  The axion is expected to solve the strong CP problem of quantum
chromodynamics and is one of the leading candidates for dark matter. CAPP in
South Korea has several axion search experiments based on cavity haloscopes in
the frequency range of 1-6 GHz. The main effort focuses on operation of the
experiments with the highest possible sensitivity. It requires maintenance of
the haloscopes at the lowest physical temperature in the range of mK and usage
of low noise components to amplify the weak axion signal. We report development
and operation of low noise amplifiers for 5 haloscope experiments targeting at
different frequency ranges. The amplifiers show noise temperatures approaching
the quantum limit.
