The OMLmic is a low distortion implementation of an electret microphone. The design uses a simple modification to a common electret microphone capsule to achieve good sound quality at a very low cost. This is particularly useful for high volume applications, such as close-mic'ing acoustic instruments and drums. A discussion of electret microphones is given here, and is a good background for some of the technical details to follow.
Electret microphones have very low pick-up noise, and a good SNR for high signal levels. They also have a wide frequency range which can be relatively flat, depending upon the capsule. This makes them very useful, especially considering their small size. But, their application in this regime (high signal levels) is limited due to their early distortion characteristics. Luckily, there are some easy ways to extend this range.
There are 3 main sources of distortion in an electret microphone application. The first is non-linear flexure of the electret diaphragm. This is impossible to change for a particular element without attenuating the sound waves before they hit the diaphragm. Fortunately, this distortion is usually much smaller than the second contributor, which is distortion in the JFET amplifier within the capsule. Reducing this source of distortion is the main focus of the OMLmic. The last source of distortion is the final amplification stage, which should be negligible in a good mixer or pre-amp.
The OMLmic uses 2 methods for JFET distortion reduction. The first is a common technique of converting a 2 terminal electret module into a 3 terminal module so the internal amplifier can be run as a source-follower (rather than the standard common-source configuration). According to the datasheet for the JFET used inside of an electret microphone (2SK596 - linked below), the input signal level can only go to 10mV before the output distortion reaches 1%. Not only is this a very low sound level, equivalent to someone talking at normal volume at a distance of a few feet, 1% distortion is already a very large level, and far better is achievable.
Cutting a trace on the bottom PCB of the electret capsule allows the JFET source to be pinned out, and a seperate ground lead attached. But, due to the low bias current of the JFET (usually around 200uA), the added benefit is quite minimal. The reason for this, is that in a source-follower configuration, the distortion is due to gate to source voltage (Vgs) variations as the drain current (Id) varies. In an ideal situation, Vgs would remain constant, and distortion would be eliminated. But, since there is no access to the gate terminal within the capsule, it is difficult to exert any control over Id (which can vary by up to 50%), and the resulting Vgs.
The OMLmic's second method overcomes this problem by using an op-amp to monitor Id outside of the microphone capsule, and then provide a feedback signal to keep Id and Vgs essentially constant. The schematic is linked below, and shows a low distortion op-amp (TLC071) measuring the current at the drain, and compensating the source voltage to keep the drain current constant. This reduces JFET distortion to .05%, and can provide moderate gain in the process.
The dynamic range of the OMLmic can be extended by changing a few resistors. Depending upon your needs, this can be done with minimal change to the SNR or THD.
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