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, 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 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.