This is done because there is a significant difference between the DC and AC solution. Note that DC values are in upper case and AC or signal values are in lower case. 1Ģ Figure 1: Basic transistor characteristics 2Ĥ Transistor Nomenclature Bipolar Junction Transistor Basics Figure 3 illustrates standard transistor nomenclature. For most normal cases these two values are practically identical and for this course we will not make a distinction. Note that there is a DC beta (I C / I B ) term and an AC beta (ΔI C / ΔI B ) term. Figure 2 shows how beta varies with temperature, voltage, and current through the transistor. At very low voltages across the transistor the collector current has a strong dependency on voltage thus the name, ohmic region.
In this region the collector current is very independent of the voltage across the transistor thus the name, current saturation. Linear operation is in the current saturation region as shown on the plots. The plots at the bottom show a family of characteristic curves. Figure 1 shows the construction and schematic symbols of NPN and PNP structures. But, with proper design techniques, the bias design for a BJT circuit can be made to be very independent of beta. At first it would seem that it would be difficult to design a BJT circuit since the exact value of beta is not known for a particular transistor other than roughly the broad range described above. Although we treat beta as if it were a constant, we must keep in mind that beta is a complex function of temperature, collector current, and collector to emitter voltage. An amplifier is based on a small base-emitter current controlling a much larger collector-emitter current. All of the math we use is based on this and will produce incorrect results if either condition is not true. For linear amplifiers the base-emitter junction is always forward biased and the basecollector junction is always reversed biased. The beta for typical small signal BJTs ranges from about 50 to 300. The collector current is the base current multiplied by beta. The symbol for beta is β although B is often used too. A current between the base and emitter terminals controls a current between the collector and emitter terminals.
There are three terminals described as follows: Base Emitter Collector the control terminal the source of majority carriers the collector of majority carriers The basic mathematical model of the BJT is a current controlled current source. Both constructions have the identical mathematical model but differ in that the terminal currents are inverted from each other. 29, 2001, rev 1 Introduction A bipolar junction transistor (BJT) is a three layer semiconductor device with either NPN or PNP construction.
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