A 87 dB, 2.3 GHz, SiGe BiCMOS operational transconductance amplifier

This paper describes a 3.3 V, fully differential SiGe BiCMOS operational transconductance amplifier (OTA) for switched capacitor applications. The simulated DC gain of the OTA is 87 dB and its unity gain frequency is 2.3 GHz at a phase margin of 63 degrees. The OTA is optimally compensated for fast settling using a 0.6 pF capacitor, dissipates 17 mW power, has a differential output swing of 2.4 V and the differential input-referred noise PSD is 2.7 nV/√Hz. This OTA settles to 0.01% accuracy within 4.3 ns when configured in a closed loop with feedback factor β = 1/2 . The above specifications will allow realization of a 12-bit, 115 MS/s gain-of-2 sample and hold amplifier for use at the front end of a pipeline ADC. This design uses a BiCMOS technology that provides a 47 GHz f_t SiGe HBTs and 250 nm CMOS. Results are based on circuit level SpectreS simulations in Cadence, with foundry provided transistor models.