Bias analysis – E-MOSFET drain voltage at Q-point: For VDD = 30 V and a drain resistor RD = 8 kΩ, find the drain voltage at the Q point if the drain current is ID = 3 mA (assume source at ground and neglect channel-length modulation).

Introduction / Context:
Finding node voltages at a MOSFET operating point is a basic step in biasing and verifying headroom for signal swing. With a known drain current and resistor, the drain voltage follows directly from Ohm’s law.

Given Data / Assumptions:

• VDD = 30 V.
• RD = 8 kΩ.
• ID = 3 mA.
• Source at ground; ignore channel-length modulation so ID is set by the bias network.

Concept / Approach:
The drain resistor drops Vdrop = ID * RD. The drain node voltage is then VD = VDD − Vdrop. This is a straight substitution problem.

Step-by-Step Solution:
Compute drop: Vdrop = 0.003 A * 8000 Ω = 24 V.Compute node: VD = 30 V − 24 V = 6 V.Conclusion: Drain sits at approximately 6 V.

Verification / Alternative check:
Check power in RD: P = ID^2 * RD = (0.003)^2 * 8000 ≈ 0.072 W → a 1/4 W resistor is adequate with margin.

Why Other Options Are Wrong:
10 V and 24 V result from arithmetic errors.

30 V would require zero current (no drop), not the stated 3 mA.

0 V is impossible with positive VDD and finite drop.

Common Pitfalls:
Forgetting to subtract the drop from VDD or mixing units (kΩ vs Ω) leads to wrong drain voltages.

Final Answer:
6 V