FET: How to understand noise specification?

I was reading an article about electret microphones and there was a link to a datasheet of the FET which was used for impedance matching. It is a popular Fet which is used in many electret microphones.

Here is the link to the datasheet:

There is a noise specification which I am trying to understand. Here is what is written there:

Output noise voltage Vno (Vin-0, A curve) -110 dB

What does that mean? dB is specified as a ratio between two values.

I am working with a high-gain amplifier and trying to evaluate the noise level in the voltages the FET generates. How can I recalculate that ratio into voltage? What does "A curve" mean?

Thanks.

I would read that datasheet as -110dB(V) noise output for that FET connected in the specified test circuit. That is, -110dB below 1V rms, or 3uV rms (A weighted), for Vin=0V.

"Specified test circuit" is given on page 1 of that datasheet, "Output noise voltage" near top of page 2, and V(NO) vs I(DSS) graph at bottom of page 3.

This graph shows how the noise voltage varies as I(DSS) for a FET connected as specified. This I(DSS) variation is a function of the threshold voltage variation during FET manufacture.

NOTE that connecting the FET differently will produce different results : a higher load resistance will increase the gain but also increase the output noise voltage.

There is no reference to the mic capsule in this datasheet; it may contribute additional noise.

• I didn't know that -110dB means -110dB(V). It is relative to the 1V. How do you figured that out? Nov 29 '21 at 21:43
• Because the units of output voltage are volts, and there is no further annotation to "dB". If they had quoted a noise power, there would be an annotation such as "dBm" which is dB referred to 1 mW. Nov 29 '21 at 21:51
• Thanks a lot for a descriptive answer!!! Dec 1 '21 at 18:49

@user_1818839's answer is correct and exhaustive. The visitors to this page with little or no experience in audio engineering may find useful an explanation of an A weighted qualifier of the phrase 3uV rms (A weighted), for Vin=0V.

To measure the noise of an elecret preamp, the input is shorted. A spectrum analyser is used to measure rms voltage giving results in nV per root herts. The squared results are convolved with the so-called A-weighting curve (A curve) over the frequency range of human hearing, giving a parameter (VNO)2 that measures in square volts. A-weighting is a standard curve defined in the International standard IEC 61672:2003 and used to modify the microphone-recorded sound pressure levels to more closely match the perception of the human ear.

The ratio VNO/1V expressed in dB is an output noise voltage of the datasheet.