Common-emitter amplifier signal phase: what is the phase difference between the small-signal input voltage and the output voltage of a properly biased common-emitter BJT amplifier?

Introduction / Context:
Phase relationships are central to amplifier design, feedback analysis, and signal interpretation. The common-emitter (CE) amplifier is known for providing substantial voltage gain and inverting the signal. Knowing the expected phase inversion helps when cascading stages and when designing negative-feedback loops that must account for phase to ensure stability.

Given Data / Assumptions:

• BJT biased in its active region (not saturating or cutting off in normal operation).
• Small-signal sinusoidal analysis around the bias point.
• Load and coupling capacitors chosen so midband behavior applies (no extra phase shift from reactive elements at midband).

Concept / Approach:
The CE topology places the output at the collector. An increase in base voltage (and hence base current) increases collector current, which causes a larger voltage drop across the collector resistor RC. The collector node voltage therefore drops as the input rises. This results in an output that is inverted relative to the input, which corresponds to a 180º phase shift in sinusoidal steady state for the midband region. Additional capacitive effects at low or high frequencies can add phase lag or lead, but the ideal midband CE amplifier is considered a 180º inverter.

Step-by-Step Solution:

Verification / Alternative check:
Small-signal gain Av of a CE stage is negative (Av ≈ −gm * RC in simplified form), and a negative gain is equivalent to a 180º phase inversion for sinusoidal signals. Oscilloscope measurements on lab CE amplifiers show output peaks when input is at troughs, confirming inversion.

Why Other Options Are Wrong:
0º: describes common-collector or common-drain follower behavior.
90º: indicates quadrature typically due to reactive networks, not a midband CE amplifier.
360º: implies no net phase shift (equivalent to 0º) and is not characteristic of CE midband operation.
Depends on load only: while loading affects magnitude, the inversion is intrinsic to CE topology.

Common Pitfalls:
Observing extra phase lag at frequency extremes and assuming the midband inversion is not 180º; forgetting coupling-capacitor and bypass-capacitor phase effects outside midband.

Final Answer:
180º