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Early Voltage Effect

The reduction in the effective base-emitter voltage due to the change in collector-to-base voltage (VCBV_{CB}), modeled by the Early voltage VAV_A. This effect limits the output resistance of BJT stages.
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The statement of the theorem

The Early voltage effect describes the finite output resistance of a BJT, limiting the collector current ICI_C as a function of VCBV_{CB}. The effective output resistance ror_o is determined by the Early voltage VAV_A and the DC current gain β\beta: \nro≈VAICβr_o \approx \frac{V_A}{I_{C} \beta} \nMathematically, the collector current ICI_C is modified from the ideal constant current assumption by the term (1+VDCVA)−1\left( 1 + \frac{V_{DC}}{V_A} \right)^{-1}, where VDCV_{DC} is the collector-base voltage VCBV_{CB}.