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Engineering LibreTexts

15.1: Theory Overview

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  • For a bipolar junction transistor to operate properly, the base-emitter junction must be forward biased while the collector-base junction must be reverse biased. This will place \(V_{BE}\) at approximately 0.7 volts and the collector current \(I_C\) will be equal to the base current \(I_B\) times the current gain \(beta\). For small signal devices, the current gain is greater than 100 typically. Thus, \(I_C \gg I_B\) and \(I_C \approx I_E\).

    The common emitter configuration places the emitter terminal at ground. The base terminal is seen as the input and the collector as the output. Using a fixed base supply, the base current is dependent on the value of the base resistor via Ohm’s law. Consequently, any variation in current gain across a batch of transistors will show up as an equivalent variation in collector current, and by extension, a variation in collector-emitter voltage \(V_{CE}\).

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