Question

When a positive voltage signal is applied to the base of a common emitter npn amplifier

• A. The emitter current decreases.
• B. The collector voltage becomes more positive.
• C. The collector voltage becomes less positive.
• D. The collector current decreases.

Answer 1

Answer: (C)

The collector voltage becomes less positive.

Let $I_B$ and $I_c$ be the base current and collector current respectively.

Using Kirchhoff's loop law in loop (1) we get, $-V_{BB}+I_B{R}_s+V_{BE}=0$

$⟹I_B=\frac{V_{BB}-V_{BE}}{R_s}$ .......... (1)

Thus, if $V_{BB}$ increases then base current $\left(I_B\right)$ increases

Also collector current, $I_c=\beta I_B$, where $\beta$ is the current gain.

Hence $I_C$ also increases due to increase in base current

Using Kirchhoff's loop law in loop (2) we get, $-V_{CC}+I_C{R}_L+V_C=0$

$⟹V_C=V_{CC}-I_C{R}_L$ ..........(2)

$⟹$ $V_C$ decreases due to increase in collector current $\left(I_C\right)$

Hence, collector voltage $\left(V_C\right)$ becomes less positive when $V_{BB}$ is applied.