Saturation voltages: Ideally, a saturated transistor acts as a closed switch contact between collector and emitter, dropping zero voltage at full collector current. Just something to keep in mind when designing a transistor circuit! If the transistor is not saturated, and in fact is dropping substantial voltage between collector and emitter, the maximum power dissipation rating will probably be exceeded before the maximum collector current rating will. Understand that this maximum figure assumes a saturated state (minimum collector-emitter voltage drop). This rating is of particular importance when using a bipolar transistor as a switch.Ĭollector current: A maximum value for collector current will be given by the manufacturer in amps. The rating for maximum collector-emitter voltage can be thought of in terms of the maximum voltage it can withstand while in full-cutoff mode (no base current). This includes voltage ratings for the base-emitter junction, base-collector junction, and also from collector to emitter. Reverse voltages: As with diodes, bipolar transistors are rated for maximum allowable reverse-bias voltage across their PN junctions. When transistors are to be used in hotter-than-normal environments, their power ratings must be derated to avoid a shortened service life. Power ratings are always given in reference to the temperature of ambient (surrounding) air. High temperature is the mortal enemy of all semiconductor devices, and bipolar transistors tend to be more susceptible to thermal damage than most. Just like resistors, transistors are rated in terms of how many watts they can safely dissipate without sustaining damage. At any given time, the power dissipated by a transistor is equal to the product (multiplication) of collector current and collector-emitter voltage. Power dissipation: When a transistor conducts current between collector and emitter, it also drops voltage between those two points. What follows is an itemized description of some typical transistor ratings. Since transistors are a bit more complex than some of the other components you're used to seeing at this point, they tend to have more kinds of ratings. Like all electrical and electronic components, transistors are limited in the amounts of voltage and current they can handle without sustaining damage. Volume III - Semiconductors » BIPOLAR JUNCTION TRANSISTORS
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