If the emitter resistance increases, the collector voltage

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Examoogle · Accepted Answer

Increases

Explanation: When emitter resistance ($R_E$) increases, the emitter current ($I_E$) decreases due to increased negative feedback. Consequently, the collector current ($I_C \approx I_E$) decreases, leading to a smaller voltage drop across the collector resistor ($R_C$), which causes the collector-to-ground voltage ($V_C$) to increase toward the supply voltage ($V_{CC}$).

Working Principle: In a common-emitter BJT amplifier with emitter degeneration, the circuit follows the loop equation $V_{CC} = I_C R_C + V_{CE} + I_E R_E$. Since $I_C \approx I_E$, we can write $V_C = V_{CC} - I_C R_C$. Increasing $R_E$ reduces the overall gain and decreases the quiescent collector current $I_C$. As $I_C$ drops, the drop $I_C R_C$ decreases, resulting in a higher collector potential $V_C$ relative to the ground.

Key Formulas: $V_C = V_{CC} - I_C R_C$ — Collector voltage equation; $I_E \approx \frac{V_B - V_{BE}}{R_E}$ — Emitter current dependency on $R_E$

Key Points: Emitter resistance provides negative feedback which stabilizes the operating point.; The collector voltage is determined by the voltage drop across the collector load resistor.; Increased $R_E$ leads to a decrease in the transconductance of the BJT stage.; The collector potential rises when the transistor is less 'on'.

Advantages: Improved thermal stability of the bias point; Increased input impedance of the amplifier

Disadvantages: Reduction in voltage gain ($A_v$); Reduction in dynamic output voltage swing

Applications: Stabilizing biasing circuits in BJT amplifiers; Emitter follower or common-emitter stages with local feedback

Additional Info: The value of $R_E$ is usually chosen to ensure the transistor remains in the active region.. Option A is incorrect because a decrease in $I_C$ (caused by higher $R_E$) results in less voltage drop across $R_C$, leaving a higher residual voltage at the collector.

Answer: B is correct — Increasing the emitter resistance reduces the collector current, thereby decreasing the voltage drop across the collector resistor and raising the collector voltage.

Exam Tip: Always remember that in BJT amplifier circuits, any factor that reduces the collector current will directly cause the collector voltage to swing closer to the supply voltage $V_{CC}$.

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