We present measurements of an amplifier operating at 3.8 GHz with 150 MHz of
bandwidth based on the microstrip input-coil resonance of a dc superconducting
quantum interference device (SQUID) with submicron Josephson junctions. The
noise temperature is measured using two methods: comparing the signal-to-noise
ratio of the system with and without the SQUID in the amplifier chain, and
using a modified Y-factor technique where calibrated narrowband noise is mixed
up to the SQUID amplifier operating frequency. With the SQUID cooled to 0.35 K
we observe a minimum system noise temperature of 0.55 ± 0.13 K, dominated
by the contribution from the SQUID amplifier