  We describe a microwave amplifier based on the Superconducting Low-inductance
Undulatory Galvanometer (SLUG). The SLUG is embedded in a microstrip resonator,
and the signal current is injected directly into the device loop. Measurements
at 30 mK show gains of 25 dB at 3 GHz and 15 dB at 9 GHz. Amplifier performance
is well described by a simple numerical model based on the Josephson junction
phase dynamics. We expect optimized devices based on high critical current
junctions to achieve gain greater than 15 dB, bandwidth of several hundred MHz,
and added noise of order one quantum in the frequency range of 5-10 GHz.

Superconductivity;Quantum Physics