Transconductance curve of a JFET: Identify the quantities plotted to obtain the standard JFET “transconductance (transfer) characteristic” curve used in bias design and small-signal modeling.

Introduction / Context:For JFET biasing, the key device curve is the transfer (transconductance) characteristic, which shows how the control voltage VGS sets the drain current ID. Designers use it to select the Q-point and to estimate small-signal parameters.

Given Data / Assumptions:

• JFET operating in its normal region with gate reverse-biased.
• Transfer curve taken at sufficiently high VDS to ensure saturation behavior.
• We seek the standard axes used.

Concept / Approach:The JFET transfer characteristic plots ID as a function of VGS, often following a square-law-like relation between 0 and the pinch-off voltage VP. This is distinct from the output characteristic, which plots ID versus VDS at fixed VGS.

Step-by-Step Solution:Hold VDS large enough for saturation.Sweep VGS and measure ID.Plot ID (vertical axis) versus VGS (horizontal axis) → transfer curve.

Verification / Alternative check:Datasheets show both families: output characteristics (ID–VDS) and transfer characteristics (ID–VGS). The latter is used to derive gm as the slope dID/dVGS.

Why Other Options Are Wrong:Options involving IC/VCE refer to BJTs, not JFETs.

IS versus VDS is not a standard JFET characterization.

ID × RDS is not a device characteristic axis pair.

Common Pitfalls:Confusing the transfer curve with the output characteristic; they serve different purposes in design.

Final Answer:ID versus VGS