The commercial availability of field effect transistors (FET) made it possible to design a transistorized amplifier for a condenser microphone which takes advantage of the FET's high input impedance and low noise combined with gain stability vs temperature changes. The correlation between circuit constants and the noise generated in the FET and in the next stage is explored by means of a suitable manipulation of the equivalent circuit representation of the preamplifier. Experimental results show that the insertion of a fairly large resistor between the source and the gate of the FET results in a good signal-to-noise ratio.
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