General Considerations of Noise in Microphone Preamplifiers

In this paper a study of the noise performance of electret microphone systems as a part of hearing aids is presented. The signalto-noise ratio of the microphone-preamplifier combination, containing a field-effect transistor (FET) and a high value resistive bias element in a hybrid configuration, is mainly determined by the noise generated in the preamplifier circuit. A theoretical analysis of the noise sources in a source follower is given. The dominating noise sources are the channel noise of the FET, the thermal noise of the gate bias element, and finally the noise due to the gate leakage current of the FET and its package. It is shown that for the systems investigated, the noise performance does not depend on the choice of the amplifying device (JFET or MOSFET) itself, but only on its packages. Besides this, it is found that it is necessary to keep the parasitic capacitances as small as possible and to make the resistance of the bias element as large as possible.