Audio Power Amplifier (APA)
Operation and Measurement
Stephen Crump
http://e2e.ti.com
Audio Power Amplifier Applications
Audio and Imaging Products
18 August 2010
Contents
• Audio Power Amplifier Operation
• Class-D APA Operation
• Measuring Class-D and Class-AB Outputs
Audio Power Amplifier
Operation
APA Classes
Input Configurations
Output Configurations
Fully Differential APAs
Audio Power Amplifier Classes
• There are two classes of audio power amplifiers
in common use.
– Class-AB – continuous
output. The traditional
configuration.
Continuous output,
amplitude proportional to input
– Class-D – switching
output. We will examine
Class-D in detail later.
– Class-D output is the
short-term average of
the switching waveform.
Switched output,
duty cycle and short-term
average proportional to input
Advantages and Disadvantages
• Advantages and disadvantages of Class-AB.
– Simple application.
– Inexpensive (but not necessarily in SYSTEM cost).
– Low efficiency, high power drain and heat generation.
• Advantages and disadvantages of Class-D.
– High efficiency, low power drain and heat generation.
– Somewhat more expensive (but not in SYSTEM cost).
– More complicated application.
• Class-D advantages usually are compelling.
APA Input Configurations
– Differential – a pair of
input lines. A superior
configuration.
– May be connected to a
differential source OR
a single-ended source.
Input referred
to APA ground
S
– Single-ended – single
input line referred to
ground. Traditional
configuration.
S
• There are two common input configurations.
Input referred
to the source,
not APA ground
Single-Ended Inputs: Disadvantages
• Input DC blocking cap are practically always
required in single-supply systems.
• No rejection of input noise or interference.
S
Noisy power currents in the
ground between source &
APA produce voltages that
add to the APA input signal.
APA input = sum
of these signals
Differential Inputs: Advantages
• Input blocking caps may not be required.
• High rejection of input noise and interference.
net input = only
intended signal
S
With ground reference at
source, voltages induced by
noisy ground currents are
the same at both inputs &
are rejected by APA CMRR.
Differential Inputs Cont’d.
• Differential inputs may be connected to either
differential or single-ended sources.
caps may be optional
Differential
Source
Singleended
Source
- If the audio source DC is within
the APA common-mode range,
DC blocking caps are optional.
- However, be sure source DC
offset is not a problem!
- Input caps may still be used if
high-pass filtering is needed.
- Input DC blocking caps are
required with a single-supply
single-ended source.
Differential Inputs Cont’d.
• Psuedo-differential sources use a single output
with a midrail bias.
caps may be optional
PsuedoDifferential
Source
• Treat these like differential sources for wiring to
the differential inputs of APAs.
Differential Input Connections
• Keep the 2 input leads close together.
• With single-ended sources connect the APA
input ground lead at source ground, NOT at APA
ground.
• This lets the CMRR of the APA reject any
common-mode radiation or any ground noise
between the APA and the source.
APA Output Configurations
– Differential – a pair of
output lines. Also called
BTL (for Bridge-Tied
Load).
– Must be connected to a
floating load.
Output referred
to APA ground
Zload
– Single-ended – single
output line. Traditional
configuration.
Zload
• There are two common output configurations.
Output independent
of APA ground
Single-Ended Outputs: Disadvantages
Zload
Zload
• A large output DC blocking cap is required in
single-supply systems.
The blocking capacitor is
required to prevent high DC
current through the load.
The cap must be very big for
good low-frequency response:
- 8Ω, 50Hz: C ~= 390μF.
- 4Ω, 20Hz: C ~= 2200μF!
Differential Outputs: Advantages
+
2x
-
Zload
• Output power is nearly 4 times S/E output
power.
Outputs sum to
2 x S/E outputs
The 2 outputs are opposite in
phase, so their voltages sum
across the load to provide
twice the voltage and 4 times
the power of single-ended.
Zload
• DC blocking capacitor is not required.
DC load current
is negligible
NO output cap is required –
when input is zero, output is
a small DC offset, so DC
load current is negligible.
Fully Differential APAs
Zload
S
• Fully differential APAs use differential circuits at
inputs, outputs and all intermediate stages.
• They have all the advantages of differential
inputs and outputs, with increased CMRR,
PSRR and RF immunity from balanced
differential operation throughout the IC.
• All recent differential APAs from TI use fully
differential architecture.
Fully Differential vs. Traditional
• APAs with differential inputs and outputs, like
master-slave IC’s, may not be fully differential.
• These cannot match the performance of fully
differential APAs.
Noise on input
RF
VDD/2
Gain
amp
RIN
1
2
Noise coupled
into inputs is
amplified to
the outputs
Input Signal
1
A
+
R
2
Noise on
output
CBYPASS
RF coupled into inputs
or outputs can cause
RF Rectification –
BAD!
-
R
2
B
+
Inverting amp
Noise on
output
Class-D Audio Power
Amplifier Operation
Benefits
Block Diagram and Circuit
Description of Operation
Output Waveforms
AD and BD Modulation
Class-D APA Benefits
• Class-D audio power amplifiers offer greater
efficiency than amplifiers like Class-AB.
• They therefore reduce power consumption of
products in which they’re used.
– Product power budgets are reduced.
– Battery life is extended in portable products.
– Heat generation is reduced.
• These benefits reduce product cost and improve
product performance.
Class-D APA Block Diagram
• Below is a block diagram for a fully differential
Class-D audio power amplifier.
feedback
Vcc
OUT+
PGA
Vin
-
-
-
+
+
+
+
-
feedback
PWM
LOGIC
Vcc
H-Bridge
OUT-
• Most TI Class-D amplifiers are fully differential.
• Single-ended implementations are possible.
Class-D Differential APA Circuits
• A programmable-gain differential amplifier feeds
a differential integrator and comparator.
• The integrator takes feedback from the output
pulse train, subtracts it from the input signal and
low-pass filters the result.
• The comparator compares integrator output to a
triangle wave to set output pulse width.
• PWM (pulse width modulation) interface logic
drives output FET gates.
• A MOSFET bridge supplies switching pulses to a
loudspeaker, which low-pass filters them to
produce an audio output.
Class-D Analog/PWM Conversion
• The integrator produces an error voltage at its
output that reflects the input after feedback.
OUT+
Error
Voltage
-
Triangle
Wave
+
+
-
OUT-
Comparator
Outputs
• The comparator switches when the error voltage
crosses the output of the triangle wave oscillator.
• PWM logic converts the comparator outputs to
gate drive signals for the H-bridge.
Class-D Output Waveforms
• The PWM output switches at a frequency well
above the audio frequency range.
• Its short-term average is the audio-band output.
Vcc
ON
Q1
+ +
- -
Q4
off
Q3
ON
off
Q1
Vcc
+
Positive
Output
Polarity
ON
Q2
Q4
ON
off
Q2
- +
Q3
off
Negative
Output
Polarity
Duty cycle determines the
short-term average, the
amplitude of the audio output
AD Modulation
• AD modulation, the simplest technique, puts the
full differential output voltage across the load at
all times, varying the duty cycle to control output.
– (Differential or BTL AD modulation is shown on the
preceding page. In differential AD modulation the
outputs are always switched in opposite phase.)
• AD modulation is a powerful technique, but it
can generate high ripple current in the load at
the switching frequency.
• So AD modulation generally requires an LC filter
before the load to eliminate the ripple current.
AD Modulation Ripple Current
• Without the LC filter, AD modulation ripple
current wastes power and may increase the
power handling requirement of the speaker.
With no LC filter, ripple current
is limited only by loudspeaker
inductance, usually 20 to 60 uH.
With no input signal,
switching at 250kHz,
approximate peak ripple
current would be
Vcc * 1uS / L.
For Vcc = 12V and L = 30uH,
peak ripple current would be
~ 0.4A.
With an 8Ω load, including
extra power burned in the
APA, this would waste nearly
1/2 watt.
BD (Filter-Free) Modulation
• A newer technique, BD modulation, permits
operation without an output LC filter.
ON
Q1
OUTP
Vcc
+
-
Q4
OUTP
Q4
ON
OUTN
Q3
off
off
off
Q1
ON
Q2
Vcc
+
-
off
Q2
OUTN
Q3
ON
OUTP
OUTN
Differential
Load
Voltage
Ripple
Current
BD Modulation Characteristics
• BD modulation requires a differential output.
• When there is no input, BD modulation switches
the opposing outputs nearly in parallel.
• So the differential voltage across the load is
limited to very low duty cycle and ripple current
is reduced dramatically.
BD Modulation Waveforms
• As input increases, output duty cycles are
modulated in opposite phase to produce a net
load voltage at twice the switching frequency.
OUTP
OUTN
Differential
Load
Voltage
+5V
0V
-5V
Load Current
Current
Current
Increasing Decaying
Filter free modulation output voltage and
current waveforms, example signal
A Note About Output Filtering
• BD modulation eliminates the problem of ripple
current without an output LC filter.
• However, a output filter may be required for
EMC even with BD modulation.
• This will depend on the system or product
configuration!
Measuring Class-D and
Class-AB Outputs
Viewing Class-D Outputs
• Look again at an earlier graph of Class-D output.
Positive
Output
Polarity
Negative
Output
Polarity
Duty cycle determines the
short-term average, the
audio output amplitude
The audio output
may be an ordinary
1kHz signal.
However, it’s very
difficult to see any
audio output in the
switching
waveforms!
• The switching waveform doesn’t look much like
the audio output.
RC Filter for Viewing Class-D Output
• To view the audio content of a Class-D output
use an RC low-pass filter at each output.
Rflt
Cflt
Rflt
Cflt
Audio unclear in switching output
Class-D
RC filter shows audio with small ripple
• Filter frequency should be 30 to 40 kHz.
• Recent work shows that 330Ω+15nF works best.
Measuring Differential Outputs
• Single-ended outputs are measured between
output and ground.
• HOWEVER ! – measure differential outputs
BETWEEN the 2 output lines to be accurate.
Rflt
Cflt
 Measure single-ended outputs to ground.
 Measuring a differential output vs.
ground is NOT accurate, and it
overlooks half the output voltage.
Class-D
Rflt
Cflt
 Connect a scope probe to each side
and use a math difference function.
• Do not connect probe ground to a differential
output – that will short it to ground.
Class-D Output Rise and Fall
• A Class-D switching waveform has very fast rise
and fall, or equivalent slew rate.
• Very few other devices can match this.
The audio output may be an
ordinary 1kHz signal.
But the switching output may
rise and fall in 10nS.
With a typical 12V supply,
this means an equivalent
slew rate of 1200V/uS!
Filters for Measuring Class-D APAs
• Many audio analyzers require filtering because
extreme slew rates of Class-D waveforms cause
slew-induced distortion in their input stages.
• A first-order RC filter with time constant around
4.7μS eliminates this problem in most cases.
• At high gains such analyzers may require
second-order filters. These may be cascaded
RCs, with time constants around 2.7μS.
• Be aware that there is some frequency response
rolloff in the audio band! It is generally not large
enough to cause significant loss in results.
1st and 2nd Order Filter Responses
• Schematics
and frequency
responses for
suggested 1st
and 2nd order
filters appear
at right.
0
-0
-10
-10
-20
-20
-30
-30
-40
-40
1.0KHz
DB(V(_V1.1))
1kHz
3.0KHz
DB(V(_V2.2))
3kHz
10KHz
10kHz
30KHz
30kHz
Frequency
100KHz
100kHz
300KHz
300kHz
1.0MHz
1MHz
Other Filter Possibilities
• It’s possible active filters could be used for
measuring outputs of Class-D amplifiers.
– HOWEVER, active filters can have the
same slewing problems as analyzers.
• It’s possible transformers could be used for
measuring outputs of Class-D amplifiers.
– HOWEVER, transformers often have
problems like saturation and overshoot.
• MAKE SURE YOUR FILTER DOES NOT ADD
TROUBLE !
QUESTIONS ?