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The frequency-domain passivity theory offers an effective way to assess the stability of inverters in a complex grid. In this article, a unified impedance model, suitable for either inverter current control (ICC) or grid current control (GCC) of LCL-type grid-connected inverters with observer-based capacitor current feedback active damping, is built to facilitate the passivity-based stability assessment and controller parameter design. With the passivity analysis, it is found that when the antiresonant frequency of LCL filter is in certain ranges, i.e., (0.056 $\omega _s$ , 0.20 $\omega _s$) for ICC and (0.046 $\omega _s$ , 0.23 $\omega _s$) for GCC, all frequencies’ passive output admittance of the inverter can be achieved via proposed parameter design guidelines. Due to the utilization of the observer and all frequencies’ passive output admittance property, not only extra current sensors for active damping can be saved, but also the inverter can be connected and stably operated in a grid regardless of the grid impedance. The validity of the theoretical analysis and effectiveness of the proposed method are verified by using experimental results on a laboratory prototype. [ABSTRACT FROM AUTHOR]