Stiff source rule-of-thumb (1% regulation criterion): a 15 V source has internal resistance Rs = 0.2 Ω. Which load resistance is the smallest that still qualifies the source as “stiff” under the 1% rule (Rs ≤ 0.01 * RL)?

Introduction / Context:
A voltage source is considered “stiff” if its terminal voltage changes very little when the load varies. A common engineering rule uses a ratio between source resistance and load resistance to quickly judge stiffness and expected regulation without a full analysis.

Given Data / Assumptions:

• Ideal source voltage Vs = 15 V (magnitude does not affect the ratio test).
• Internal (source) resistance Rs = 0.2 Ω.
• Stiffness rule-of-thumb: Rs ≤ 0.01 * RL (approximately 1% regulation).

Concept / Approach:
Using the voltage divider model, Vout = Vs * RL / (Rs + RL). If Rs is much smaller than RL, Vout ≈ Vs with minimal drop. The 1% rule requires Rs to be at most 1% of RL, i.e., RL ≥ 100 * Rs. For a given Rs, we compute the minimum RL that satisfies the criterion and select that from the options.

Step-by-Step Solution:

Verification / Alternative check:
Compute regulation at RL = 20 Ω: Vout = 15 * 20 / (0.2 + 20) ≈ 15 * 20 / 20.2 ≈ 14.85 V (about 1% drop), confirming stiffness at the threshold.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring that the question asks for the minimum qualifying value; mixing absolute numbers with ratio-based criteria; forgetting that Vs magnitude does not change the ratio test.

Final Answer:
20 Ohm.