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Exploring Class-D Amplifiers
By David Bates CET, CA
The Class-B or Class-AB amplifier has
been the main choice for many designers of
audio amplifiers. This linear amplifier configuration
has been able to provide the necessary
conventional performance and cost
requirements. Now, products such as LDC
TVs, plasma TVs, and desktop PCs are driving
the necessity for greater power output
while maintaining or reducing the form-factor,
without increasing costs. Portable powered
devices, such as PDAs, cell phones,
and notebook PCs, are demanding higher
circuit efficiencies. Due to very high efficiency
and low heat dissipation, the Class-D
amplifier is now challenging the Class-AB
amplifiers in many applications.
A Class-D amplifier uses output transistors
operated as switches. This enables the
transistor to either be in a cutoff or saturated
mode. When cutoff, its current is zero.
When it is saturated, the voltage across it is
low. In each mode, its power dissipation is
very low. This concept increases the circuit
efficiency, therefore, requires less power
from the power supply and enables the use
of smaller heat sinks for the amplifier.
A basic Class-D amplifier is shown in
Figure 1. The amplifier consists of an opamp
comparator driving two MOSFETs
operating as switches. The comparator has
two input signals. One signal is the audio
signal VA, while the other input is a triangle
wave VT with a much higher frequency.
The voltage value out of the comparator,
VC, will be approximately at either +VCC
Figure 1
or -VEE. When VA > VT, VC = +VCC.
When VA < VT, VC = -VEE.
The comparator's positive or negative
output voltage drives two complementary
common-source MOSFETs. When VC is
positive, Q1 is switched on and Q2 is off.
When VC is negative, Q2 is switched on and
Q1 is off. The output voltage of each transistor
will be slightly less that their +V and
-V supply values. L1 and C1 act as a lowpass
filter. When their values are properly
chosen, this filter passes the average value
of the switching transistors' output to the
speaker. If the audio input signal VA was
zero, VO would be a symmetrical squarewave
with an average value of zero volts.
To illustrate the operation of this circuit,
examine Figure 2. A 1 kHz sine-wave is
applied to the input at VA, while a 20 kHz
triangle-wave is applied to input VT. In
practice, the triangle-wave input frequency
would be many times higher than shown in
this illustration. A frequency of 250-300
kHz is often used. The frequency should be
as high as possible, compared to the cutoff
frequency, fc, of L1C1 for minimum output
distortion. Also, note that the maximum
voltage of VA is at approximately 70 % of
VT.
The resulting output VO of the switching
transistors is a pulse-width-modulated
(PWM) waveform. The duty cycle of the
waveform produces an output whose average
value follows the audio input signal.
This is shown in Figure 3.
More sophisticated Class-D amplifiers
use a MOSFET H-bridge circuit configura
tion for the switching devices and incorporate
active low-pass filters. Resulting efficiencies
can reach upwards from 85-90 %,
even at lower power levels. This exceeds
the efficiency of the Class-AB amplifier
whose efficiency reaches a theoretical maximum
of 78 % at high output levels and is
much less efficient at lower power levels.
New generation IC Class-D amplifiers,
such as the NJU8755, amplify analog input
signals and produce PWM digital output
signals. This provides for a fusion between
digital and analog systems. The NJU8755,
configured as a stereo BLT (Bridge-tied
Load) and connected to an analog input signal,
is capable of delivering 1.2 W/ch at 5 V
into 8 ohms. This type of circuit also
employs a standby mode designed to reduce
power consumption to minimum levels during
silent periods. Applications would
include cell phones, PDAs, portable audio
applications, and toys.
Figure 2 — Input Waveforms
Figure 3 — Output Waveform
Following the Input
4— February 2004
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