Biasing methods for JFETs: Which JFET biasing scheme specifically requires a separate negative gate supply to establish the operating point (for an n-channel device)?

Introduction / Context:
Biasing a JFET can be done in several ways: gate-bias (fixed bias), self-bias using a source resistor, voltage-divider bias, or feedback arrangements. Knowing which approach needs a negative gate supply (for n-channel devices) is key when selecting a topology compatible with available rails.

Given Data / Assumptions:

• n-channel JFET requiring VGS < 0 to set the quiescent current.
• Single-supply versus dual-supply design considerations.
• Idealized components for conceptual clarity.

Concept / Approach:

Gate-bias (fixed-bias) sets the gate at a fixed negative potential relative to the source, typically using a dedicated negative supply VGG. Self-bias uses a source resistor to raise VS and naturally make VGS negative without a negative supply. Voltage-divider bias can reference the gate to a mid-rail point to achieve the desired VGS using one supply.

Step-by-Step Solution:

Verification / Alternative check:

Typical textbook schematics show VGG negative for n-channel JFET fixed-bias. SPICE simulations confirm identical operating points can be reached via self-bias without a negative supply by choosing an appropriate source resistor.

Why Other Options Are Wrong:

• Source and voltage-divider bias: designed to avoid a negative rail, using resistor networks.
• Feedback bias: often leverages drain feedback and source resistor, again avoiding a negative supply.
• Current-source self-bias: similarly avoids a separate negative rail.

Common Pitfalls:

• Assuming all JFET biases need dual supplies; many single-supply options exist.
• Confusing MOSFET threshold behavior with JFET depletion-mode bias needs.

Final Answer:

gate