Optically Powered Sub-1 dB Noise Figure CMOS LNA for Magnetic Resonance Imaging

This paper presents two low-voltage, sub-1 dB Noise Figure (NF) CMOS Low-Noise-Amplifiers which have been designed to be used as a preamp for an optically powered magnetic resonance imaging (MRI) front-end. The operation frequency is 64 MHz, which is the 1.5 T MRI Larmor Frequency of a hydrogen atom. The design is shown to satisfy the NF requirements at 3T & 7T as well. By utilizing a photodiode as an optical power supply, the design uses 2mA of total bias current and only 0.7V power supply to attain the awaited noise requirements. The first topology utilizes a telescopic cascode structure with transformer-based inductive source degeneration and a common-mode feedback loop. The initial system attains a similar to 0.147 dB noise figure at 64MHz with direct 50 Omega matching. The second topology is an NMOS cascode stage with resistive loads. This topology attains a noise figure of similar to 0.352 dB when optimized for noise matching.