designing a low noise amplifier

100 1k 10k 100k 1M 10M Os (0) 85455-1 To design a low noise amplifier, it does not suffice to choose a low noise opamp, because the components associatedwiththeopamp, particularly resistors, are themselves sources of noise. The noise in a resistor, which is caused by random movement of electrons, increases by the square root of the increase in resistance. Figure 1 shows a very convenient characteristic for determining opti- mum values of input resistance. The y-axis gives the square of the sum total of noise voltage produced in a circuit (in nV over the bandwidth con- sidered), while the x-axis gives the value of the source resistance. For instance, a noisy opamp like the 741, which produces some 70 nV of noise over its bandwidth, can cope withaninputimpedanceofsome 200 k (higher values would cause the input impedance to generate more noise than the opamp!). On the other hand, the less noisy TCA 520, which generates about 30 nV of noise over its bandwidth, should have an input imp...