Circuit noise is determined by the thermal noise of external resistors and operational amplifier noise. Operational amplifier noise is described by two parameters: noise voltage and noise current. The total noise is quantified by the equation:

With low source impedance, the current noise term is insignificant and voltage noise dominates the noise performance. At high source impedance, the current noise term becomes the dominant contributor.

Low-noise bipolar operational amplifiers such as the OPA27 and OPA37 provide low voltage noise at the expense of a higher current noise. However, OPAx134 series operational amplifiers provide both low voltage noise and low current noise. This provides optimum noise performance over a wide range of sources, including reactive source impedances; refer to Figure 5-6. Above 2kΩ source resistance, the operational amplifier contributes little additional noise; the voltage and current terms in the total noise equation become insignificant and the source resistance term dominates. Below 2kΩ, operational amplifier voltage noise dominates over the resistor noise, but compares favorably with other audio operational amplifiers such as the OP176.