This configuration is mainly employed in impedance matching applications and it comes with high input impedance.īJT devices are mainly categorized into two main types as follow: In a Common-collector configuration, also known as voltage follower circuit, we will keep the collector terminal common between the output and input signals. In the saturation region, both emitter and collector junctions are forward biased. In the cutoff region, the collector current is not entirely cut off and the emitter junction is not completely reverse biased. In the active region, the emitter junction is forward biased and the collector junction is reverse biased. The common-emitter configuration also comes with three regions. The output characteristics curve is a plot between the collector current IC on the y-axis and collector-emitter voltage VCE on the x-axis. The input characteristics of the Common Emitter configuration is a plot between the base current IB on the y-axis and the base-emitter voltage VBE on the x-axis. In the Common-emitter configuration, we will keep the emitter terminal common between the output and input signals. In the active region, the emitter junction is reverse biased, in the saturation region both collector and emitter junctions are forward biased and in the cutoff region, both collector and emitter junctions are reverse biased. There are three different regions on the curve named an active region, saturation region, and cutoff region. The output characteristics of the Common Base configuration will be drawn between the collector current IC on the y-axis and the collector-base voltage VCB on the x-axis, as shown below. The following input characteristic curve for the Common Base configurations is a graph between the emitter current IE on the y-axis and the base-emitter voltage VEB on the x-axis. In a Common-base configuration, we will keep the base terminal common between the output and input signals. The BJTs are connected in three different configurations as follow: The collector side is moderately doped while the emitter terminal is highly doped and the base terminal, on the other hand, is lightly doped. The BJT is also known as an active semiconductor device where all three terminals are different in terms of doping concentration. The PNP transistors work in the same way but here current direction and voltage polarities are reversed. A large number of leftover electrons at the base region that don’t combine with the holes, then start to enter the collector side, in the form of collector current.Īccording to Kirchoff’s Current Law, the emitter current is the combination of collector current and base current. ![]() These electrons will combine with the holes present at the base region and start to flow out of the base region as a base current. The base region is lightly doped and is very thin, hence it struggles to hold the number of electrons for the maximum time. The barrier potential is decreased at the BE junction that is forward biased, and as a result, the electrons will move from the emitter to the base region. The depletion region width at the collector-base junction is higher, compared to the depletion region width of the base-emitter junction. As this is an NPN transistor, here collector-base junction is reverse-biased and the base-emitter junction is forward-biased. ![]() This current is known as the collector current and is denoted by Ic. When voltage is applied at the base terminal in the NPN transistor, it turns ON the transistor and as a result, the current starts to flow from the collector to the emitter terminal. The working of BJT starts from the base pin. Current flows from base to emitter in NPN transistor and it flows from emitter to base in PNP transistor. The arrows in the symbol show the direction of the current. The left one is the symbol of the NPN transistor and the right one is the symbol of the PNP transistor. The following figure shows the symbols of BJT. ![]() Earlier BJTs were composed of germanium, however, recently silicon is used for the making of BJTs. The small current at the base side is used to control the large current at the remaining terminals. It comes with three terminals called emitter, base, and collector. The BJT (bipolar junction transistor) is a current-controlled electronic device main employed for amplification and switching purpose. In this post today, we’ll discuss BJT (Bipolar Junction Transistor) in detail, including BJT definition, symbol, working, characteristics, types & applications.
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