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Bipolar Junction Transistor (BJT)

(External reference on Wikipedia)

A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. In an NPN transistor, a thin and lightly doped P-type base is sandwiched between a heavily doped N-type emitter and another N-type collector; while in a PNP transistor, a thin and lightly doped N-type base is sandwiched between a heavily doped P-type emitter and another P-type collector. In the following we will only consider NPN BJTs.



In many schematics of transistor circuits (especially when there exist a large number of transistors in the circuit), the circle in the symbol of a transistor is omitted. The figures below show the cross section of two NPN transistors. Note that although both the collector and emitter of a transistor are made of N-type semiconductor material, they have totally different geometry and therefore can not be interchanged.



All previously considered components (resistor, capacitor, inductor, and diode) have two terminals (leads) and can therefore be characterized by the single relationship between the current going through and the voltage across the two leads. Differently, a transistor is a three-terminal component, which could be considered as a two-port network with an input-port and an output-port, each formed by two of the three terminals, and characterized by the relationships of both input and output currents and voltages.

Depending on which of the three terminals is used as common terminal, there can be three possible configurations for the two-port network formed by a transistor:


The relationship between the input and output currents of both CB and CE configurations can be summarized below:

The collector characteristics of the common-base (CB) and common-emitter (CE) configurations have the following differences:



Various parameters of a transistor change as functions of temperature. For example, $\beta$ increases along with temperature.

Load line and DC operating point

A typical CE circuit is shown in the figure below, where $I_e=I_b+I_c$, $V_{in}=V_{be}=V_B$, and $V_{out}=V_{ce}=V_C$.


The DC operating point (also known as bias point, quiescent point, or Q-point) is the DC steady-state operating condition of a transistor, (with no AC input signal applied), determined by $I_b$ and $V_{be}$ of the input port, and $I_c$ and $V_{ce}$ of the output port.

The output characteristic plot of the transistor can be divided into three regions:

Switch Circuit

From the current-voltage plot of the output characteristics, we see that the operation of a transistor can be in one of the three possible regions:

Example: In the CE circuit shown below, $V_{cc}=12V$, $R_B=6 K\Omega$, $R_C=2 K\Omega$, $\beta=60$. The load line can be determined by two points: $(V_{ce}=0,\;I_c=V_{cc}/R_C=6\;mA)$ and $(I_c=0,\;V_{ce}=V_{cc}=12\;V)$. Find output voltage $V_{out}=V_{ce}$ when $V_{in}$ takes the following values:


next up previous
Next: Dynamics with AC input Up: ch4 Previous: Diodes
Ruye Wang 2014-08-02