1.2.1 Bipolar Junction Transistors (BJT)


• Bipolar Junction Transistors (BJTs) are made with three layers of doped silicon. The layers are either doped to be positive (p-type) or negative (n-type) using low concentrations of elements mixed with the silicon.


• There are two basic types, PNP and NPN. Their names come from the sequence of doped layers in the transistor. The schematic symbols for these transistors are shown below.



• The base-emitter voltage is ussualy given as a constant. This junction acts much like a diode, and will on average have voltages around 0.7V.


• Transistors are highly non-linear, but they are often biased by carefully applying voltages and currents to put them in a roughly linear range.


• A designer will depend heavily upon specifications. These are often in the form of graphs for different transistor applications.


• Except for applications such as switching, most transistor configurations are used for sinusoidal signals. As a result there is ussualy a DC design, as well as AC. - Biasing Common Emitter Transistors


• A common emitter configuration is shown in the figure below.



• Consider the common emitter amplifier shown. The resistors provide DC biasing to select an operating point. The capacitor Ce is used to allow the AC to bypass Re.


• To perform the design we must first bias the transistor using the curves below.