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Abstract

An audio frequency amplification circuit for use with low level, high impedance signal sources, such as a miniature electret microphone, particularly useful where the power supply range is variable or may be limited to a single cell. The preferred embodiments of the circuit include a field effect transistor preamplifier stage operating as a voltage follower which is biased by a first and second impedance network coupled to the gate electrode of the FET transistor. A first impedance network is coupled between the power source terminals to provide a floating bias point. A second, high impedance network comprising (in one embodiment) a pair of parallel, reverse oriented diodes and a high impedance resistor is coupled between the bias point and the gate terminal of the transistor. In a further embodiment, the first impedance network comprises series connected high impedance resistors, and the second impedance network includes only the parallel, reverse oriented diodes. In either embodiment, extremely low capacitance elements may be coupled across the high impedance resistors, the elements preferably comprising stray capacitance from circuit conductor layout. Such source-follower biasing networks provides high input impedance and extended dynamic range without introducing noise or distortion. The audio frequency amplification circuit is preferably used as the preamplifier stage of a miniature electret microphone of extremely low source capacitance and mounted within the microphone in close proximity to the electret. Such a miniaturized, wide dynamic range microphone is especially adapted for use mounted on acoustic or electric musical instruments.

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First Claim