An Ultralow-Noise Phantom-Powered Microphone Amplifier

Low-noise amplifier design inherently conflicts with low power consumption. This trade-off becomes particularly critical in phantom-powered audio equipment, where the available power is severely limited and must be carefully preserved. An optimized design for achieving ultralow-noise amplifiers with a noise floor below 1 nV/√Hz while operating under phantom power is presented. It is shown that, when the total collector current is limited, paralleling N transistors to reduce amplifier noise is effective only up to a certain number of devices, beyond which increasing N no longer improves noise performance. As an example, a double-balanced microphone amplifier was designed, built, and tested. It exhibits an input-referred noise density of 0.6 nV/√Hz over the 20 Hz–20 kHz bandwidth (−141 dBV or −139 dBu) while consuming less than 8.5 mA. A noise floor level of 0.55 nV/√Hz (−140 dBu) can be achieved by accepting a phantom current of 9.5 mA. A detailed circuit analysis and design equations are provided to adapt the circuit to different requirements and design constraints.