Bruce Trump, Texas Instruments
EDN
The electret microphone capsule is similar to those commonly used in telephones, cassette recorders, and computers. The element functions as a capacitor with a fixed trapped charge. Sound pressure moves a diaphragm, producing variations in the capacitance. This action produces an ac-output voltage with an extremely high source impedance. A FET inside the capsule uses an external-resistor drain load (Figure 1). R1 and R2 provide an appropriate load impedance and voltage from the 10 V supply. The basic performance of this simple capsule is excellent, but it requires further signal processing to conform to professional phantom-powered-microphone standards.
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| Figure 1. | This microphone system derives its power from the receiving-end circuitry through the leads that carry the audio signal. |
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The output of a phantom-powered microphone is a low-impedance differential signal. IC1 is a simple voltage buffer that provides low-impedance drive for one output. IC2 is a unity-gain inverter that derives its drive from the output of IC1. Bias for the noninverting input of IC2 comes from a heavily filtered output of IC1. We selected the dual op-amp IC1/IC2 for its low noise and low distortion properties. R6 and R7 provide immunity from long-line capacitance, RF interference, and transients that occur when you "hot- plug" the microphone into a live phantom-power source. The amplifier outputs use ac coupling, C2 and C3, to the microphone's output terminals to block the dc phantom-power voltage on the audio lines. Differential-output voltage capability is limited to approximately 2 V p-p because of the limited power supply available to drive the op-amp output currents. This level is adequate, because it corresponds to an extraordinary sound level beyond the linear range of the capsule.
Phantom-powered microphones derive power for their active circuitry from the receiving-end circuit through the same leads that transmit the audio signal. The 48 V phantom-power supply couples through two 6.8-kΩ resistors, R10 and R11, to both signal lines. This coupling allows the microphone's low output impedance to drive a differential ac signal on the relatively "soft" impedance of the phantom supply voltage. In the microphone, power comes from the signal lines through resistors R8 and R9. Zener diode D1 regulates the voltage. These resistors also provide a soft impedance on the balanced line, allowing the outputs of IC1 and IC2 to inject their differential ac-output signal. You can locate the microphone hundreds of feet from the receiving-end phantom power and amplifier and still obtain excellent performance.
The receiving-end amplifier, IC3, is a low-noise instrumentation amplifier with three internal op amps. Its configuration and laser-trimmed resistors provide excellent CMR (common-mode-rejection) properties. The high CMR rejects noise and power-line hum that appear equally in both signal lines. Low noise (1 nV/√Hz), though unnecessary for high-output microphones such as those described here, is necessary in professional- audio equipment to accommodate the use of low-output ribbon and dynamic microphones. These microphone types are strictly passive electromechanical generators and do not require a power source. Phantom power earns its name from the fact that these microphone types "float" at 48 V without harm. The electret capsules are available in various sizes and physical configurations. They include both omnidirectional and directional (cardioid) types. Directional capsules have a vent in the rear; you must mount them with free access to both the front and the back to obtain proper characteristics.
