Rainbow Electronics MAX9814 - Manual (Page 7)

MAX9814
Microphone Amplifier with AGC and
Low-Noise Microphone Bias
_______________________________________________________________________________________ 7
MAX9814 AGC DISABLED
400 µs/div
V MICIN
100mV/div
V MICOUT
(AC-COUPLED)
1V/div
MAX9814 fig01a
MAX9814 AGC ENABLED
400 µs/div
V MICIN
100mV/div
V MICOUT
(AC-COUPLED)
1V/div
MAX9814 fig01b
0V
0V
0V
0V
Detailed Description
The MAX9814 is a low-cost, high-quality microphone
amplifier with automatic gain control (AGC) and a low-
noise microphone bias. The MAX9814 consists of sever-
al distinct circuits: a low-noise preamplifier, a variable
gain amplifier (VGA), an output amplifier, a microphone-
bias-voltage generator, and AGC control circuitry.
An internal microphone bias voltage generator pro-
vides a 2V bias that is suitable for most electret con-
denser microphones. The MAX9814 amplifies the input
in three distinct stages. In the first stage, the input is
buffered and amplified through the low-noise preampli-
fier with a gain of 12dB. The second stage consists of
the VGA controlled by the AGC. The VGA/AGC combi-
nation is capable of varying the gain from 20dB to 0dB.
The output amplifier is the final stage in which a fixed
gain of 8dB, 18dB, 20dB is programmed through a sin-
gle tri-level logic input. With no compression from the
AGC, the MAX9814 is capable of providing 40dB,
50dB, or 60dB gain.
Automatic Gain Control (AGC)
A device without AGC experiences clipping at the output
when too much gain is applied to the input. AGC pre-
vents clipping at the output when too much gain is
applied to the input, eliminating output clipping. Figure 1
shows a comparison of an over-gained microphone
input with and without AGC.
The MAX9814’s AGC controls the gain by first detect-
ing that the output voltage has exceeded a preset limit.
The microphone amplifier gain is then reduced with a
selectable time constant to correct for the excessive
output-voltage amplitude. This process is known as the
attack time. When the output signal subsequently low-
ers in amplitude, the gain is held at the reduced state
for a short period before slowly increasing to the nor-
mal value. This process is known as the hold and
release time. The speed at which the amplifiers adjust
to changing input signals is set by the external timing
capacitor C
CT and the voltage applied to A/R. The
AGC threshold can be set by adjusting V TH . Gain
reduction is a function of input signal amplitude with a
maximum AGC attenuation of 20dB. Figure 2 shows the
effect of an input burst exceeding the preset limit, out-
put attack, hold and release times.
If the attack and release times are configured to
respond too fast, audible artifacts often described as
“pumping” or “breathing” can occur as the gain is
rapidly adjusted to follow the dynamics of the signal.
For best results, adjust the time constant of the AGC to
accommodate the source material. For applications in
which music CDs are the main audio source, a 160µs
attack time with an 80ms release time is recommend-
ed. Music applications typically require a shorter
release time than voice or movie content.
Figure 1. Microphone Input with and Without AGC