Output impedance (Zout) is the internal impedance of a power supply or signal generator as seen from its output terminals. It is a critical parameter that determines how the output voltage changes in response to variations in the load current. Output impedance is generally expressed by the following relationship:
where Δ V represents the change in output voltage due to load current variations and Δ I represents the change in load current. Lower output impedance results in minimal output voltage changes despite load fluctuations, enabling stable voltage delivery.
High-quality regulated power supplies typically have an output impedance of just a few milliohms (mΩ), which ensures the output voltage remains virtually constant even with significant load variations. This stability is essential for high-precision electronic circuits and experimental equipment. Devices with a low Zout generally offer superior performance but are often more expensive due to their more complex design.
Output impedance is closely related to load regulation, which is a measure of a power supply's ability to maintain a constant output voltage under varying load conditions. A lower Zout results in better load regulation. Conversely, a high Zout causes significant output voltage fluctuations as the load changes, which can compromise system stability.
When selecting a high-quality power supply or signal source, it is essential to carefully review both the output impedance and load regulation specifications. Additionally, for high-frequency applications, impedance matching between the source's output impedance and the load's impedance is critical to ensure maximum power transfer and prevent signal reflections, thereby optimizing performance.