0.9-V class-AB Miller OTA in 0.35-μm CMOS with threshold-lowered non-tailed differential pair

This paper presents a CMOS operational transconductance amplifier (OTA), suitable for sub-1-V supply applications, whose (input) common-mode voltage can be set to (VDD+VSS)/2 thanks to two combined techniques applied to the differential pair, namely, threshold voltage lowering and elimination of the tail current generator. Both techniques are implemented through a single common-mode feedback loop, which embeds the shared bulk terminal of the pair. In contrast to other low-voltage approaches employing bulk driving, the proposed OTA is driven from the gate terminals and exploits only MOS transistors in strong inversion. Therefore, effective values of dc gain, gain bandwidth, and noise are found, suitable for high-accuracy switched-capacitor applications. Using a standard 0.35-μm technology with nominal MOS transistors threshold around 0.7 V, a 0.9-V OTA with 0.45-V analog ground was designed and successfully tested. The measured gain and unity gain frequency were 65 dB and 1 MHz with phase margin of 60° for a capacitive load of 10 pF.