What determines an electret microphone voltage?

Question

I am trying to get an electret microphone working with my Raspberry Pi Pico's ADC pin. I am trying to detect the output by passing it through a transistor.

I have a 4.5V voltage source. I split this with a voltage divider to create a 3V input for the microphone (labelled V1 on the diagram.) I then pass this point as the base of a transistor.

I set a resistor at the transistor's emitter to fix the current over the transistor. I placed a resistor at the transistor's collector which - with the current provided by the transistor - should set a voltage at the collector.

The ADC is connected to the collector.

The circuit diagram is here:

The circuit values are:

• R1=2.2k
• R2=1k
• R3=100
• R4=220

The transistor is NPN. The emitter is connected to ground over R4.

Electret microphone datasheet.

A few things are happening which seem to indicate there's something Im misunderstanding:

1. The voltage powering the microphone is about 0.75V. This is different to the voltage divider's output when it is independent.
2. The output at the transistor's collector is 4.5V (source voltage.) That's not what I expected. Itcould be for the same reason as (1.)

I've tried varying all the resistors, but that didn't change (1) or (2).

I'm not sure why the electret is altering the voltage divider provided voltage from 3V to 0.75V so that leads me to the title question:

What sets the voltage drop over an electret microphone?

Input on any other issue with my design, with an explanation, would be appreciated.

Note on capacitor at Vb:

I have seen multiple diagrams online showing a capacitor to remove the DC offset at Vb. I don't have a capacitor to use currently, and to my reasoning - as I am setting the ADC voltage based on the current through the transistor, which is determined by the change in voltage at Vb, this should be independent of the DC offset at Vb.

Again, if there seems to be an issue with my reasoning here, please do comment on it.

Answer 1

Some electret microphone capsules come with a preamplifier, and some don't. If your capsule does not come with a pre-amplifier, you should add one with a circuit like this

^{simulate this circuit – Schematic created using CircuitLab}

Then, either using your electret mic with built in pre-amp, or with added preamp, add the following components.

^{simulate this circuit}

Connect the node \$Audio\$ to the base of your transistor in your amplifier, and connect \$AudioGND\$ to the ground of your amplifier. Your final circuit should look something like this

^{simulate this circuit}

Answer 2

You should use the circuit on the mic datasheet but increase its 1k resistor to 4.7k which will quadruple the tiny level of the output signal.

Your transistor has signal loss instead of gain so the signal output level will be very low. its voltage gain should be 100 times or more.

Answer 3

Your transistor has no emitter and you have specified none of the resistors you are using.

An electret capsule has a nominal current that it will draw when the minimal operating voltage has been reached: that is due to the internal FET operating in transconductance mode where the output current more or less just depends on the gate voltage.

Typical are 0.5mA of operating current and a specified minimum voltage somewhere between 1.1V and 2V. With larger voltages, the current will be just about the same, with smaller voltages, the FET gets into non-linear regions where the gate voltage increasingly determines conductance instead of current.

The capsule tends to operate down to very small voltages; it just delivers small signals then and has non-linear response.

Since signal and operating voltage are shared, you'd put a low LSR capacitor (22uF or so) at the point where you write VI and connect that smoothed voltage source then with a 2.2kOhm resistor (typical load/feed for electret capsules) to Vb.

You probably don't need R1: no electret condenser capsule I know would mind 4.5V. R2 can be 470ohms or so if it is just used for keeping the charge on the capacitor providing the smoothed voltage supply for the capsule. If it is lower, you get a higher operating voltage for the capsule but you may want to increase the capacitor then to keep voltage fluctuations on the DC rail from becoming audible.

There are better single-transistor amplifiers for electret microphones and I would actually check whether you don't already get sufficient signal by not using a transistor at all but directly going into VADC from Vb.