9.2.2 Phantom Power in Microphone Preamplifier Circuit

+

–

0.1 μF

0.1 μF

6.2 k

6.2 k

IN+

IN

48-V

GND

RG

RG

REF

INA849

47 µF

47 µF

47 k

3 k

3 k

GND

R1

R2

R3

C3

C4

R4

R5

C5

C6

R7

2 k

R6

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 to provide 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 needs to be

powered. C3 and C4 are therefore used as blocking capacitors to protect the INA849. In a fault scenario when

the input connections are shorted a large surge current discharges the dc blocking capacitor through the shottky

diodes. In case of a 48-V phantom power the surge current exceed 4 A for a short duration of time. It is

recommended to use shottky diodes that are specified for at least 10 A surge current. Additional series

resistance with the dc blocking capacitor limits the surge current but needs to be traded off as 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. To

make sure that the thermal noise of these resistors does not dominate, use the lowest possible value resistors.

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.

INA849

SBOS945A – NOVEMBER 2020 – REVISED DECEMBER 2020

www.ti.com

24

Submit Document Feedback

Copyright © 2020 Texas Instruments Incorporated

Product Folder Links: INA849