Microphone amplifier with AGC and low-noise microphone bias

The MAX9814 is a low-cost, high-quality microphone amplifier with automatic gain control (AGC) and low-noise microphone bias. The device features a low-noise preamplifier, variable gain amplifier (VGA), output amplifier, microphone-bias-voltage generator and AGC control circuitry.

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 trilevel digital input programs the output amplifier gain. An external resistive divider controls the AGC threshold and a single capacitor programs the attack/release times. A trilevel 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.

The MAX9814 is available in the space-saving, 14-pin TDFN package. This device is specified over the -40°C to +85°C extended temperature range.

Key Features
 • Automatic Gain Control (AGC)
 • Three Gain Settings (40dB, 50dB, 60dB)
 • Programmable Attack Time
 • Programmable Attack and Release Ratio
 • 2.7V to 5.5V Supply Voltage Range
 • Low THD: 0.04% (typ)
 • Low-Power Shutdown Mode
 • Internal Low-Noise Microphone Bias, 2V
 • Available in the Space-Saving, 14-Pin TDFN (3mm x 3mm) Package
 • -40°C to +85°C Extended Temperature Range