[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Sat Jun 12 15:11:43 PDT 2021

Where does that come from?  The lowest noise at the output of an amplifier results from the source impedance being equal to or less than the "optimum" for a particular amplifier - where e-noise and I-noise are equal contributors to total noise. This is determined by the first-stage device and its operating point.  The amplifier's input impedance is irrelevant.  For example, a transistor with a collector current of 1 mA may have equal e and i noise with a 500 Ω source impedance, but the input impedance of the stage could be 100 kΩ!  In most amplifiers, an intentional resistor is added only to lower input impedance for mic loading purposes 
See Motchenbacher & Fitchen, "Low Noise Electronic System Design" page 62 and others.
Bill Whitlock

-----Original Message (in part)-----
From: Scott Dorsey via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com; chris at chriscaudle.org; 6807.chris at pop.powweb.com
Sent: Sat, Jun 12, 2021 2:42 pm
Subject: Re: [ProAudio] Microphones question

In a perfect world the lowest noise would be when the input impedance of the
preamp matches the output impedance of the microphone, ....

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