MAX9814 Microphone Amplifier

The MAX9814 is a low cost, high-performance microphone amplifier with automatic gain control (AGC) and low noise microphone bias. The device features a low-noise front-end amplifier, a variable gain amplifier (VGA), an output amplifier, a microphone bias voltage generator, and an AGC control circuit.

The low-noise preamplifier has a fixed 12dB gain, while the VGA gain automatically adjusts from 20dB to 0dB, depending on the output voltage and the AGC threshold. The output amplifier offers selectable gains of 8dB, 18dB, and 28dB. With no compression, the cascade of the amplifiers results in an overall gain of 40dB, 50dB, or 60dB. A tri-level digital input programs the output amplifier gain.

An external resistive divider controls the AGC threshold and a single capacitor program the attack/release times. A tri-level digital input programs the ratio of attack-to-release time. The hold time of the AGC is fixed at 30ms. The low-noise microphone-bias-voltage generator can bias most electret microphones.

You can also change the Attack/Release ratio, from the default 1:4000 to 1:2000 or 1:500. The output from the amp is about 2Vpp max on a 1.25V DC bias, so it can be easily used with any Analog/Digital converter that is up to 3.3V input. If you want to pipe it into a Line Input, just use a 1-100uF blocking capacitor in series (100uF sounds best).

Features:​

• Automatic Gain Control (AGC)
• programmable action time
• programmable action and release time ratio
• Low-power shutdown mode
• Built-in 2 Volt low noise microphone bias

Specifications:

• 3 kinds of gain settings (40dB, 50dB, 60dB)
• Supply Voltage: 2.7v – 5.5v @ 3mA current
• Output: 2Vpp on 1.25V bias
• Frequency Response: 20Hz – 20 KHz
• Low Input-Referred Noise Density of 30nV
• Low THD: 0.04% (typical)

Application:

• Speech Recognition
• Bluetooth Headsets
• Two-Way Communicators
• High-Quality Portable Recorders
• IP Phones/Telephone Conferencing