How is Thevenin resistance R_th defined?

How is Thevenin resistance R_th defined?

• A. The resistance of the load in the circuit.
• B. The resistance of the source.
• C. The resistance seen from the output terminals with independent sources deactivated, ignoring dependent sources.
• D. The resistance seen from the output terminals with independent sources deactivated and dependent sources left active.

Answer: (D)

Thevenin resistance is the effective impedance the circuit presents when you look into its output terminals and connect a load. To find it, turn off all independent sources (replace voltage sources with shorts and current sources with opens) but keep any dependent sources active, because their behavior depends on circuit variables and can change the impedance seen by the load. Then determine the resistance seen from the terminals, typically by applying a test source and measuring the resulting voltage or current. This approach includes the influence of dependent sources, which is essential for circuits that contain them. If there are no dependent sources, you can simply use the network with sources off to get R_th. The other ideas—resistance of the load or the source itself—don’t capture what the Thevenin resistance represents.

Thevenin resistance is the effective impedance the circuit presents when you look into its output terminals and connect a load. To find it, turn off all independent sources (replace voltage sources with shorts and current sources with opens) but keep any dependent sources active, because their behavior depends on circuit variables and can change the impedance seen by the load. Then determine the resistance seen from the terminals, typically by applying a test source and measuring the resulting voltage or current. This approach includes the influence of dependent sources, which is essential for circuits that contain them. If there are no dependent sources, you can simply use the network with sources off to get R_th. The other ideas—resistance of the load or the source itself—don’t capture what the Thevenin resistance represents.