Transistors perform two primary functions. They amplify electronic signals and act as switches. A BC108 transistor is a general-purpose transistor made from silicon. The "B" letter code represents silicon, and the "C" letter code represents low-power audio frequency. The numbers identify the particular transistor model.
During manufacturing, the pure silicon used to make transistors is doped with other elements to change the neutral property of the silicon into positive and negative material. Positively doped material is labeled "P," and the negatively doped material is labeled "N." An NPN transistor, thus, has two negative layers and one positive layer. Conversely, a PNP transistor has two positive layers and one negative layer. The BC108 transistor is an NPN transistor.
The transistor base sits between the collector and the emitter. In the two main types of transistors, the base is made of either positive or negative material. An NPN BC108 transistor has a positive material base, and the collector and emitter are negative materials. The opposite is true for a PNP transistor.
A transistor schematic shows the transistor base as a straight line with the lead perpendicular to it in the center. The schematic shows the collector as a line connecting to the base at an angle. The emitter is indicated with an arrow pointing at the base for a PNP transistor and away from the base for an NPN transistor. Some transistor schematics put a circle around the symbol.
Applying a current to the base of the BC108 transistor causes the current to flow through the base and out the emitter. The base-emitter current allows current to flow from the collector through the emitter. The two currents add together, and the result is amplification of the current applied to the collector. This is how a transistor like the BC108 amplifies a signal.
Current cannot flow from the collector unless the base-emitter current is present. Switching applications use this BC108 transistor characteristic to control current flow. Transistors that can switch on and off quickly are better suited for this application than those that amplify signals.
The characteristics of the BC108 transistor make it a general purpose transistor with some properties suitable for low-power amplification on frequencies in the audio range. The total power capability is limited to 300 milliwatts in the standard version, and the BC108C can handle as much as 600 milliwatts. The audio range limitation suggests that it is best suited to applications that limit frequencies to 20 hertz through 20,000 hertz.
With total power limited to 300 milliwatts, the BC108 transistor isn't suited for use as the main amplifier in a home or car stereo. Rather, it is better suited for low-power, non-critical switching or to boost a signal's power for processing by another circuit. Other transistors are better suited for high-speed switching or high-power amplification.