Microphone assembly with P-type preamplifier input stage

Abstract

A microphone assembly is provided that includes a condenser transducer element having a displaceable diaphragm and a back-plate. The displaceable diaphragm and the back-plate are arranged to form a capacitor in combination. A preamplifier circuit has an input stage, the input stage comprising a P-type field effect transistor. The displaceable diaphragm and the back-plate are operatively connected between a source input and a gate input of the P-type field effect transistor.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims priority to, and hereby incorporates by reference, U.S. Provisional Application No. 60 / 696,910, entitled “Microphone Assembly With P-Type Preamplifier Input Stage,” filed Jul. 6, 2005 with the United States Patent and Trademark Office.FIELD OF THE INVENTION[0002]The present invention relates to a microphone assembly comprising a condenser transducer element having a diaphragm, a back-plate and a preamplifier circuit that has an input stage with a P-type field effect transistor. The diaphragm and back-plate are operatively connected between the source input of the P-type field effect transistor and the gate input of the P-type field effect transistor, so that input-referred noise is low and noise induced from the supply line is significantly attenuated as improved power supply rejection is obtained.BACKGROUND OF THE INVENTION[0003]Various microphone assemblies in the art disclose how a diaphragm and a back-plate of ...

AI Technical Summary

Benefits of technology

[0010]There are many advantages afforded by embodiments of the present invention. For example, electronic input referred noise of the preamplifier may be minimized by using a P-type field effect input transistor and by improving power supply noise rejection of the microphone assembly. Another advantage is the reduction of light induced noise in certain silicon microphone assemblies. Experimental results indicate a noise reduction in the order of 20-30 dB has been achieved.

[0014]The condenser transducer element may include an electret transducer element type comprising an electrically pre-charged layer of material providing a build-in or permanent electrical field between the diaphragm and the back-plate. The permanent electrical field may be provided by an electrically pre-charged layer, such as a Teflon coating with implanted electrical charges, arranged on either the diaphragm or back-plate. The condenser transducer element may alternatively be of the type requiring an external high impedance bias voltage source for generating an electrical field between the diaphragm and the back-plate. Such an external high impedance bias voltage source may comprise a Dickson voltage pump followed by a smoothing type of filter, such as a low pass filter. The external high impedance bias voltage source is preferably arranged inside a common housing with the condenser transducer element to avoid EMI problems that could be associated with long leads between the bias voltage source and the condenser transducer element.

[0018]Preferably, the condenser transducer element is a silicon-based condenser transducer element with an external DC bias voltage source. Silicon-based condenser transducer elements, where the diaphragm or the back-plate is directly exposed to the environment, tend to be sensitive to light exposure in that electronic noise is superimposed onto the output signal from such transducers. The origin of this light induced noise is believed to be due to the semiconductor properties and thereby the semiconductor behavior of silicon. However, by grounding or virtually grounding the diaphragm in transducer elements having the diaphragm physically facing the environment and where the diaphragm essentially overlaps the back-plate area, the electrically conductive diaphragm will act as an EMI shield so that problems relating to light-induced noise in silicon-based transducers can be significantly reduced.

Problems solved by technology

A disadvantage of this coupling or electrical interface is that noise applied or injected at the source input is amplified because ground acts as a signal reference terminal.