BJT

The Bipolar Junction Transistor (BJT) is an extremely common electronic device to all forms of electronic circuits. It can be used for a number of useful applications such as an amplifier, a switch, a buffer, an oscillator, a nonlinear circuit – so forth.

The BJT is made by P and N type semiconductor material, which should be familiar from the study of diodes. The BJT is a three terminal device (Fig 1).

Figure 1 – The BJT

The three terminals are Base, Collector and Emitter. The emitter terminal always has an arrow. The collector is always on the opposite side of the emitter and the base is the other remaining terminal on the left. Note that this is the conventional schematic diagram of a BJT transistor. Furthermore, there are two types of BJT transistors. They are the NPN type, and the PNP type. Figure 2 illustrates this:

(a) NPN (b) PNP

Figure 2 – The two types of BJT


The letters b, e, and c have been used for abbreviations for the base, emitter and collector terminals respectively. An NPN transistor is always drawn with the arrow pointing outwards whilst the PNP transistor always has the arrow pointing inwards. And of course, remember that the arrow is always the emitter terminal. So which type is the transistor in Fig 1? – It is NPN. The other diagrams shown in Figure 2 illustrate why the transistors are called either NPN or PNP. It’s simply due to the semiconductor material used for each terminal.

Now, lets take a more detailed look:

Figure 3.


The arrows show the direction of DC current flow for both the NPN and PNP cases. In both cases the base current (Ib) is a very small current in the order of microamps whilst the collector current (Ic) and emitter current (Ie) are larger and in the order of milliamps. Note that for the NPN transistor, the base current flows into the transistor but for the PNP transistor, the base current flows out the transistor. Also note Ic and Ie always flow in the same direction and in the direction of the (black) arrow, the same arrow that tells us whether the transistor is PNP or NPN.

Now for the voltages:

The voltage at the base is normally written as Vb.
The voltage at the collector is normally written as Vc.
The voltage at the emitter is normally written Ve.

That part was easy, but what about the voltage between the collector and the emitter? Is it written as Vce or Vec? The convention is that the first subscript letter is the voltage that you are measuring and the second subscript letter is the reference. That means, if:

Vc = 6V (The voltage at the collector is 6 volts)
Ve = 2V (The voltage at the emitter is 2 volts)

Then Vce is 4V because the voltage at the collector is 4V higher than the voltage at the emitter. Also, Vec = -4V because the voltage at the emitter (measuring point) is 4V lower than the voltage at the collector (reference point). This concept is important and If you’re a bit lost read it again. The following diagram should summarize. This is the convention used for measuring voltages between terminals of the NPN and PNP transistors. The reason for this is that in these examples the first subscript letter is usually of higher voltage than the second, hence all variables listed below will have positive values.

Figure 4.