What is Output Resistance: Understanding Input-Output Impedance

In the world of electronics and electrical systems, understanding the concept of output resistance is crucial for comprehending how devices interact and exchange signals. Exploring the realms of input and output resistance, impedance matching, and its context under Thevenin’s theorem. How these elements impact the efficiency and performance of electronic systems.

What is Output Resistance?

Output resistance refers to the resistance seen by a source when it delivers a signal to a load. In simpler terms, it’s the resistance that the source device exhibits when an external load is connected to it. This resistance affects how effectively the source can transfer power to the load.

Difference between Input and Output Resistance

Input Resistance

Every electronic system possesses input and output resistances. Input resistance is the resistance offered by the input terminals of a system when a signal source is connected to it. It determines how much current is drawn from the signal source.

Output Resistance

Output resistance, as mentioned earlier, characterizes the resistance offered by the output terminals of a system when a load is connected. It influences how much voltage will drop when connected to a load.

Input-Output Impedance and Thevenin Theorem

The Thevenin theorem is a fundamental concept in circuit analysis that simplifies complex networks into an equivalent circuit comprising a single voltage source and series resistance. This theorem is invaluable in understanding input-output impedance relationships.

As per the Thevenin equivalents, the output impedance of a source becomes crucial. It is the internal resistance of the source model as a Thevenin equivalent circuit when viewed from the load terminals.

Source and Load Impedance Matching

Impedance matching plays a pivotal role in maximizing power transfer between source and load. When the output impedance of a source matches the input impedance of the load, it results in maximum power transfer efficiency. This matching minimizes signal loss and distortion.

However, mismatching impedances can lead to signal reflections, reduced power transfer, and distortion in the transferred signal. For instance, if the source impedance is much higher than the load impedance, it can cause signal loss due to internal resistance.

Effect of Mismatched Resistance

Mismatched resistances between source and load can lead to various adverse effects. Significant reflections can occur, resulting in signal distortion and loss. This phenomenon becomes more pronounced at higher frequencies and in transmission lines.

When the output resistance is much higher than the input resistance of the load, the voltage drop across the output resistance can significantly affect the voltage received by the load. This voltage division causes inefficiency in power transfer and affects signal integrity.

Mismatched resistance can also cause standing waves, leading to impedance mismatches at different points along the transmission line, further deteriorating signal quality.


Understanding output resistance, input-output impedance, and impedance matching is vital in optimizing the performance of electronic systems. The Thevenin theorem provides a valuable tool for simplifying complex circuits to their equivalent forms, aiding in analyzing and designing efficient systems.

Efficient power transfer, reduced signal distortion, and improved signal integrity can be achieved through proper impedance matching between source and load. Mismatched resistances can significantly impact signal quality and efficiency, emphasizing the importance of impedance matching in electronic systems.

In conclusion, mastering the intricacies of output resistance, input-output impedance, and impedance matching empowers engineers to design and optimize electronic systems for maximum efficiency and performance.


How do you find the output resistance?

The output resistance of a circuit can be found by using Thevenin’s theorem, experimental measurements, analytical analysis, or circuit simulation tools like SPICE.

What is the output resistance of a power supply?

For most practical power supplies, the output resistance is relatively low but not zero just like an ideal power supply. It might vary depending on the type of power supply, its design, and the components used.

What is the best output impedance?

The output impedance should be around ten times lower than the input impedance which it will feed to in audio electronics ensuring minimal impact on system performance and optimal signal transfer.

Which power supply has lower output resistance?

Typically, linear power supplies tend to have lower output resistance compared to switching-mode power supplies.

What happens if there is an impedance mismatch?

An impedance mismatch in a circuit or transmission line causes signal reflection back to the source, leading to reduced power transfer to the load.


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.