The standard bipolar or BJT transistor is presented in two basic forms. An NPN type (Negative-Positive-Negative) and a PNP type (Positive-Negative-Positive).
The most commonly used transistor configuration is the NPN transistor. We have also learned that the bipolar transistor junctions can be polarized in one of three ways: Common Base, Common Emitter and Common Collector.
In this tutorial on bipolar transistors we will examine more closely the configuration of “Common Emitter” using the bipolar NPN transistor with an example of the construction of an NPN transistor along with the current flow characteristics of the transistors.
The construction and voltages of the terminals for a bipolar NPN transistor are shown above. The voltage between Base and Emitter (VBE), is positive at the Base and negative at the Emitter because for an NPN transistor, the Base terminal is always positive compared to the Emitter. The manifold supply voltage is also positive compared to the Emitter (VCE). Thus, for a bipolar NPN transistor conducting the Collector is always more positive than both the Base and the Emitter.
Thus the voltage sources are connected to an NPN transistor as shown. The collector is connected to the supply voltage VCC through the load resistor, RL which also acts to limit the maximum current flowing through the device. The base supply voltage VB is connected to the base resistor RB, which is used again to limit the maximum base current.
Thus in an NPN transistor is the movement of negative current carriers (electrons) through the base region which constitutes the action of the transistors, since these movable electrons provide the connection between the collector and emitter circuits. This connection between the input and output circuits is the main feature of the transistor action, since the amplifying properties of the transistors derive from the consequent control that the base exerts on the collector-to-emitter current.
So we can see that the transistor is a current device (Beta model) and that a large current (Ic) flows freely through the device between the collector and the emitter terminals when the transistor is switched “completely ON”. However, this only happens when a small bias current (Ib) flows into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input.
The current transistor in a bipolar NPN transistor is the ratio of these two currents (Ic / Ib), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, (β). The value of β can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the bipolar NPN transistor a useful amplification device when used in its active region as Ib provides the input and Ic provides the output. Note that the Beta has no unit as it is a relationship.