Question

In a common emitter amplifier circuit using n-p-n transistor, the phase difference between the input and the output voltages will be

• A. ${45}^0$
• B. ${90}^0$
• C. ${135}^0$
• D. ${180}^0$

Answer 1

The correct option is D ${180}^0$


$\Delta V_{CC}=\Delta V_{CE}+R_L\Delta I_C=0$

$\Delta V_{CE}=-R_L\Delta I_C=-\beta R_L\Delta I_B$

Voltage gain is $\frac{V_0}{V_i}=\frac{\Delta V_{CE}}{r_i\Delta I_B}=\frac{-\beta R_L\Delta I_B}{r_i\Delta I_B}=\frac{-\beta R_L}{r_i}$


The negative sign indicates that the output voltage and input voltage are out of phase.

Hence, $\left(A\right)$ is the correct answer.