Op-amp dc-gain is nonlinear, and this can cause distortions in various applications. However, so far no efficient tool is available for analyzing dc-gain nonlinearity; the dc-gain curve can only be generated from a designed op-amp. If the dc-gain nonlinearity is too serious, one has to redesign op-amps. Time-consuming iterations involving transistor-level designs and simulations may be needed before an acceptable result is achieved. In this brief, an analytical op-amp dc-gain curve model is proposed. After entering circuit parameters and process parameters into this model, it can generate op-amp nonlinear dc-gain curves to facilitate subsequent distortion analyses, requiring no transistor- level designs and simulations. This dc-gain model applies to general two-stage op-amps, and can accommodate various 1st stage configurations, including cascode and folded cascode ones. Our analyses show that the smaller VDD is, the more precise the analytical model becomes. Simulations demonstrate that, when VDD ≤ 1.8V, the error between the curve from the model and the curve generated from SPICE simulation is less than 2%.