AN1042/D
High Fidelity Switching
Audio Amplifiers Using
TMOS Power MOSFETs
Prepared by: Donald E. Pauly
ON Semiconductor
http://onsemi.com
Special Consultant
APPLICATION NOTE
Almost all switching amplifiers operate by generating a
high frequency square wave of variable duty cycle. This
square wave can be generated much more efficiently than
an analog waveform. By varying the duty cycle from 0 to
100%, a net dc component is created that ranges between
the negative and positive supply voltages. A low pass filter
delivers this dc component to the speaker. The square wave
must be generated at a frequency well above the range of
hearing in order to be able to cover the full audio spectrum
from dc to 20 kHz. Figure 1 shows a square wave
generating a sine wave of one–ninth its frequency as its
duty cycle is varied.
With the introduction of complementary bipolar power
transistors in the late 1960s, switching amplifiers became
theoretically practical. At low frequencies, bipolar transistors
have switching efficiencies of 99% and will directly drive
a low impedance speaker filter. The requirement for
switching frequencies above 100 kHz resulted in excessive
losses however. Bipolar drive circuitry was also complex
because of its large base current requirement.
With the advent of complementary (voltage/current
ratings) TMOS power MOSFETs, gate drive circuitry has
been simplified. These MOS devices are very efficient as
switches and they can operate at higher frequencies.
A block diagram of the amplifier is shown in Figure 2.
An output switch connects either +44 or –44 volts to the
input of the low pass filter. This switch operates at a carrier
frequency of 120 kHz. Its duty cycle can vary from 5% to
95% which allows the speaker voltage to reach 90% of
either the positive or negative supplies. The filter has a
response in the audio frequency range that is as flat as
possible, with high attenuation of the carrier frequency and
its harmonics. A 0.05 ohm current sense resistor (R27) is
used in the ground return of the filter and speaker to provide
short circuit protection.
The negative feedback loop is closed before the filter to
prevent instabilities. Feedback cannot be taken from the
speaker because of the phase shift of the output filter, which
varies from 0° at dc to nearly 360° at 120 kHz. Since the
filter is linear, feedback may be taken from the filter input,
which has no phase shift. Unfortunately, this point is a high
frequency square wave which must be integrated to
determine its average voltage. The input is mixed with the
square wave output by resistors R4 and R5 shown in Figure
2. The resultant signal is integrated, which accurately
simulates the effect of the output filter. The output of the
integrator will be zero only if the filter input is an accurate
inverted reproduction of the amplifier input. If the output
is higher or lower than desired, the integrator will generate
a negative or positive error voltage. This error voltage is
applied to the input of the switch controller, which makes
the required correction. The integrator introduces a 90°
phase shift at high frequencies which leaves a phase margin
of nearly 90°.
Input
1.0
Switching Frequency =
9X Modulation Frequency
0.75
0.5
Output
+1
0
0.25
0
–1
–0.25
–0.5
–0.75
–1.0
0°
90°
180°
270°
360° 420°
Figure 1. Switching Amplifier Basic Waveforms
The concept of switching amplifiers has been around for
about 50 years but they were impractical before the advent
of complementary TMOS power MOSFETs. Vacuum tubes
were fast enough but they were rather poor switches. A
totem pole circuit with supply voltages of±250 volts would
drop about 50 volts when switching a current of 200
milliamps. The efficiency of a tube switching amp could
therefore not exceed 80%. The transformer needed to
match the high plate impedance to the low impedance
speaker filter was impractical as well.
This document may contain references to devices which are no
longer offered. Please contact your ON Semiconductor represen-
tative for information on possible replacement devices.
Semiconductor Components Industries, LLC, 2002
1
Publication Order Number:
August, 2002 – Rev. 3
AN1042/D
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