|Created||November 13, 2011|
|Last modified||June 08, 2017|
|Tags||amplifier audio bias-point jfet|
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.
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.
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.
Run the DC simulation and you'll see the bias current running through the microphone, as well as the resulting output voltage.
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.
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.
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.
March 04, 2012
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