  We develop Johnson noise thermometry applicable to mesoscopic devices with
variable source impedance with high bandwidth for fast data acquisition. By
implementing differential noise measurement and two-stage impedance matching,
we demonstrate noise measurement in the frequency range 120-250 MHz with a wide
sample resistance range 30 {\Omega}-100 k{\Omega} tuned by gate voltages and
temperature. We employ high-frequency, single-ended low noise amplifiers
maintained at a constant cryogenic temperature in order to maintain the desired
low noise temperature. We achieve thermometer calibration with temperature
precision up to 650 mK on a 10 K background with 30 s of averaging. Using this
differential noise thermometry technique, we measure thermal conductivity on a
bilayer graphene sample spanning the metallic and semiconducting regimes in a
wide resistance range, and we compare it to the electrical conductivity.
