Identifying the JFET amplifier configuration with voltage gain less than unity Among the standard JFET small-signal stages (common-source, common-gate, and common-drain), which configuration typically provides a voltage gain magnitude less than one while offering a high input impedance and low output impedance?

Introduction / Context:
JFET amplifiers come in three classic topologies: common-source (CS), common-gate (CG), and common-drain (CD, also called source follower). Each has distinct input/output impedances and characteristic voltage gain. Recognizing these properties is critical when choosing a stage for buffering, voltage amplification, or impedance transformation.

Given Data / Assumptions:

• Small-signal operation of a JFET around a proper DC bias point.
• Idealized, midband conditions (capacitors as shorts, no loading beyond stated impedances).
• Focus on the intrinsic voltage gain magnitude and impedance characteristics.

Concept / Approach:
The common-drain (source follower) provides near-unity but slightly less than one voltage gain because the output (source) follows the input (gate) minus the small vgs required to support the drain current. It exhibits very high input impedance (at the gate) and low output impedance (at the source), making it an excellent buffer. In contrast, common-source provides significant gain (often much greater than one), and common-gate typically has a gain around or above unity with low input impedance.

Step-by-Step Solution:

Verification / Alternative check:
Small-signal equations show Av ≈ gm * RS / (1 + gm * RS), which approaches but remains less than 1 for finite gm * RS, confirming the “slightly under unity” behavior.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing “source follower” (CD) with “emitter follower” (BJT): they are analogous but not identical devices; both are buffers with Av slightly less than one.

Final Answer:
common-drain (source follower)