The most commonly used configuration of the transistor is the NPN transistor. We also found that the bipolar transistor intersections can be biased in one of three different ways – common ground, common transmitter and common collector.
In this bipolar transistor tutorial we will examine the “Common Emitter” configuration more closely using the Bipolar NPN transistor with an example of construction of an NPN transistor along with the current flow characteristic of the transistors given below.
Then the voltage sources are connected to an NPN transistor as shown. The collector is connected to the VCC supply voltage via the RL load resistance which also acts to limit the maximum current flowing through the device. The supply voltage VB is connected to the base resistor RB, which is again used to limit the maximum base current.
Thus, in an NPN transistor is the movement of the negative current carriers (electrons) through the base region constituting the action of the transistor, since these mobile electrons provide the connection between the collector and the transmitter circuits. This connection between the input and output circuits is the main feature of the transistor’s action because the transistor amplification properties come from the control the Base exercises over the Collector-Emitter current.
Then we can see that the transistor is a current driven device (Beta model) and that a high current (Ic) flows freely through the device between the collector and the transmitter terminals when the transistor is switched “completely ON”. However, this only happens when a small polarization current (Ib) flows into the base terminal of the transistor at the same time, allowing the Base to act as a sort of current command input.
The current of the transistor in a bipolar NPN transistor is the ratio between these two currents (Ic / Ib), called the current gain of the device and given to the current hfe or Beta symbol, (b). The value of β can be high up to 200 for standard transistors, and this high ratio between Ic and Ib makes the NPN bipolar transistor a useful amplifier when used in its active region because Ib supplies the input and Ic supplies the output. Note that Beta does not have units because it is a report.
Also, the current gain of the transistor from the collector terminal to the Emitter, Ic / Ie terminal is called Alpha, (a) and is a function of the transistor itself (electrons that diffuse between junctions). Since the emitter current Ie is the sum of a very low base current plus a very large collector current, the alpha value is very close to the unit, and for a typical low power signal transistor this value ranges from about 0.950 to 0.999