emitter, unlike PNP transistors where current flows from emitter to collector.
• Though both electrons and holes play a key role in conductivity, in the case of NPN transistors, electrons are major charge carriers.
• The base terminal controls the number of electrons passing from collector to emitter and appears positive with respect to the emitter terminal.
• The applied voltage at the base terminals draws a small current which is then used to police the large current at the collector and emitter terminals.

4. BC548 Power Ratings

• Collector-Base voltage is 30V. While Emitter-Base voltage and Collector-Emitter voltages are 5V and 30V respectively.
• The Collector current is 500mA with maximum power dissipation 625mW at temperature 25C.
• And the current gain ranges from 110 to 800.
• It’s important to note that these values are called stress ratings. Before you employ this transistor into the required circuit, make sure these ratings don’t cross the absolute maximum ratings, else you can risk your transistor and thus the entire project.
• Plus, if these ratings are applied for an extended period, they can influence the device reliability.
• Note: these values are measured at T = 25 C

5. BC548 Alternatives

• 2N2222
• 2N3904
• BC547
• BC549
• BC550

6. BC548 Applications

• Used in Darlington pairs to amplify weak signals.
• It can be employed in sensor circuits.
• Driving loads under 500mA.
• Used in audio amplification.
• Used in audio Amplifier Stages.

7. BC548 Physical dimensions