Common-gate JFET characteristics: What is the typical input impedance seen at the input of a common-gate configured JFET stage?

Introduction / Context:
Different FET configurations (common-source, common-gate, common-drain) present different input impedances, gains, and bandwidths. The common-gate (CG) configuration is often used for wideband or low-noise RF front-ends precisely because of its relatively low input impedance at the source node, which can match transmission lines.

Given Data / Assumptions:

• Idealized small-signal JFET model.
• Gate is AC-grounded; signal applied to the source; output taken at the drain.
• Neglect parasitic capacitances for the basic impedance discussion.

Concept / Approach:

The small-signal input resistance of a common-gate JFET is approximately 1/gm (the inverse of transconductance), often only a few hundred ohms to a few kilo-ohms, which is much lower than the megaohm-level input resistance of common-source or source-follower stages. Hence, the qualitative description is “low” input impedance, not “very high.”

Step-by-Step Solution:

Verification / Alternative check:

RF amplifier design texts use CG JFETs and MOSFETs to match 50 Ω or similar lines by choosing devices with suitable gm, confirming the “low input impedance” characterization.

Why Other Options Are Wrong:

• Very high/high: describe common-source or common-drain behavior, not common-gate.
• Very low: overly strong; while low, it is typically not near-zero ohms.
• Approximately infinite: applies to gate input of CS, not CG source input.

Common Pitfalls:

• Confusing gate input resistance (very high) with source input resistance in CG (low).
• Ignoring frequency-dependent capacitive reactance that can further shape input impedance.

Final Answer:

low