BJT phase relationship — in a common-collector (emitter-follower) voltage amplifier, the output voltage is in phase with the input voltage. Is this statement correct?

Introduction / Context:
Different BJT amplifier topologies exhibit different phase relationships between input and output. Recognizing these signatures helps during troubleshooting and when choosing stages for buffering or gain. The common-collector stage (emitter follower) is especially valued as a buffer because of its high input impedance, low output impedance, and non-inverting behavior.

Given Data / Assumptions:

• Topology: common-collector (input at base, output at emitter).
• Small-signal AC operation in the passband.
• Biasing set to linear active region.

Concept / Approach:
In a common-collector amplifier, the emitter voltage follows the base voltage minus approximately one diode drop (V_BE) for BJTs. Small-signal changes at the base produce similar-polarity changes at the emitter; therefore, the stage is non-inverting (input and output are in phase). By contrast, the common-emitter stage inverts phase by about 180 degrees for voltage signals.

Step-by-Step Solution:

Verification / Alternative check:
Small-signal hybrid-π models show voltage gain ≈ R_L / (r_e + R_L) which is positive; scope measurements confirm in-phase waveforms within the midband region.

Why Other Options Are Wrong:

• Incorrect: contradicts the well-known non-inverting property of emitter followers.
• “Only midband” or “only infinite load”: while extreme frequency or heavy loading alters magnitude, the sign remains non-inverting within the useful band.

Common Pitfalls:
Assuming all transistor amplifiers invert because common-emitter does; overlooking the buffering role of the emitter follower.

Final Answer:
Correct — the emitter follower's output is in phase with its input.