Base-width modulation/ Early Effect

Posted By on December 15, 2014


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As the voltages applied to the base-emitter and base-collector junctions are changed, the depletion layer widths and the quasi-neutral regions vary as well. This causes the collector current to vary with the collector-emitter voltage as illustrated in Figure

 

Figure Variation of the minority-carrier distribution in the base quasi-neutral region due to a variation of the base-collector voltage.
A variation of the base-collector voltage results in a variation of the quasi-neutral width in the base. The gradient of the minority-carrier density in the base therefore changes, yielding an increased collector current as the collector-base current is increased. This effect is referred to as the Early effect. The Early effect is observed as an increase in the collector current with increasing collector-emitter voltage as illustrated with Figure . The Early voltage, VA, is obtained by drawing a line tangential to the transistor IV characteristic at the point of interest. The Early voltage equals the horizontal distance between the point chosen on the IV characteristics and the intersection between the tangential line and the horizontal axis. It is indicated on the figure by the horizontal arrow.
Figure
Collector current increase with an increase of the collector-emitter voltage due to the Early effect. The Early voltage, VA, is also indicated on the figure.
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Posted by Akash Kurup

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