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Engineering LibreTexts

19.1: Theory Overview

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  • By inserting a resistor in either the emitter or collector portions of the transistor circuit, it is possible to partially control the base current in such a way that an increase in beta will cause a decrease in base current which in turn helps to mitigate the tendency of collector current to increase. This will result in circuits that have greater Q point stability than simple base bias circuits although for certain practical reasons they might not be as stable as voltage divider or dual supply emitter bias schemes.

    In the collector feedback arrangement, the base resistor is connected from the collector to the base. Therefore, its voltage is one base-emitter drop less than the collector voltage. The collector voltage, in turn, is simply the supply potential minus the collector resistor’s drop. Therefore, as the collector current rises, the collector resistor’s drop increases, forcing the collector voltage down and thus reducing the base resistor’s voltage. By Ohm’s law, this means that the base current must decrease. This decrease helps to limit the overall increase in collector current.

    The emitter feedback situation is similar. In this instance, as collector current increases the drop across the emitter resistor rises. This will result in an increase in base voltage as it is locked to one base-emitter drop above the emitter. Consequently, as the collector current increases, the voltage across the base resistor decreases which helps to compensate for the original increase in collector current.

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