When a transistor is used as an Amplifier, the phase difference between the input and the output signal voltage is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \pi \)
A transistor amplifier is normally used in the common-emitter arrangement. A rise in the input voltage increases the base current, which raises the collector current \( I_C \). The output voltage taken across the collector is \( V_{CE} = V_{CC} - I_CR_L \), so it falls when \( I_C \) rises.
The output therefore moves opposite to the input: the amplified signal is inverted, a phase difference of \( 180^\circ = \pi \).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Transistor as an Amplifier (CE-Configuration) – “… The voltage gain of the amplifier is
\( \displaystyle A_v = \frac{v_0}{v_i} = \frac{\Delta V_{CE}}{r\,\Delta I_B} = -\frac{\beta_{ac} R_L}{r} \qquad (14.18) \)
The negative sign represents that output voltage is opposite with phase with the input voltage.”