Microphone

The microphone is a very useful electronic device because it is able to convert sound into an electrical signal. Microphones carry out this transduction of sound to electrical energy by using one of many different methods such as a variable capacitor diagram in an "Electret Condenser" type for which I am using in this project.

An electret microphone's ability to transduce is brought about by having its diaphragm act as one of the plates of a built in capacitor. As the diaphragm vibrates, the inward and outward movement results in a change in the distance between two plates (the other plate is fixed) and, in turn, varies the voltage across the capacitor. The relationship between the distance between the plates, d, and the voltage, V, is shown in the following equation.

By simple inspection, one can see that as the distance between the plates becomes smaller, the voltage across the capacitor goes down and vice versa assuming all other variables are held constant. It is important to note that an electric condensor microphone is technically not the same a condensor type microphone because, for the former, the voltage across the capacitor is not set by an outside supply voltage. Instead, the voltage applied to the electret microphone is used to bias the internal amplifier -- a common-source FET.

For biasing the microphone correctly, I chose the classic method of tieing a 2.2K ohm resistor, from Vdd, to the V+ terminal of the microphone. Output was then taken from the same V+ of the microphone through a coupling capacitor with a value of 10uF. The choice of the microphone's bias configuration was borrowed from Epanorama.net. The resulting circuit is shown below.

The value of 2.2K ohms was chosen by observing that the datasheet of the microphone used for this project indicated that the impedance of the the unit itself is 2.2K ohms. The value of the coupling capacitor was chosen by using the fact that a capacitor in series with the output of the microphone and a load resistance forms a passive high pass filter. By varying the value of the coupling capacitor, the 3dB roll off of the high pass filter can be moved so that its effect on the low end frequency response of the device is mitigated.

Why use a coupling capacitor in the first place? The role of the coupling capacitor in this application is to block the DC portion of the signal and allow only the AC portion through. By doing so, the amplifier that is present in the next stage will not be affected by the microphone's biasing. The choice of the capacitor's dielectric was of no concern during the design of this project and thus an electrolytic was used.