"Quantifying Resistor Noise with a Low-Noise Amplifier"
[Limpkin] designed a low-noise amplifier and signal buffer to measure the noise produced by a 50 Ohm resistor, which is the impedance most commonly seen on the inputs and outputs of RF systems. The amplifier cranks up the noise signal by a factor of 100, using ultra-low noise op amps running off a pair of nine-volt batteries. A clipper circuit was added to the amplifier to prevent any stray DC voltage present at its input from damaging the analyzer’s sensitive input port. The end result was about 0.89 nV, just as predicted by theory.