Low-noise FET input amplifier for applications in the audio and subaudio frequency ranges

A low-noise FET input amplifier is described, which has been designed to be used in all physical measurements where high internal resistance sources are employed and exceptional noise performances of the amplifier, from a few hertz to some hundred kilohertz, are required. The noise figure of the amplifier is less than 0.5 dB at 10 Hz for source resistances ranging from 30 kΩ to 10 MΩ. The midband input noise voltage spectral density is 2.2 nV/(Hz) and rises to 7.3 nV/(Hz) at 10 Hz with a lower corner frequency of 2 kHz. The amplifier is stabilized against variations in supply voltage and temperature. A short analysis of the general noise characteristics of transistor linear amplifiers precedes the description of the low-noise amplifier.