An Isolated Frequency Compensation Technique for Ultra-Low-Power Low-Noise Two-Stage OTAs

This brief presents a novel isolated frequency compensation (IFC) technique for ultra-low-power (ULP) low-noise two-stage operational transconductance amplifiers (OTAs) with high gain-bandwidth (GBW) products. The IFC technique stabilizes the OTA in two steps. First, it pushes the first non-dominant pole of the OTA to higher frequencies using an isolated Miller capacitor. Second, it employs a local feed-forward path to create a zero and nullify the first non-dominant pole through pole-zero cancellation. This allows the compensated OTA to reach higher GBW for a given bias current compared to conventional frequency-compensation methods. Fabricated in a standard 180-nm CMOS technology, the proposed circuit operates in sub-threshold and achieves 3 MHz of GBW and 2.3 V/
$\mu s$
of average slew rate for a 45 pF capacitive load, while consuming a total current of
$1.8 ~\mu \text{A}$
from a 1.8-V supply. In addition, the proposed circuit achieves a measured common-mode rejection ratio (CMRR) of ≥ 70 dB for a bandwidth of up to 100 kHz.