IEEE Solid-State Circuits Magazine - Summer 2017 - 24

0

Audio Precision

PSRR

-20
-40
(dB)

-60

96.8 dB

-80
-100
-120
-140
-160
100

120

140

160

180 200 220
Frequency (Hz)

240

260

280

300

FIGURE 28: Measured PSRR.

100
90
80
Efficiency (%)

70
60
50
40
30
20
10
0

0

0.2

0.4

0.6
0.8
1
1.2
Output Power (W)

1.4

1.6

1.8

FIGURE 29: Measured class D efficiency.

Electrical Field Strength (dBµV/m)

80
70
60
50
40

EN55022 Class B Envelope

30
20
10
0
30

100
Frequency (MHz)

FIGURE 30: Measured radiated emission.

24

15 dB

SU M M E R 2 0 17

IEEE SOLID-STATE CIRCUITS MAGAZINE

1,000

inductor and the 8-Ω resistor are
used to emulate the 8-Ω loudspeaker.
The design supports battery voltages
from 2.5 to 5.5 V. Figure 27 shows a
plot of the A-weighted THD+N with
a 1-kHz input signal using audio
precision equipment. The signalto-noise ratio (SNR) is calculated by
adding 60 dB to the absolute value of
the THD+N at -60 dBFS. The class D
amplifier achieves over 105 dB SNR
when driving an 8-Ω loudspeaker.
The minimum THD+N is 0.004%. At
-40 dB THD+N, the output power
corresponds to 1.75 W. The power
supply rejection is measured with a
400 mVPP disturbance on the battery
supply and a -60 dBFS input signal.
Figure 28 shows the measured
spectrum of both the supply and the
class D amplifier output. A PSR of
96.8 dB is achieved at the GSM time
division multiple access frequency of
217 Hz. The class D efficiency versus
output power is shown in Figure 29.
A peak efficiency of 95% is achieved
at a 1.75-W power output. The radiated electromagnetic emission was
measured with a commercial smartphone that uses this class  D amplifier. The measured class D radiation
is shown in Figure 30. Edge-rate control enables the system to meet the is
European emission standard EN55022
class B standard with a 15-dB margin. The EN55022 class B standard
is more stringent compared to North
American standards. The class D amplifier pop-and-click noise level was
measured with an audio precision
analyzer, with A-weighted filtering
applied to emphasize the frequency
range over which the human ear is
sensitive. For a class D amplifier with
a 4-V achievable output voltage level,
the amplifier pop-click-noise level was
reduced from 27 mV, shown at the
top of Figure 31 with pop-and-click
suppression disabled, to 1 mV with
the suppression circuit enabled, as
shown at the bottom.
In summary, compared to other amplifier classes such as AB, G, and H, class
D has superior efficiency. Audio class D
amplifiers can be realized using digital
PWM architecture and analog feedback



Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Summer 2017

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