JFET-based electret microphone amplifier PUBLIC

Most electret microphone modules today come with a built-in JFET to help provide a low-impedance output and noise resistance, but these must be biased properly.

Vmic represents the electret element inside a typical electret microphone. J1 represents the built-in n-channel JFET that provides some impedance transformation before the sound signal ever leaves the microphone capsule.

Also see this teardown of a typical electret microphone, as well as this datasheet of a JFET commonly built into microphone modules.

R1 represents an external resistor, which provides both gain (current to voltage), as well as provides the path for the JFET's bias current to flow.

C1 and Rload represent a high-pass filter as one would probably find between the microphone and the input to an ADC, further amplification, etc.

A few things to try in CircuitLab

This circuit highlights several cool features of CircuitLab simulations that you can quickly try. Load this circuit with the "Open in editor" button above, and then click the "Simulate" tab to get started.

DC

Run the DC simulation and you'll see the bias current running through the microphone, as well as the resulting output voltage.

DC Sweep

JFETs can vary over a wide range from part to part. We can model that by sweeping one of the JFET's parameters, V_TO, over a range, and see what kind of effect that has on the resulting circuit bias.

Time

You can plot the input versus output over time, although the Vmic is really a "hidden" signal that isn't exposed directly to the engineer.

Frequency response

If we generate a Bode plot, the only frequency-dependent element we're really modeling here is the high-pass filter, so we'll see the effect of C1 and decide how big we need to make it to pass lower frequency bands downstream into our signal chain.