Biasing a depletion-mode MOSFET (D-MOSFET): What is the very simple bias method, often used because the device conducts at VGS = 0 V?

Introduction / Context:
D-MOSFETs, like JFETs, are depletion-mode devices with a conducting channel at zero gate bias. This lets designers use extremely simple biasing in some applications, particularly as variable-resistance elements or for constant-current configurations with minimal parts.

Given Data / Assumptions:

• D-MOSFET with channel present at VGS = 0 V.
• Small-signal or low-power usage where drift and tolerance are acceptable.
• Temperature effects ignored for the conceptual answer.

Concept / Approach:

Because the device naturally conducts at VGS = 0 V, one can simply tie the gate to the source (or otherwise ensure VGS ≈ 0) to obtain a baseline operating current. More elaborate schemes (self-bias, voltage-divider) add resistors to stabilize the operating point, but the simplest is often called zero-biasing.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer application notes show D-MOSFETs used as two-terminal current regulators or variable resistors with VGS around 0 V, demonstrating zero-bias practicality.

Why Other Options Are Wrong:

• Self biasing and voltage-divider biasing: valid, but not the “very simple” method exploiting VGS = 0 conduction.
• Gate biasing/fixed negative-gate bias: require an additional supply or network, defeating the simplicity goal.

Common Pitfalls:

• Expecting identical current across parts; zero-bias current varies with device IDSS and temperature.
• Using zero-bias in precision circuits without feedback or trimming.

Final Answer:

zero biasing