# Signal amplitude of electret microphone

Discussion in 'Electronic Basics' started by Rune D. Jørgensen, Mar 6, 2006.

1. ### Rune D. JørgensenGuest

What kind of signal amplitude can I expect from an electret microphone?

A google search on "CZ034D" yielded a datacheet:
http://akamai.globalsources.com.edgesuite.net/f/593/3445/5d/pdt.static.glob
alsources.com/IMAGES/PDT/SPEC/272/K1000674272.pdf

At http://www.epanorama.net/circuits/microphone_powering.html
I found a basic circuit for powering the mike. If I use an R1 that is
higher than 2.2K, what will happen? Am I right in assuming that the only
difference will be a higher 3dB frequency of the mike?

Basic circuit

+---------------------------- battery +ve (3 to 12 Volts)
|
2k2 R1
|
o---------- 10uF ------o----- output
|+ |
CAPSULE 10k R2
|- |
+----------------------o----- GND, and battery -ve

2. ### Walter HarleyGuest

The electret mic needs a certain amount of current to power its internal
circuitry. If R1 is too high, it will not be able to pass enough current.
The datasheet for a particular element will tell you how much current it
needs.

3. ### Anthony FremontGuest

microphone?

I haven't done much playing with them yet, but it looks like mV are what
is to be expected. From looking around lately it looks like a wide
variation exists.
The impedance will go up with the value of the resistor.
I have one that I've only just toyed with that uses a 10k resistor and
I'm using a 5V supply.

4. ### tlbsGuest

Anywhere from 3 to 10 millivolts per Pascal of sound-pressure-level,
depending on the particular electret element. This is assuming you
bias the microphone with the normal operating current according to the
manufacturer (usually 2 to 5 volts at around 500 microamps), using a
circuit similar to the one shown in your post.

5. ### Rune D. JørgensenGuest

Det var smuk og solrig dag da tlbs skrev
i
sci.electronics.basics:
The datasheet specifies max 500 microamps, and I have measured the DC
resistance to 1.8K ohm. My power supply is 12V, so I need an R1 of at 22.2K
ohm, to get a 500 microAmp bias current. Am I right? I'm in doubt because
it's so far from the circuit I found.

Another question.
Sensitivity (0dB=1V/pa, 1KHz)
-40dB for my specific mike. Which I translate into 10mV/pa. I have no idea
what the sound pressure level is. Does anyone know how many pascal you
would get in a motorcycle helmet? Or just in an ordinary noisy environment.

+---------------------------- battery +ve (3 to 12 Volts)
|
2k2 R1
|
o---------- 10uF ------o----- output
|+ |
CAPSULE 10k R2
|- |
+----------------------o----- GND, and battery -ve

6. ### tlbsGuest

Rune D. Jørgensen wrote
0.1 Pascal equals 1 dynes/cm^2

0.0002 dyne/cm^2 is defined the threshold of human hearing, or 0 dB_spl

That microphone is good for 100 mV / dyne / cm^2 (at 1 kHz), so at 0
dB_spl it will produce 20 microvolts.

At 80 dB_spl (a relatively noisy environment), it should produce 200
mV.

At 100 dB_spl (a very loud environment), it should produce 2 V, but now
you have to make sure the circuit has enough "headroom" in the power
supply, the bias and the load.

I would change the 2k2 resistor to at least 22k. In most electret
circuits I have seen, the bias resistor is at least twice as large
(47k). The load resistance is probably OK, just make sure that the
load resistance of the amplifier is taken into account, also.

Tom

7. ### ehsjrGuest

Great! In general, keep the current through the electret
to 1 mA or less. You can always depart from that general
figure when you have a datasheet for the element. The 10 uF
is normally polarized - you want its (+) side connected
to the capsule in your diagram.

Next: It turns out that it doesn't much matter what amplitude
the electret provides. All electrets produce signals that must
be amplified in order to do anything worthwhile with them.
The output from the electret is fed into an amplifying stage
(often called a pre-amp). You get the level you need for whatever
circuit you are driving from the pre-amp. (Sometimes the pre-amp
is part of the circuit you are driving.) There is no need to
control the signal level coming out of the element itself -
controlling the amplitude in the pre-amp is all that is required.
http://www.epanorama.net/circuits/micamp.html

For a kit that has everything you need, including an electret
element, get # 3378 RB from MPJA
http://www.mpja.com/productview.asp?product=3378+RB

(You may need to put 3378 in the "stock no. direct" box,
The kit includes a pre-amp and an amp that delivers 1.5
watts output, all on one small PC board, and costs \$10.95.

Ed

8. ### BanGuest

1 Pascal are 93.97dB SPL. this is quite high, but should be in the ballpark
if your mike is close. With 12V you should split the required resistor of
22k into 2 parts and use first an 18k into a grounded cap(10u) and then the
recommended or a bit bigger load resistor like 4k7 or 6k8. You do not need
R2, it will only dampen the signal.

9. ### Phil AllisonGuest

"tlbs"

** WRONG.

At 0.1 Pascal the mic outputs only 1 mV !!

You *multiplied* by 10 when you should have divided.

** Also WRONG .

The calculated output is actually 0.2 microvolts @ 0 dB SPL.

However, self generated electrical noise will be WAY higher than that
burying any output signal.

** WRONG again - it should be 2 mV.

** WRONG again - it will be 200mV @ 100 dB SPL.

........... Phil

10. ### Phil AllisonGuest

"Phil Allison"

Correction:

** WRONG again - it will be 20mV @ 100 dB SPL.

........ Phil

11. ### Rune D. JørgensenGuest

Ban wrote
in sci.electronics.basics:

Is this it? Wouldn't that still give a bias current of 12V/(6k2+1k8)=1.5mA?

+---------------------------- battery +ve (3 to 12 Volts)
|
6k2 R1
R3 |
+-18K---o---------- 10uF ------o----- output
| |+ |
10uF CAPSULE 10k R2
| |- |
+-------+----------------------o----- GND, and battery -ve

12. ### BanGuest

No, here it is. the bias current is always 0.5mA, as the output behaves more
like a current source. the recommended voltage across the Mic is usually
less than 5V, so your circuit might damage certain mics.

.---o
| +12V
.-.
| |
| |18k
'-'
|
o------o---------. ||15p
| |+ | .---||---.
.-. === | | || |
| |4k7 /-\ 47u | | ___ |
| | | | o-|___|--o
'-' === | | 100k|
| GND \] | | |\ | 47u
o-----------|]---)-o--|-\ | +[/ out
_ | /]+ | | >--o----[|--o-----o
| / \--' 10u '----|+/ [\ |
|(Mic) |/ .-.
| \_/-- | |
| 100k| |
=== '-'
GND |
===
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

13. ### Rune D. JørgensenGuest

Det var smuk og solrig dag da Ban skrev
i sci.electronics.basics:
Thanks. I have tried your circuit and it seems to work.