Comparison of Three Preamplifier Technologies: Variation of Input Impedance and Noise Figure With B0 Field Strength

Introduction Array coil preamplifiers are subjected to large magnetic fields since they are placed close to each coil to minimize SNR losses due to cabling. Noise figure (NF) and input impedance are key parameters that must vary minimally with magnetic field strength and orientation [1,2] to ensure there are no differences in performance between the bench and the bore. In this work we compare the sensitivity to magnetic fields of three amplifiers based on different semiconductor technologies. Methods The following commercially available devices (DUTs) were chosen for comparison (in order of decreasing electron mobility): MGA-53543 (Avago, USA) GaAs pHEMT RFIC; MAR-8A+ (Mini-Circuits, USA) InGaP HBT in Darlington configuration; and BF998 (various vendors) N-channel Si dual-gate MOSFET. An initial baseline noise figure measurement was performed in a radiofrequency (RF) lab in order to obtain a reference zero-field value. The measurement system (Fig. 1, [3]) was then placed outside the 5 gauss field line of a 9.4T magnet (205 mm bore, Magnex, UK). Double-shielded coaxial cables (RG223) 6 meters in length were used to connect the measurement system to the DUT through the magnet’s RF cage. Each amplifier was positioned in three orthogonal orientations within the magnet bore and scattering (S) parameters were measured at 130 MHz at nine B0 field strengths between 0 T and 9.4 T, varying the field as described in [1]. For each amplifier the orientation that produced the greatest change in S-