For maximum power transfer, how must the load reactance relate to the source reactance?

For maximum power transfer, how must the load reactance relate to the source reactance?

• A. It must be equal to the source reactance
• B. It must be zero
• C. It must be twice the source reactance
• D. It must be the negative of the source reactance

Answer: (D)

Maximum power transfer occurs when the load presents the complex conjugate of the source impedance, so all reactive charging is canceled and the circuit looks purely resistive to the source. If the source has a reactance X_S, the load must have a reactance X_L = -X_S. This opposite sign cancels the reactance, leaving a real impedance and allowing the maximum possible real power to be delivered (with the resistive parts also matched, R_L = R_S). That’s why the load reactance should be the negative of the source reactance.

Maximum power transfer occurs when the load presents the complex conjugate of the source impedance, so all reactive charging is canceled and the circuit looks purely resistive to the source. If the source has a reactance X_S, the load must have a reactance X_L = -X_S. This opposite sign cancels the reactance, leaving a real impedance and allowing the maximum possible real power to be delivered (with the resistive parts also matched, R_L = R_S). That’s why the load reactance should be the negative of the source reactance.