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 significantly constrained, more than any other performance parameter, by the exclusion of a complimentary active load. The PSRR for an inherently 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 represents a “gain” constraint imposed by the coincidence of signal and bias paths in bipolar circuits which employ current differencing or bootstrapping techniques. Base current errors can significantly degrade PSRR. Two all-npn circuits are presented with PSRR ⩽0.1 mV/V that exhibit a single operating point under most circumstances