Analytically modeling nonlinear DC-gain for Op-amps

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 1^st stage configurations, including cascode and folded cascode ones. Our analyses show that the smaller V_DD is, the more precise the analytical model becomes. Simulations demonstrate that, when V_DD ≤ 1.8V, the error between the curve from the model and the curve generated from SPICE simulation is less than 2%.