A 1.2-V pseudo-differential OTA with common-mode feedforward in 65-NM CMOS

In this work, we describe the implementation of a 1.2-V pseudo-differential operational transconductance amplifier (OTA) with common-mode feedforward (CMFF) and inherent common-mode feedback (CMFB) in a 65-nm, digital CMOS process. The OTA architecture provides an inherent CMFB when cascaded OTA structures are utilized and this work has studied a cascaded amplifier consisting of four stages. Due to the low-gain using core 65-nm circuit devices, the overall gain must be distributed on all four stages to acquire a gain of more than 60 dB, while maintaining a -3-dB bandwidth of 200 MHz. To achieve high gain, we propose using a modified, positive-feedback, cross-coupled input differential stage. The modified OTA achieves a high output swing of ±0.85 V due to only two stacked transistors, 88 dB DC gain and a third-order harmonic of -60 dB for 800 mV_pp at 30 MHz. Further on, in a capacitive buffer configuration, we achieve a high slew rate of 1240 V/μs, -3-dB bandwidth of 509 MHz, signal-to-noise ratio of 63 dB while consuming 10.4 mW power.