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. Understand that this maximum figure assumes a saturated state (minimum collector-emitter voltage drop). Typical values for small signal transistors are 10s to 100s of mA, 10s of A for power transistors. In power transistors, this could range to 1000 V, for example, a horizontal deflection transistor in a cathode ray tube display.Ĭollector current: A maximum value for collector current I C will be given by the manufacturer in amps. A typical value for a small signal transistor is 60 to 80 V. This rating is of particular importance when using a bipolar transistor as a switch. The rating for maximum collector-emitter voltage V CE can be thought of as the maximum voltage it can withstand while in full-cutoff mode (no base current). Transistor inputs to analog integrated circuits also have a V EB rating, which if exceeded will cause damage, no zenering of the inputs is allowed. Some circuit designers use discrete BJTs as 7 V zener diodes with a series current limiting resistor. V EB, the maximum reverse voltage from emitter to base is approximately 7 V for some small signal transistors. This includes voltage ratings for the emitter-base junction V EB, collector-base junction V CB, and also from collector to emitter V CE. 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 environments (>25 o, their power ratings must be derated to avoid a shortened service life. Power ratings are always referenced 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 for how many watts each 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.
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