ERROR AMPLIFIER LIMITATIONS IN LOW-VOLTAGE REGULATORS USING ONLY NPN TRANSISTORS

The performance of low-voltage bandgap regulators built with only npn transistors is examined. The power supply rejection ratio (PSRR) is si gnificantly constrained, more than any other performance parameter, by the exclusion of a complimentary active load. The PSRR for an inheren tly stable (single operating point) bandgap regulator built using only npn transistors is bounded below; by 2.5(nV(T)/V-cc)(n) = 0.7 mV/V or 0.04 mV/V for circuits with n = 2 or 3 stages, respectively, This rep resents a ''gain'' constraint imposed by the coincidence of signal and bias paths in bipolar circuits which employ current differencing or b ootstrapping techniques. Base current errors can significantly degrade PSRR. Two all npn circuits are presented with PSRR less than or equal to 0.1 mV/V that exhibit a single operating point under most circumst ances.