The phase difference between the output and input of a common emitter amplifier is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 180°
In a common emitter (CE) amplifier the input is applied between base and emitter and the output is taken between collector and emitter, with the output voltage \( V_o = V_{CC} - I_CR_C \).
When the input signal rises, the base current and hence the collector current \( I_C \) rise, so the drop \( I_CR_C \) rises and the output voltage falls. When the input falls, the output rises.
The output is therefore the inverted copy of the amplified input, a phase difference of \( 180^\circ \).
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 a Device – “… That is why as input voltage of the CE amplifier increases its output voltage decreases and the output is said to be out of phase with the input.”
Transistor as an Amplifier (CE-Configuration) – “… The negative sign represents that output voltage is opposite with phase with the input voltage.”