UAM2
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5
First, your audio input is compared to a high frequency reference signal. This
generates two squarewave outputs. These outputs change phase in
response to the audio input.
When the audio input is 0 volts, the two outputs are nearly in phase and the
differential voltage is almost zero most of the time, just little spikes. But, when
the audio input is greater than zero volts, the two outputs are out of phase
and the differential voltage becomes greater, that is, its duty cycle increases.
The voltage is high a greater percentage of the time. This differential voltage
is filtered to produce a sine wave that is sent to the speaker. The magic of all
this is that the circuitry needed to carry out these operations can be digital
because all the waveforms are squarewaves; the voltage is either high or
low, on or off. until they go through the filter. This means that the circuitry
need only have switches to turn the voltage on and off. Switches don’t
consume power so a tiny chip can be used without getting hot. This is in
contrast to other types of amplifiers that have resistive elements in them
which burn up lots of power. They have to have big fat transistors in them
that can handle lots of heat. For more info on class D amps check out the
TPA3001D1 data sheet at:
http://focus.ti.com/lit/ds/symlink/tpa3001d1.pdf
Or a Georgia Tech site:
http://users.ece.gatech.edu/~mleach/ece4435/f01/ClassD2.pdf
audio input = 0 volts
audio input > 0