Bjt transistor calculations8/8/2023 ![]() ![]() Standard signal amplification may be in the range of 50, but this value can be less for high-power usage. ![]() Β (beta) - Represents the common-emitter gain, or the ratio of collector terminal DC to base terminal DC in the forward active mode. Ideally 1, but practically the value approaches 1 from the left due to electron and electron-hole recombination. Α (alpha) - Represents the common-base gain or the ratio of emitter terminal DC to collector terminal DC in the forward active mode. ![]() This construction prevents a loss of current due to recombination and the greater mobility of the electrons improves the diffusion rate to the collector across the base. In an NPN BJT, the heavier doped n-type emitter region can provide more electrons than the lightly-doped p-type base can provide holes. The transistor characteristics relay the efficiency of the BJT due to its uneven doped region. Saturation - A forward bias at both base-emitter and base-collector junctions acts as a closed switch for the BJT, effectively a logical high state.Ĭut-off - A reverse bias at both base-emitter and base-collector junctions acts as an open switch for the BJT, effectively a logical low state.Īs the BJT can be utilized in several configurations, multiple metrics measure its performance. Traditionally, the amplification is much less effective than in the forward direction, as the heavily doped region of the emitter cannot be optimized in this orientation. Reverse - A seldom used mode where the base-emitter junction is reverse-biased and the base-collector junction is forward-biased. Almost all BJTs provide the greatest common-emitter current gain in the forward direction. However, it is often considered and treated as a current-controlled current source due to the low impedance of the base terminal.ĭepending on the biasing direction between terminals and the relative voltage on the pins, there are four distinct operational modes for BJTs:įorward - The standard amplification mode where the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. As BJTs require a biasing voltage to operate, the device is technically a voltage-controlled current source (like MOSFETs). The major difference between these two BJT styles is the direction of current flow: emitter to base for NPN and base to emitter for PNP. It surrounds the base and emitter to prevent the escape of injected electrons.Įmitter - A heavily doped region that enables high current gain by injecting the bulk of charge carriers into the base-emitter junction. A biasing voltage placed on the base will have the current run either from collector to emitter (typically the preferred direction for maximum current gain) or emitter to collector.Ĭollector - The largest semiconductor region of the BJT. Together, these two regions are alternated to make a PNP or NPN BJT.īase - The control terminal of the BJT that is lightly doped opposite to the surrounding collector and emitter regions. The bipolar junction is built of three alternating doped regions that are either electron-rich (n-type) or rich in the absence of electrons from available positions (p-type). Until the maturation of MOSFET technology in the 1970s, BJTs were the primary active element in electronics and are still seen to this day in certain high-frequency applications where CMOS exhibit large switching losses. The BJT is an active current-controlled element in circuits, able to take an input current on the base terminal and effectively multiply it for switching and amplification purposes.
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