Microstrip superconducting quantum interference device radio-frequency amplifier: Effects of negative feedback on input impedance

We present the results of measurements of the scattering parameters of microstrip amplifiers (MSAs) based on the dc superconducting quantum interference device. The amplifier input impedance is poorly matched to typical transmission line impedances, resulting in high input return loss around -2 dB. We show that negative feedback can lower the MSA input impedance to achieve a robust match to 50 Omega. In the presence of capacitive and inductive feedback, the input return loss of the MSA can be reduced below -10 dB, opening the door to the practical use of the MSA for a variety of demanding applications.