Maxim's 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 12 dB gain, while the VGA gain automatically adjusts from 20 dB to 0 dB, depending on the output voltage and the AGC threshold. The output amplifier offers selectable gains of 8 dB, 18 dB, and 28 dB. With no compression, the cascade of the amplifiers results in an overall gain of 40 dB, 50 dB, or 60 dB. 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 30 ms. The low-noise microphone-bias-voltage generator can bias most electret microphones.