Common-source JFET amplifier characteristics: Which description best matches the typical properties of a properly biased common-source JFET voltage amplifier?

Introduction / Context:
The common-source (CS) configuration is the JFET analog of the BJT common-emitter and the MOSFET common-source stage. It provides voltage gain with phase inversion and is widely used as the basic voltage-amplifying stage.

Given Data / Assumptions:

• JFET operated in the saturation region at a suitable Q-point.
• Drain load resistance RD chosen for gain.
• Signal small enough for linear approximation.

Concept / Approach:
Voltage gain magnitude for a CS JFET is approximately |Av| ≈ gm * RD (neglecting ro). JFETs have high input impedance because the gate–channel junction is reverse-biased, so gate current is very small. With proper gm and RD, high voltage gain is achievable (often much greater than unity), unlike the source follower which has Av < 1.

Step-by-Step Solution:
Bias JFET so that it operates in saturation.Select RD to set gain: larger RD → larger |Av| within headroom limits.Recognize high input Z due to reverse-biased gate.

Verification / Alternative check:
AC analysis or measurements will show phase inversion and |Av| often in the tens, validating “very high voltage gain” relative to unity.

Why Other Options Are Wrong:
“Voltage gain less than 1” describes a source follower (common-drain).

“No voltage gain” is false for CS.

“Very low input impedance” contradicts JFET characteristics.

Common Pitfalls:
Ignoring output resistance ro and bypass capacitor choices, which affect actual gain and bandwidth.

Final Answer:
a high input impedance and a very high voltage gain