Amplifying the human voice
presents some tough challenges.
One of the toughest of
these is providing enough
amplification to make sure a
soft-spoken person can be
heard while allowing sufficient
headroom for people who
speak loudly. If the amplification
is too high, a loud voice
or noise causes clipping of the
audio waveform, which makes
the output signal unintelligible
and harsh to the ears.
One solution to this dilemma
is to limit the signal with a
nonlinear transfer function,
which is implemented by
adding a few components to a
standard application circuit
(Fig. 1). When a positive or
negative peak of the audio
input exceeds the turn-on voltage
of one of the back-to-back
diodes (D1), the diode conducts
and attenuates any further
increase in the signal
amplitude that’s presented to
the MAX9700A class D audio power amplifier. Resistors R1
and R2 add series impedance
that prevents excessive loading
of the audio source.
During normal operation, the
incoming audio signal sees little
attenuation by the diode network,
and the high input
impedance of U1 prevents
attenuation due to the higher
source impedance. As input
amplitudes increase, the transfer
function shifts (Fig. 2).
As you can see, the output
tracks the input for low-level signals.
Above input levels of
about 0.5 V rms, there’s a
decrease in circuit gain. Gain
compression is independent of
the peak output voltage, as
illustrated by the responses for
various supply voltages. Note
that the clamped 3.3- and 5-V
inputs have the same response
and are therefore superimposed.
Also, the 2.5- and 3.3-V
inputs show compression due to
swing limits, even though no
clamp is present.
JOHN GUY, an applications manager,
received a BSEE from San Jose State
University, Calif
