The superconducting quantum interference device microstrip amplifier: Computer models

Computer models are presented for a microstrip amplifier based on a de supe rconducting quantum interference device (SQUID). In this device, the signal is applied between one end of the spiral input coil and the square washer on which it is deposited. The amplifier exhibits substantial power gain whe n the signal frequency is such that a half wavelength is approximately equa l to the length of the microstrip formed by the coil and the groundplane. T he resonant frequency is lowered significantly by the inductance of the squ are washer transformed into the input coil; this reduction is consistent wi th predictions of a simple model and with analog simulations. With the wash er grounded, the gain of the amplifier peaks at a frequency that is lowered from the unloaded resonant frequency by the damping of the resistance asso ciated with the source. The position and magnitude of the peak are in good agreement with both a lumped circuit model and with a model representing th e microstrip as a transmission line. When the counter electrode of the SQUI D is grounded and the washer floats, feedback from the output of the SQUID to the input via the capacitance of the microstrip plays a major role and i s well described by simulations using the transmission line model. Measurem ents of the input impedance of the microstrip amplifier show that the retur n loss can be positive or negative, depending on the sign of the feedback a nd whether the frequency is above or below the resonant frequency. This beh avior is in good accord with simulations. (C) 2000 American Institute of Ph ysics. [S0021-8979 (00)06523-3].