BJT amplifier naming — a “common-emitter” (C-E) amplifier is so named because the emitter node is common (shared) to both the input and the output circuits. Is this statement accurate?

Introduction / Context:
Bipolar junction transistor (BJT) amplifier stages are categorized by which terminal serves as a reference for both input and output: common-emitter, common-collector, and common-base. The naming convention reflects how signals are applied and measured. This question asks if the common-emitter name indicates the emitter is shared between the input and output paths.

Given Data / Assumptions:

• Small-signal AC view: coupling capacitors and bias networks present the emitter as a reference node.
• Input taken at base–emitter, output taken at collector–emitter.
• Applies to NPN and PNP with appropriate polarity conventions.

Concept / Approach:
In a C-E amplifier, the emitter terminal is the common reference for signal injection and signal extraction. The input signal perturbs base–emitter voltage, and the output is observed as a voltage change at the collector with respect to the emitter. This topology provides voltage and current gain, with a 180-degree phase inversion between input and output voltages.

Step-by-Step Solution:

Verification / Alternative check:
Comparing topologies: common-collector (emitter follower) has collector as common node for DC but the output is taken at the emitter; common-base has the base as the common node. Naming aligns with the node common to signal paths.

Why Other Options Are Wrong:

• Incorrect: contradicts the standard definition.
• “Only low frequency” or “only PNP”: the naming is independent of frequency and device polarity.

Common Pitfalls:
Confusing “common” with “ground”; in practice, the common node is a reference that may or may not be chassis ground.

Final Answer:
Correct — the emitter is common to both input and output, hence “common-emitter.”