Can I assume that the signal from an electret microphone capsule is inverted due to common source configuration of jfet?
I would like to have non-inverted signal on my sound's card line input. Then I pick up inverting preamplifier project. What about microphone input. Does it have inverting op-amp?
As others have stated, in general, in many situations not only does not matter as absolute phase is impossible to guarantee with audio over more than a short distance.
However, if you you wish to combine the output with other microphones then it is necessary to ensure that they all have the same polarity signal with a pressure wavefront.
This would be used for example in beam-forming or to locate a source of sound. Most cell-phones use multiple microphones to aid in noise cancellation to reject background noise.
Especially at low frequencies multiple microphones can be coherent over several feet.
Although the FET amplifying stage does invert the signal it also depends upon how the elecctret microphone capsule is designed and connected. If the capsule gives out a negative going signal for a positive pressure change the output from the combination will be positive going.
The information you need might be in the datasheet.
I would like to have non-inverted signal on my sound's card line input.
You are asking for an impossibility. The wavelength at 1 kHz is 340 mm hence, if your microphone is 170 mm from your speaker and the sound is 1 kHz, the signal received will be inverted. In fact it will be inverted every repeated 340 mm from the initial half wavelength of 170 mm. It will be in-phase every 340 mm from the speaker. That's 1 kHz and, for a complex audio signal the shape of the microphone signal will not at all look like the original signal fed to the loudspeaker.
You appear to be on a quest of disappointment.
I would like to have non-inverted signal on my sound's card line input.
If you are using a microphone to pick up this signal then.... it's not going to happen. Be realistic and think about it.
Then of course you have to remember that as the air warms the wavelength changes. Wavelength calculator. Speed of sound changes with air pressure too so, wavelength will also change with weather conditions.
It inverts.
Applying voltage (properly) to a JFET game makes it conduct less unlike most other devices available. With no gate-source voltage it conducts maximally. On an N-channel JFET, the gate should be more negative than the source. That means the gate is biased negative.
So with a positive signal relative to the bias, the gate-source voltage gets less negative and the JFET conducts more. That causes more current to flow through the resistor which causes a larger voltage drop at the output away from +V which makes the output more negative.
With a negative signal relative to the bias, the gate source voltage gets more negative and the JFET conduct less. That causes less current to flow through the resistor which causes voltage drop at the output that is more towards +V which makes the output more positive.
Microphones, if configured correctly, register a positive pressure wave as a positive voltage (transducer moves in.) Likewise, a positive speaker drive pushes a positive wave toward the audience (voice coil moves out.) More about that here: https://www.sweetwater.com/insync/polarity-does-it-really-matter/
With that out of the way, you’re good to go with the circuit as-is. Positive pressure wave is positive output. The details of whether the JFET inverts or not aren’t important (it does, kinda: the varying capacitance of the electret cell causes displacement current at the JFET gate.)
Note also that your sound card probably has a mic input that injects a bias (called phantom power) to power the electret capsule, so you only need two wires to the capsule. The load resistor and DC blocking cap that inject and isolate phantom power are already on the sound card. All the other discussion about sound polarity aside, you do need to ensure the correct polarity for phantom power, otherwise the mic won’t work at all.
So, what actually happens at the JFET gate?
The electret structure consists of a permanently-charged capacitor in series with a variable capacitor. The moving part can be either the electret or the opposing plate. Works the same either way: the assembly has a fixed charge \$Q = CV\$. It would be modeled as a fixed voltage source (the pre-charged electret) in series with the variable capacitor.
The capsule JFET, with nothing connected to the gate, will self-bias with the gate close to ground via the miniscule gate leakage. This puts the JFET in the conducting region, as the pinch-off (threshold) for a depletion-mode n-JFET is below the source / drain voltage (pinch-off will be about -1V for the popular 2SK170 device), but still below the forward diode conduction threshold of 700mV or so.
With the electret connected between JFET source and gate, the electret assembly will also bias to about 0V across it via JFET gate leakage. It would be the same as just connecting a capacitor from drain to gate.
Now, here's the magic. When a positive pressure sound wave hits the diaphragm, it will push the electret assembly's opposing capacitor plates together. You may recall that the closer the plates are to each other, the higher the capacitance. This can be expressed more formally as follows:
where \$C\$ is the capacitance, \$V\$ the bias voltage, \$A\$ the plate area, \$\epsilon\$ is the dielectric constant, and - most relevant here - \$d\$ the separation of the plates.
What happens? Plates closer together, capacitance gets bigger, voltage decreases to maintain the constant charge. How?
See that equation above? The distance \$d\$ is in the denominator. Smaller \$d\$, bigger capacitance. Meanwhile, the electret's two terminals connected between the JFET source and gate are essentially open circuit. To maintain a constant charge Q across both the charged electret and the varying C, the voltage must change. So, the V becomes smaller.
The JFET gate-source voltage will see its voltage decrease with a positive-pressure impulse. Likewise, a negative pressure will pull the plates apart, and you will see an increase in gate voltage.
If you were to probe the gate with a special high-impedance probe you would see an inverted waveform vs. the instantaneous sound pressure impinging on the diaphragm. The JFET will invert this, producing a waveform that follows the sound pulse: positive pressure is more positive, negative pressure is less positive.
So the n-JFET buffered electret mic polarity is same as a dynamic mic: diaphragm moving in, positive-going voltage.
