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. R1 can be 470ohms or so.

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.

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.