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The noise figure of a traveling-wave amplifier has been derived as a function of circuit loss and space charge. The minimum-obtainable noise figure of the backward-wave amplifier is shown to be the same as the minimum-obtainable noise figure of the forward-wave amplifier, i.e., about 6 db. The noise figure of an ordinary backward-wave amplifier has been measured as a function of gain. The calculated noise figure checks well with the measured values.