NPN Transistor
In the preceding tutorial it was presented that the typical Bipolar Transistor or BJT comes in two major forms, the NPN (Negative-Positive-Negative) type and the PNP (Positive-Negative-Positive) type with the NPN Transistor being the most commonly used transistor type. We also find out that the transistor junctions can be biased in one of the three different ways namely Common Base, Common Emitter and Common Collector. Now in this tutorial, we will study in detail the Common-Emitter configuration using NPN Transistors. Shown below is an example of its current flow characteristics.
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An NPN Transistor Configuration
Apparently, transistor is a “CURRENT” operated device which has a very large amount of current (Ic) which flows without restraint through the device between the collector and emitter terminals. But this is only possible if a small amount of biasing current (Ib) is present in the base terminal of the transistor making the base to act as a current control input. The symbol hfe or sometimes referred to as Beta (β) is actually the ratio of these two currents (Ic/Ib) and is described as the DC Current Gain of the device. Therefore making Beta as unitless since it is expressed as a ratio. Alpha (α) on the other hand is the current gain form the emitter to the collector terminal, Ic/Ie, and is a function of the transistor itself. Typically the value of the parameter α is almost close to unity since the emitter current Ie is the product of a very small base current Ib and very large collector current Ic. In general, the value of α in a low-power signal transistor ranges only from about 0.959 to 0.999.
The α and β Relationships
Two mathematical equations can be derived by combining the two parameters α and β which will describe the relationship between the different currents flowing in the transistors.
The figure Beta ranges from about 20 to 1000 for high current power transistors and for high frequency low power type bipolar transistors respectively. By rearranging the equation of Beta to make it a function of the collector current Ic, and then equating the base current to zero (Ib = 0) would eventually result to zero collector current (Ic = β x 0). In addition, as base current increases collector current also increases thus making the base current as the controlling parameter for the collector current. Moreover, the most significant feature of the Bipolar Junction Transistor is that only a very small amount of base current is needed to control a very high collector current.
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