JFET small-signal parameter: The ratio of a small change in output drain current to a small change in input gate-to-source voltage (at a defined operating point) is called:

Introduction / Context:
Designers characterize FET amplifiers using small-signal parameters measured about a quiescent point. For JFETs, one key figure is how effectively gate-to-source voltage variations control drain current.

Given Data / Assumptions:

• We consider small-signal incremental behavior around the Q-point.
• Output quantity is drain current ID; input control is VGS.
• Linearization applies for small perturbations.

Concept / Approach:
Transconductance, symbol gm, is defined as gm = dID/dVGS at the operating point, with units of siemens (A/V). “Siemens” is the unit, not the parameter. “Gain” is generic; “resistivity” is a material property.

Step-by-Step Solution:
Define parameter: gm = change in ID / change in VGS.Units check: amperes per volt → siemens.Apply in design: voltage gain of a common-source stage ≈ −gm * RD (simplified).

Verification / Alternative check:
Datasheets specify gm versus ID; bench measurements can extract gm from small AC sweeps around the bias point.

Why Other Options Are Wrong:
“Siemens” is the unit, not the parameter name.

“Resistivity” is unrelated to the device small-signal transfer.

“Gain” is ambiguous without specifying transconductance.

Common Pitfalls:
Confusing transconductance with output conductance (dID/dVDS) or with static ratios like ID/VGS; gm is a derivative at the bias point.

Final Answer:
transconductance