OPTIMUM TAPERED BUFFER

Driver stages in MOS circuitry have been extensively investigated during the last decade. Recently a tapering rule for CMOS buffers was derived showing that the tapering factor (beta) is determined by the ratio of output to input capacitance. The derivation fails to account for the correlation between the short-circuit current and beta. As a result, the derived formula consistently overpredicts the value of optimum-beta, especially for large output/input capacitance ratios. We present a modified formula and a method to account for the effect of the short-circuit current that is viable for buffer stages over a wide range of output/input capacitance ratios; this newly derived formula accurately predicts the optimum tapering factors for BiCMOS as well as CMOS buffer chains.