This work focuses on the subthreshold design of ultra low-voltage low-power operational amplifiers. A well-defined procedure for the systematic design of subthreshold operational amplifiers (op-amps) is introduced. The design of a 0.5-V two-stage Miller-compensated amplifier fabricated with a 0.18-mu m complementary metal-oxide-semiconductor process is presented. The op-amp operates with all transistors in subthreshold region and achieves a DC gain of 70dB and a gain-bandwidth product of 18kHz, dissipating just 75nW. The active area of the chip is approximate to 0.057mm(2). Experimental results demonstrate that well-designed subthreshold op-amps are a very attractive solution to implement sub-1-V energy-efficient applications for modern portable electronic systems. A comparative analysis with low-voltage, low-power op-amp designs available in the literature highlights that subthreshold op-amps designed according to the proposed design procedure achieve a better trade-off among speed, power, and load capacitance. Copyright (c) 2013 John Wiley & Sons, Ltd.