Which condition yields maximum power transfer from a source to a load in an AC circuit?

• A. Load resistance equals source resistance and load reactance equals source reactance
• B. Load resistance equals source resistance and load reactance equals negative of source reactance
• C. Load resistance equals zero and load reactance equals the source reactance
• D. Load resistance equals twice the source resistance and load reactance equals negative of source reactance

Answer: (B)

Maximum power transfer occurs when the load impedance is the complex conjugate of the source impedance. That means the real part of the load equals the source’s real part, and the load’s reactance is the opposite sign of the source’s reactance, so the reactive powers cancel. If the source impedance is Rs + jXs, the ideal load is RL = Rs and XL = -Xs. This makes the total impedance seen by the source purely real (2Rs) and prevents reactive power from circulating, which lets the available power be fully delivered to the load.

Choosing RL = Rs and XL = -Xs produces this conjugate match, delivering the maximum possible real power to the load. Other options either leave a reactive component, short the load, or mismatch the real part, all of which reduce the power actually delivered to the load.