Electret Condenser Microphones

Condenser microphones use variations in capacitance to convert an acoustic signal into an electrical signal. The capacitor used is usually of the parallel plate type with the two plates shaped like circles. The capacitance value of such a capacitor can be expressed as

C = ε A / d

where ε is the permitivity of the material between the plates. This is a constant with units of Farads per meter. The variable d represents the distance between the plates and A represents the area of the plates. To be clear, the value of A is not exactly equal to the area of the plates except in the limit as d becomes vanishingly small.

Mechanically, the way the microphone works is that a sound wave, which is a longitudinal pressure wave, impinges upon one of the plates of the capacitor. This forces that plate to move toward the other plate, reducing the distance d between the plates and thus changing the capacitance.

This can be converted into an electric signal because the capacitance is also a function of the charge and the voltage across the plates. We have C = Q / V where Q is the charge and V is the voltage. Solving for V we have V = Q / C and substituting the above equation for C we get

V = Q d / ε A

Everything in this equation can be held constant except for d which varies according to the sound wave. The simplest way to hold Q constant is to put an electret on one of the plates of the capacitor. An electret is the electric charge equivalent of a magnet. It is a dielectric (insulator) that has a permanent charge at its surface. With an electret on one of the plates we have what is called an electret condenser microphone.

The most common electret microphone comes in a small cylindrical capsule with two terminals. Inside, in addition to the capacitor, there is a JFET transistor with the capacitor attached to its gate, as shown in the figure below.

The pin marked 1 in the figure will sometimes be marked with a + on the capsule. The pin marked 2 in the figure will always be connected to the aluminium case of the capsule. The most common way to interface this capsule to a circuit is shown in the figure below.

This is called a common source amplifier and it provides some gain for the voltage signal produced by the capacitor. The amount of gain is determined by the resistance RD and characteristics of the transistor. Most capsule datasheets will not tell you which particular trasistor is used in the capsule but it will tell you the current consumption for a given value of VDD and RD. Note that these values are sometimes labeled VS and RL in the datasheet. VDD is usually in the range of 2 to 10 volts and RD is usually in the range of 1 to 10 kilo ohms.

There are also three terminal electret microphone capsules but these are much less common. They also contain a JFET and the connections are as shown in the figure below.

The pin marked 3 in the figure will be connected to the aluminium case of the capsule. One of the other two pins will usually be wider than the other. That is the pin marked 1 in the figure. The most common way to interface the three pin capsule to a circuit is shown in the figure below.

This is called a source follower circuit and it provides no gain to the capacitor signal. The primary advantage of this circuit is that it reduces distortion when there are very high sound levels and the voltage signal coming from the capacitor is high.