6.2 TRANSISTORS

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6.2.1 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.

6.2.1.1 - 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.