 Even with a hypothetical perfect amplifier, however, noise is unavoidably present due to thermal fluctuations in the signal source (usually assumed to be at room temperature, "T" = 295 K).Such fluctuations cause an electrical noise power of formula_1, where "k"B is the Boltzmann constant and "B" the bandwidth.Correspondingly, the voltage noise of a resistance "R" into an open circuit is formula_2 and the current noise into a short circuit is formula_3.