Association and Dissociation of Optimal Noise and Input Impedance for Low-Noise Amplifiers

For magnetic resonance imaging (MRI) receive coil arrays, an ideal low-noise amplifier (LNA) is noise matched while exhibiting a high-input reflection coefficient of unity or slightly higher. For this purpose, we present a design approach allowing to manipulate the optimal noise impedance and input impedance. The method is based on noise and S-parameters, hence technology independent. As an example, the method is used to design an LNA for MRI receive coil arrays operating at 32.1 MHz. The design demonstrates the highest coil decoupling published so far of 54 dB. The measured noise figure of 0.44 dB is also better than other published designs. The measured gain is 22 dB with a 1-dB compression point of -14.5 dBm. The power consumption is 81 mW. We expect this method to enable better MRI receive coil array designs resulting in lower examination time and cost due to higher quality images.