Figure 9-7 Phantom Power in Microphone Preamplifier Circuit

Figure 9-7 shows a typical application circuit for a microphone input amplifier used to generate phantom power. Phantom power is a technique that provides power and the audio signal using the same signal path.

R1 and R2 connected to the 48-V supply define the current path in the case when the microphone must be powered. Therefore, C3 and C4 are used as blocking capacitors to protect the INA849. When the input connections are shorted In a fault scenario, a large surge current discharges the dc blocking capacitor through the Shottky diodes. For 48-V phantom power, the surge current exceeds 4 A for a short duration of time. Make sure to use Shottky diodes that are specified for at least a 10-A surge current. Additional series resistance with the dc blocking capacitor limits the surge current, but must be traded off because these add noise to the circuit.

One of the key criteria for high-performance microphones is to enable an optimum source impedance throughout the audible frequency range. The exceptional ultra-low noise performance of the INA849 permits direct input without the need for a transformer.

R4 and R5 in parallel with R1 and R2 provide the bias current path for the INA849. The input bias current (maximum of 20 nA) provides a dc differential input voltage that reflects as an voltage error on the output. Use the lowest possible value resistors to make sure that the thermal noise of these resistors does not dominate.

The mismatch of the input ac-coupling capacitors (C3 and C4) can reduce the common-mode rejection ratio significantly at low frequencies. An additional resistor (R6) connected to both of the bias resistors (R4 and R5) can mitigate this effect.

Use the TINA TI™ simulation software for a detailed analysis.