Solution:
A
Common Emitter (CE) amplifier is widely used for voltage amplification. The question asks about the phase difference between the
input signal voltage and the
voltage developed across the collector load resistor (RC), not the collector terminal voltage.
When the
input signal voltage increases, the base current (
IB) increases. This causes the collector current (
IC) to increase because the collector current is directly proportional to the base current in the transistor's active region.
The voltage across the collector load resistor is given by
VRC = IC × RC. Since the collector current increases, the voltage drop across
RC also increases. Similarly, when the input signal decreases, the collector current and the voltage across
RC also decrease.
Thus, the
input signal voltage and the
voltage across RC rise and fall together. They are therefore
in phase, resulting in a phase difference of
0°.
Why the Other Options are Incorrect:
180°: This phase difference exists between the
input signal and the
collector terminal (output) voltage, not between the input signal and the voltage across
RC. The collector voltage is given by
VC = VCC − VRC, so it decreases when
VRC increases.
270°: A phase difference of 270° does not occur in the normal operation of a single-stage CE amplifier and has no significance in this context.
90°: A 90° phase shift is characteristic of purely reactive circuits containing inductors or capacitors. Since
RC is a resistor, it does not introduce a 90° phase shift.
Key Point to Remember:
Voltage across RC is
in phase (0°) with the input signal because both increase and decrease together.
Collector terminal (output) voltage is
180° out of phase with the input signal because it is equal to
VCC − ICRC.
Therefore, the phase difference between the
voltage across the collector load resistor (RC) and the
input signal voltage is
0°. Therefore,
Option D is the correct answer.
