On the instrumentation for current noise measurements in low impedance devices

The physical interpretation of the noise generated by an electron device is more easily obtained in terms of current noise. Direct current noise measurements, however, are typically performed only on high impedance devices, employing FET input operational amplifiers for the realization of the low noise transimpedance amplifiers coupled to the device under test. The relatively large value of the equivalent noise voltage in FET input operational amplifiers limits the sensitivity of current noise measurement in the case of low impedance devices. Employing BJT input operational amplifiers might allow to reduce the equivalent input noise voltage, but at the cost of an unacceptably high level of current noise. However, if a cross-correlation approach for current noise measurement is employed, the contribution from the equivalent input current noise of the operational amplifiers can be, in principle, eliminated thus allowing to reach very low level of background noise also in the case of low impedance devices. On the other hand, the rejection of the uncorrelated noise requires extended measurement time. In this paper, after reviewing the main factors affecting the balance between sensitivity and measurement time in the case of cross correlation current noise measurements, we propose an effective methodology for guiding the selection of the cross correlation front end components so that very high sensitivity can be reached also in the case of devices under test characterized by low impedances.