lowoutputimpedance

Low output impedance refers to the electrical property of a device’s output where the Thevenin equivalent impedance seen looking back into the output is small. The output impedance, Zout, is defined as the small-signal resistance seen at the output when all independent sources within the device are deactivated. A low Zout means the output voltage remains relatively constant as the load draws different currents.

Low output impedance is desirable because it enables stable voltage delivery to loads that vary in impedance or draw varying current. It helps maintain signal fidelity, improves damping of connected transducers such as loudspeakers or headphones, and reduces the sensitivity of the output to supply or load changes. It also broadens the usable bandwidth of a stage by minimizing the voltage drop and phase shift caused by loading.

Achieving low output impedance typically involves buffering or feedback techniques. Common methods include emitter or source follower stages, push-pull output stages, and other buffer configurations. In amplifiers and operational amplifiers, negative feedback is a key mechanism for reducing Zout, often by several orders of magnitude compared with the open-loop impedance. The exact low-impedance value depends on the device and operating conditions; audio power amplifiers might exhibit a few ohms or less, while op-amps in closed-loop configurations can have output impedances in the milliohm to tens of milliohms range.

Zout is frequency dependent, increasing at higher frequencies due to parasitic elements. Measurement involves small-signal testing with a load and observing the change in output voltage per injected current, typically under prescribed operating conditions.