Electronics Components: The Most Important Transistor Specifications
Transistors are more complicated devices than resistors, capacitors, inductors, and diodes. Whereas those electronic components have just a few specifications to wrangle with, such as ohms of resistance and maximum watts of power dissipation, transistors have a bevy of specifications.
You can find the complete specifications for any transistor by looking up its data sheet on the Internet; just plug the part number into your favorite search engine. The data sheet gives you dozens of interesting facts about the transistor you're interested in, with charts and graphs only a rocket scientist could love.
If you happen to be a rocket scientist and you're thinking about using the transistor in a missile, by all means please pay attention to every detail in the data sheet. But if you're just trying to do a little on-the-side circuit design, you need to pay attention to only the most important specifications — these in particular:
Current gain (HFE): This is a measure of the amplifying ability of the transistor. It refers to the ratio of the base current to the collector current. Typical values range from 50 to 200. The higher this number, the more the transistor is able to amplify an incoming signal.
Collector-emitter voltage (VCEO): The maximum voltage across the collector and the emitter. This is usually 30 V or more, which is well above the voltage levels you work with in most hobby circuits.
Emitter-base voltage (VEBO): The maximum voltage across the emitter and the base. This is usually a relatively small number, such as 6 V. Most circuits are designed to apply only small voltages to the base, so this limit isn't usually a concern.
Collector-base voltage (VCBO): The maximum voltage across the collector and the base. This is usually 50 V or more.
Collector current (IC): The maximum current that can flow through the collector-emitter path. Most circuits use a resister to limit this current flow; the value of the resistor must be calculated using Ohm's law to keep the collector current below the limit. If you exceed this limit for long, the transistor may be damaged.
Total power dissipation (PD): This is the total power that can be dissipated by the device. For most small transistors, the power rating is on the order of a few hundred milliwatts (mW).
You need to worry about these specifications only if you're designing your own circuits. If you're building a circuit you found in a book or on the Internet, all you need to know is the transistor part number specified in the circuit's schematic.