Microphones

OPENAI

Tried to ask openai about condenser mic

Very good reply you can find below in the src folder list

Src folder list: here

Condenser Mic (kondensator mikrofon)

aka electret mic

cheap
simple
simple electronics

about kuc mics

Schematics

DYI

From komponenten

A schematics

NB: for diagram below

DESCRIPTION.
The OPA344 and OPA345 series rail-to-rail CMOS operational amplifiers
are designed for
    precision
    low-power, miniature applications.

Design told on blackboard

A cartoon

The microphone capsule

A condenser microphone is a charge preloaded capacitor with a JFET sensing the voltage above the capacitor.

The moving diaphragm is made of PTFE or teflon and hold a charge (like static electricity).
The preset Vgs (by a static charge on th PTFE diagraphm) is -1.0 V to 0 V - depending on manufactor
The static/DC current Id is normally 50uF to 250 uF - dpending on manufactor
- A little bit further down on this page we try to measure it
The diagrapm capacitor sensing JFET has an imput impedance of 10^11 Ohm or more so no off charging takes place.
Operating voltage is normally 2-10 V (including voltage over Rl. (see datasheets)
Rl (load resistor) is normally 1-30 kOhjm - depending on your and manufactor
Microphone capsules are normally 6 or 9.7 mm in diameter. Bigger diameter - higher sensitivity

All these figures are quite fluffy

Some JFET figure (2SK209) datasheet

The diaphragm as a capacitor

As the formula below states
the fixed (backside) of the capacitor must have no electric connectivity to the outer tube (inolation)

The diagraphm as a microphone

Sound waves make the very thin PTFE membrame vibrate (moving up and down)
So the distance between the capacitor plates (d) will vary and there the capacitans is also varying.
Movement (delta d) is very very small compared to d

From capacitans and charge to voltage

Standard realtion between, V(voltage above C) , Q(charge on C) and C gives V = Q/C
Q (charge) is constant.
So V and Q is invers liear related
and C is related to sound presssure so sound pressure is related to V

The JFET

We dont have a real datasheet for the JFET
We need to find the proper operating area for the JFET
As seen below Vds must be above ohmic regino and below breakdown region
Id must be above cut off region

A simple test for identifying operating area

Data sheet set a max for Vcc at 10 V
9.7 mm mic says safe Vcc is 4.5V
6 mm mic says safe Vcc is 2V

We do the experiment with the 9.7 mm mic
And varying Rl within 0.2 kOhm to 28 kOhm
Measure voltage over Rl
Calculate Id
Safe area is the area where Id do not change much

Measurements on 9.7 mm Electret_condenser_microphone_schematic

It seems that Rl values in range to 2 to 20 kOhm is ok
You can observe that Id is quite stable
REMEMBER:
- Rl change the current variations (delta Vgs * gm) to a voltage.
- So by selecting a Rl value in high end gives higher signal voltage.
- So given that variation i Id is +- x mA then using Rl = 20kOhm compared to 2 kOhm gives approx 10 times higher signal voltage.
Trade-off is that Rout - which is basicly RL - will be ten times higher. This means you should select Rinput in the amplifying stage to maybe 5-20 times higher - or accept lower amplification due to voltage split between the Ro of condenser stage and Ri of the amplyfying stage.

A used design method is to choose Rl so Vo DC wise is a half of Vs(supply voltage): This will to some extent ensure symmectric clipping in cas of overload - and there for higher dynamic capabilities.

There is no free meals.

The bottom right ID/Vds picture as normally see it in educational litteratur

spreadsheet: electrecmeas1.xlsx

Microphones - electret / condenser microphone

An electret microphone is a microphone whose diaphragm forms a capacitor (historically-termed a condenser) that incorporates an electret. Imagine static elctricity on a teflon/plastic alike membrane. It will maintain the charge on the membrane and therefore the voltage.
The electret's permanent electric dipole provides a constant charge Q on the capacitor.
Sound waves move the diaphragm(vibrating), changing the capacitance C, which produces a corresponding voltage change across the capacitor of ΔV = ⁠Q/ΔC⁠.
In the case of the microphone, since the diaphragm is moving back and forth, the shape of the capacitor is changing, and its capacitance changes accordingly.
The equation for a parallel plate capacitor is C=e * A/t, where e is a material constant representing the properties of the material between the plates, A is the area of the plates, and t is the separation between the plates.
As the electret material moves due to sound pressure variations, t becomes larger and smaller, and the voltage varies linearly with this distance since V = Q/C = Q(e*At) = Q*t/e*A. fejl i indtastning ???
The electret's constant charge eliminates the need for the polarizing power supply required for non-electret condenser microphones, though a preamplifier is typically incorporated to boost the audio voltage signal.

You can find some simple test here

from or inspired by

The electret (the capacitor plate) maintains a fixed charge, and therefore maintains a voltage across the capacitor.
The mathematical equation for the voltage on a capacitor is V=Q/C, where Q is the charge on the capacitor, and C is the capacitance.