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Texas Instruments Benefits of Class-G and Class-H Boost in Audio Amplifiers Application notes
Application Report
SLAA888 – April 2019
Benefits of Class-G and Class-H Boost in Audio
Amplifiers
Scott Bryson
ABSTRACT
Internally boosted Class-D audio amplifiers such as the TAS2562 offer extended output power as well as
efficiency gains over designs that implement a fixed boost. The Class-G and Class-H modes improve
overall efficiency which ultimately extend operating battery life. In addition to extending battery life, these
modes improve the overall user experience in portable personal electronics by increasing the available
output power.
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Contents
Introduction ..................................................................................................................
Benefits of Integrated Solution ............................................................................................
Summary .....................................................................................................................
References ...................................................................................................................
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5
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7
List of Figures
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......................................................................................................................
TAS2770 Efficiency vs. Output Power ....................................................................................
TAS2562 Fixed Boost Efficiency, 100 Hz ................................................................................
Class-G Profile ...............................................................................................................
TAS2562 Class-G Efficiency, 100 Hz .....................................................................................
Class-H Profile ...............................................................................................................
TAS2562 Class-H Efficiency, 100 Hz .....................................................................................
TAS2562 Efficiency, 100 Hz ...............................................................................................
TAS2562 Fixed Boost Mode Power Recording .........................................................................
TAS2562 Class-G Mode Power Recording .............................................................................
TAS2562 Class-H Mode Power Recording ..............................................................................
H-Bridge
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Introduction
Class-D amplifiers convert an input signal into a series of output pulses that switch between GND and
supply voltages. This PWM output is the result of modulating the input signal with the duty cycle of the
output pulses. The output stage of this type of amplifier can be implemented easily using a half H-bridge
architecture for each output terminal.
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Introduction
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Figure 1. H-Bridge
Internal power losses are minimized by never simultaneously turning on both high-side and low-side
transistors which provides a significant boost to overall system efficiency compared to Class – A, Class B, and Class - AB amplifiers. Often, this output switching may occur anywhere from hundreds of kHz to a
few MHz. Driving this modulated signal through a low pass filter will remove the higher frequency content
related to the output switching and will pass only the desired signal content to the load. Class-D amplifiers
are ideal in audio applications given the relatively low frequency passband needed. This makes it easy to
separate the audio signal from the output switching with very low distortion. Many modern audio Class-D
amplifiers can utilize the inductance of the speaker coil itself to resolve the output waveform and allow the
amp to operate without any external filtering between the output and the load. However, emissions
standards often require that some EMI filtering be used to limit radiated noise.
TAS2770 output efficiency is pictured below to demonstrate the output efficiency attainable using a
class–D amplifier.
100
90
80
Efficiency (%)
70
60
50
40
30
VBAT = 4.5 V
VBAT = 8.4 V
VBAT = 12.6 V
VBAT = 16 V
20
10
0
0.0001
0.001
0.1
POUT (W)
1
10
D014
RL = 8Ω , Fin = 1 kHz
Figure 2. TAS2770 Efficiency vs. Output Power
Note that as output power increases there is a dramatic rise in overall output efficiency. In addition,
increases to the supply voltage, VBAT, shift this curve to the right. The efficiency of the amplifier depends
on how closely the outputs are being driven to the supply rail. We can observe a loss in output efficiency
when driving a fixed output power with higher supply voltages.
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Benefits of Class-G and Class-H boost in audio amplifiers
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Introduction
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In many personal electronic applications, it is also necessary to operate the system off of a battery
voltage. It is common to see operating battery voltages of handheld devices range from 3.7 V to 4.2 V.
Even when operating at full scale output, there will be obvious output power limitations when the Class-D
output is powered at this voltage. To increase the available output power, it is possible to configure the
output stage to operate off of an additional power rail at a higher voltage. This can be easily achieved by
integrating a DC-DC boost alongside the amplifier.
Consider, however, the implications this new supply voltage will have on efficiency. As mentioned above,
we will expect to see a drop in overall output efficiency when driving a fixed output power with the higher
supply voltage. TAS2562 is a Class-D amplifier designed to operate using battery voltages in the range of
2.7 - 5.5 V. It also includes an integrated boost. We can set this boost to always be on and measure total
efficiency as shown below.
100%
90%
80%
Efficiency (%)
70%
60%
50%
40%
30%
20%
10%
0
0.001 0.002
0.005 0.01 0.02
0.05
0.1
0.2 0.3 0.50.7 1
2
3 45
Output Power (W)
D003
Figure 3. TAS2562 Fixed Boost Efficiency, 100 Hz
We can see that the amplifier has a peak efficiency of about 80% at the full scale output power of 5 W.
1.1
Class-G Boosted Amplifier
Class–G amplifiers help to overcome the limitation of efficiency loss with respect to supply voltage. In a
class-G system, the supply voltage for the class–D output is only boosted when needed. This is easily
implemented in digital input amplifiers. The device is able to monitor the input stream and determine the
relative output voltages that will be driven. If the output exceeds a certain threshold then the boost will be
enabled on and provide additional headroom.
VSPK
Class-G
Time
Figure 4. Class-G Profile
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Introduction
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In this way, we maintain the higher efficiency when operating at low output power typical to the nominal
supply voltage, but also extend the maximum output power the amplifier can deliver to the speaker. In
figure 4, see the output efficiency achieved by TAS2562 in this type of application. Notice here that output
efficiency peaks around 0.4 - 0.5 W at a maximum of 87%.
100%
90%
80%
Efficiency (%)
70%
60%
50%
40%
30%
20%
10%
0
0.001 0.002
0.005 0.01 0.02
0.05
0.1
0.2 0.3 0.50.7 1
2
Output Power (W)
3 45
D001
VBAT = 4.2 V, Rload = 8 Ohm
Figure 5. TAS2562 Class-G Efficiency, 100 Hz
1.2
Class-H Boosted Amplifier
TAS2562 is also able to operate in Class–H mode. Class-H amplifiers expand on the concept introduced
with Class-G. Instead of having a single voltage, the boost level is variable. With this implementation there
can be several possible supply voltages as shown in Figure 6.
VSPK
Class-H
Time
Figure 6. Class-H Profile
This enables the output to operate with just enough headroom to maintain low distortion while achieving
the maximum output efficiency. As an example the output efficiency of TAS2562 operating in Class-H
mode is shown below. Here we see that the amplifier has a similar profile to Class-G below 0.3 W output
power. The efficiency drop-off observed in Class-G mode is not as severe in this case, and higher
efficiency is maintained while the boost is active.
4
Benefits of Class-G and Class-H boost in audio amplifiers
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Benefits of Integrated Solution
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100%
90%
80%
Efficiency (%)
70%
60%
50%
40%
30%
20%
10%
0
0.001 0.002
0.005 0.01 0.02
0.05
0.1
0.2 0.3 0.50.7 1
2
Output Power (W)
3 45
D000
VBAT = 4.2 V, Rload = 8 Ohm
Figure 7. TAS2562 Class-H Efficiency, 100 Hz
The major design trade-off when selecting which mode to operate depends on the available input current.
Class–H mode will require greater current drive to be available from the battery in order to achieve the fast
switching necessary. Class–G mode tends to be less demanding, but will not yield the same efficiency
gains as Class–H.
2
Benefits of Integrated Solution
Integrating a Class-G or Class-H boost into a Class-D audio amplifier is an excellent design choice when
faced with battery voltage limitations. The solution could be pieced together with separate devices, but this
would actually prove to be challenging in practice.
First, the two device approach would require greater PCB area typically is not available in handheld
applications. Not only would the required components require extra PCB surface area, but there may also
be placement & routing conflicts when considering the power carrying traces and resistive losses that
would result in inferior efficiency.
Secondly, the synchronization of the boost may prove challenging without dedicated signal processing.
Care needs to be taken to track the input signal to the amplifier to determine the proper time to enable the
boost while maintaining an uninterrupted listening experience for the end user. Adequate time must be
allowed for the boost to ramp and settle. This is best done with the same signal processing that is driving
the Class-D amplifier.
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Summary
Class-G and Class-H boosted amplifiers are ideal for battery operated audio devices. In the overlaid plot
below, the efficiency benefits over a fixed boost system are evident. Class-G and Class-H boost modes
allow the amplifier output stage to maximize overall efficiency while significantly increasing achievable
output power. In addition, as an integrated solution, PCB area is kept to a minimum while delivering the
best performance. In contrast, driving the output stage using a fixed boost from an external source
produces the worst case efficiency.
Class-H Boost provides better efficiency than Class-G, but this mode typically requires greater maximum
current delivery during the level switching events. Care should be taken when selecting the operating
mode to ensure that current requirements specified in the datasheet are satisfied.
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Summary
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100%
90%
Class-H
Class-G
Fixed Boost
80%
Efficiency (%)
70%
60%
50%
40%
30%
20%
10%
0
0.001
0.01
0.1
1
Output Power (W)
5
D004
Figure 8. TAS2562 Efficiency, 100 Hz
As an additional reference, the power consumption recordings shown below were taken while playing a
typical audio track. Here we can observe the benefits of Class-H and Class-G over a system designed
around a fixed boost. Notice in the data captures that the estimated battery life for a 1000 mAh battery
changes from 10.16 hours while in fixed boost mode, to 11.2 hours in Class-G mode, and 11.74 hours in
Class-H mode. Class-H mode offers approximately 95 minute (15.5%) increase in expected battery life
over the fixed boost settings.
Figure 9. TAS2562 Fixed Boost Mode Power Recording
6
Benefits of Class-G and Class-H boost in audio amplifiers
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References
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Figure 10. TAS2562 Class-G Mode Power Recording
Figure 11. TAS2562 Class-H Mode Power Recording
The data for all plots in this document were taken under the following conditions:
VBAT = 4.2 V, VDD = 1.8 V, Rload = 8 Ohm + 30 uH, FSYNC = 48 kHz, BCLK = 3.072 MHz
For more information, please see Texas Instruments' selection of Boosted Class-D amplifiers.
4
References
•
TAS2562 Datasheet
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