![]() Using Thenevin theorem we can transform the circuit into the following circuit below with equivalent resistance and voltage source. It can be transformed into circuits that are much easier to consider from the point of view of BJT operation point. Important point to consider is self-biased BJT circuit, that is depicted below. Let’s apply to the base AC signal and DC signal and we will see the amplification on the resulting I C – V E C curve, so all oscillations of the base current is resulting in amplified collector current. In standard operation the BJT transistor works as an amplifier. The most important dynamic characteristics of a BJT are it’s switching characteristics. The circuit for determination of BJT I-V curve is depicted on the figure below. If this ideal voltage source is connected to the collector circuit, i Ccurrent also can be controlled. Varying the current from ideal voltage source, you can manage current i B E. Let’s imagine ideal voltage source, injecting current i B through base region, junction base-emitter becomes forward-biased. In the active region, base-collector pn-junction is reverse-biased, and base-emitter – forward-biased. In the cut-off region both collector-base and emitter-base pn-junctions are reverse-biased – transistor is off. The output characteristics of BJT can be divided into three areas – cut-off, saturation and active region. ![]() ![]() This configuration is called that, because base is connected to both – emitter and collector. In the active region here base-collector pn-junction is reverse-biased and base-emitter pn-junction is forward-biased. Input data will be here base-emitter, and output – collector-emitter. Here collector-base junction is reverse biased, and base-emitter junction is forward-biased. PNP BJT common emitter configuration is depicted on the figure below.Ĭurrents in the circuit are related with the ratios: I E = I C + I B, I C = α I B. This configuration is called common-emitter as emitter is common for input and outp ut voltages. The main feature of BJT – small base-collector current controlling large collector current. Another relationship for BJT currents we can derive is I E = ( β + 1 ) I B. These two amplification factors are related by the formula α = β β + 1. In case of AC circuit this parameter will vary in accordance to formula ∆ I C = β A C ∆ I B. For the AC mode ratio is more complex and different for vaious values of currents ∆ I C = α A C ∆ I E.įor DC circuit there is s ratio, that shows relation between base and collector currents across the circuit I C = β I B, β is a common emitter amplification factor. In the DC mode emitter and collector currents will be related with the ratio I C = α I E. Note that collector current will consist of leakage current and majority careers current I C = I C M J + I C M N. B layer is usually thin, but characterised with high resistivity, so majority of careers will flow through into the C layer (due to different doping levels of these materials).Ĭurrents in BJT transistor can be found with Kirchhoff’s Law: I E = I C + I B. Majority of careers are moving from E across forward-biased pn-junction. ![]() One junction is forward-biased, and the other is reverse-biased. Base and collector layers are lightly doped, emitter layer – is heavily doped.Ĭharges flow at the BJT are depicted on the figure below. BJT transistor can be two types – pnp and npn BJT transistor.īipolar junction transistor (BJT) is characterised by three regions – base (B), collector (C) and emittor (E). BJT transistor is a three terminal semiconductor device, based on three layers of p and n layers, with different doping concentration. ![]()
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