Which JFET amplifier configuration is best for buffering a high-resistance signal source into a low-resistance load (i.e., high input impedance and low output impedance)?

Introduction / Context:
Many sensor and audio front ends require a buffer stage that minimally loads the source but can drive a lower-impedance load. The classic FET solution is the source follower, which offers very high input impedance and low output impedance with near-unity voltage gain.

Given Data / Assumptions:

• Signal source has high resistance (needs minimal loading).
• Load is relatively low resistance (needs drive).
• Candidate topologies: common-source, common-gate, common-drain (source follower), cascode.

Concept / Approach:
The common-drain (source follower) presents gate input impedance dominated by gate leakage and bias network (typically megaohms), while the source output follows the gate voltage with a small offset and low output impedance (approximately 1/gm in parallel with bias resistances). This makes it ideal as a buffer.

Step-by-Step Solution:
Identify need: high Zin and low Zout.Evaluate topologies: common-source has high Zin but relatively high Zout; common-gate has low Zin; cascode improves bandwidth/early voltage but not Zout like a follower.Choose common-drain (source follower) for buffering.

Verification / Alternative check:
Small-signal models show follower output resistance near 1/gm, often tens to hundreds of ohms, while input resistance is set by gate leakage and bias resistors, commonly megaohms—exactly the desired transformation.

Why Other Options Are Wrong:

• Common-source: Provides gain but higher output impedance; not the best buffer.
• Common-gate: Low input impedance; unsuitable for a high-resistance source.
• Cascode: Great for bandwidth and gain stability, but not primarily a buffer topology.

Common Pitfalls:

• Confusing “source follower” with BJT “emitter follower” names; functions are analogous but device physics differ.
• Overlooking biasing; improper bias can raise distortion or limit swing.

Final Answer:
common-drain (source follower)