Class G versus Class AB Headphone Amplifiers

PowerWise?< Class G versus Class AB Headphone Amplifiers
Literature Number: SNAA128
SIGNAL PATH designer
Expert tips and techniques for PowerWise® energy-efficient design
No. 118
Feature Article ............... 1-5
Solutions .............................2
PowerWise® Class G versus Class AB
Headphone Amplifiers
— By Richard Gomez, Audio Product Engineer
oday’s portable devices present many challenges ranging from output
power to maintaining high levels of efficiency. As devices continue to
become more feature rich, consumers will continue to demand higher
levels of performance along with minimal battery power consumption,
putting further emphasis on proper design of key components such as the
headphone amplifiers. Headphone amplifiers are migrating from Class AB to
Class G technology. The differences and design advantages of each technology
will be addressed in this article.
Class AB Headphone Amplifiers
Class AB is a compromise between Class A and Class B output stage topologies. A Class A output stage operates with both transistors biased in their
active regions, and the low-side transistor constantly biased to handle peak
load currents at all times. A Class B output stage operates with only one
transistor active at a time, improving efficiency, but at the cost of distortion.
The Class AB output stage is a compromise between Class A and Class B.
Both output devices (push-pull) conduct for a little more than 180° but
much less than 360°. The amount of bias current varies for each amplifier, but
is a balance between minimal crossover distortion and yet good efficiency.
A simplified Class AB output stage is shown below (Figure 1). Each output
device is biased such that they are both “on” at or near the zero crossing of
the output signal, reducing
crossover distortion but also
lowering efficiency.
Figure 1. Simplified Class AB Output Stage
In Figure 2, the current
waveform for a Class AB output stage. The left axis is the
emitter currents through
Q1 and Q2.The bias current
Iq flows through both Q1
and Q2 so that each device
conducts for more than
180°. Around the crossover
Extend Battery Life
National’s PowerWise® solutions provide optimal performance
aat the lowest power
© 2008, National Semiconductor Corporation. National
nal Semiconductor, , and PowerWise are registered trademarks. All rights reserved.
• Adaptive Voltage Scaling (AVS) technology reduces energy consumption in
digital subsystems via a closed-loop voltage scaling to automatically minimize
active and leakage power with minimal system overhead
• Adaptive RF Power enables energy savings in handsets by monitoring the
RF power output and dynamically adjusting the supply voltage to RF PA
• RGB LED backlighting enhances color clarity while lowering power
• Mobile Pixel Link (MPL) reduces EMI and interconnect while lowering
power consumption
• Integrated Class D audio subsystems feature low-noise and very low
quiescent current for minimum power consumption
• Analog Noise Reduction Technology reduces background noise and
delivers more natural-sounding voice quality at one-tenth the power of
DSP solutions
Find PowerWise products, metrics,
white papers, app notes and tools at:
SIGNAL PATH designer
PowerWise® Class G versus Class AB Headphone Amplifiers
Figure 2. Current Waveform for a Class AB Output Stage
area, both transistors will be “on,” which greatly
reduces the distortion inherent in Class B. Class
AB does not fully eliminate crossover distortion but
does reduce it to an acceptable level.
For Class B, the maximum efficiency is π/4 or
78.5%. Class AB will have the same maximum
efficiency if the Q current in the output stage is not
taken into account. Taking the Q current into
account lowers the maximum efficiency. The amount
of change in the efficiency depends on the amount
of Q current. A general equation will not be derived
as real transistors with saturation voltages and I x R
loss contribute more to reduced efficiency than
Q current.
Class AB power dissipation rises as amplifier output power increases, and peaks below maximum
amplifier output power. Then it drops even though
amplifier output power continues to increase. Class
AB peak power dissipation occurs at 50% efficiency,
when the amplifier output power is equal to the
power dissipation. Peak power dissipation, PDMAX,
for a mono, single-ended Class AB amplifier is
found using the derived formula:
Class G
The Class G topology is a modification of another
Class of amplifier (normally Class B or Class AB)
to increase efficiency and reduce power dissipation.
Class G takes advantage of the fact that musical and
voice signals have a high crest factor with most of
the signal content at lower amplitudes. The Class
G topology uses multiple power supplies, operating from the power rail that provides the optimum
combination of headroom and power dissipation.
The Class G topology improves amplifier efficiency
by optimizing the power supply. A Class G device
uses a minimum of two different supply rails. The
device operates from the lower supply until output
headroom becomes an issue. At this point the device
switches the output stage to the higher supply
rail. Once the output signal drops below a predetermined level, the device switches back to the
lower rail. Power dissipation is greatly reduced for
typical musical or voice sources. Figure 3 illustrates
a simplified Class G implementation with a split
supply Class AB output stage.
(2π2RL) (Watts)
PDMAX = (Total Supply Voltage)2
The amount of power dissipation is inversely
proportional to the load impedance. If the load
impedance is halved, the peak power dissipation
doubles. The amount of power dissipation increases
with the square of the supply voltage. Doubling
the supply voltage will increase the peak power
dissipation by four times.
Figure 3. Basic Class G Design Using
Split-Supply Class AB Output Stage
SIGNAL PATH designer
PowerWise® Class G versus Class AB Headphone Amplifiers
A MOSFET is used as a switch to change the supply
rails from the lower voltage (LV) to the higher supply
voltage (HV) for the output stage.
There are several ways to control the switching of
the supply rails. Feedback from the output stage
may be used and/or the input and pre-amp stage
may have the control. The control could have some
time constant so that when a switch to the higher
rails occurs, there is some delay before the FETs
turn back off and the output stage shifts back to the
lower supply rails. The speed of the change also will
need to be controlled appropriately for minimal loss
of power in the FETs.
Figure 4 shows an example of a music output signal.
The dotted lines are the supply voltages for the output stage. When the output signal requires a higher
supply voltage, the FETs are turned on and the
supply voltage increases to the higher rail. Once the
output signal falls below the internally-set threshold,
and after some delay, the device switches back down
to the lower rail, reducing power dissipation.
Designers must balance many trade-offs associated
with Class G: selecting the proper number of supplies and the voltage difference between the supplies
in order to optimize headroom at lower voltages,
while minimizing power dissipation. Two different
rails minimize the complexity of the power supplies,
while providing sufficient voltage flexibility. Additional rails may reduce power dissipation further
but at the cost of higher component count, complexity, and reliability. Another issue is the length of
time the device operates from the higher rail. While
operating from the higher supply rail, power dissipation increases. Switching back to the lower rail too
early may result in distortion due to clipping, while
remaining at the higher rail for an extended period
of time will result in a degradation of efficiency.
An example of a Class G device is National Semiconductor’s LM48824 Class G headphone amplifier.
The LM48824 amplifier operates from two voltage
supplies (1.1V and 1.8V) generated by an integrated
step-down (buck) regulator. When the audio output
exceeds an internally-set threshold, buck converter
output increases from 1.1V to 1.8V. When the audio
signal falls below the required voltage rails for a set
period of time, the buck converter output decreases
back to 1.1V. Power dissipation is greatly reduced
for typical musical or voice sources. Figure 4 shows
how a musical output may look. The green line
corresponding to HPVDD(HV) and HPVDD(LV) is the
buck converter output. The green line corresponding to HPVSS(HV) and HPVSS(LV) is the inverting
charge pump output.
Buck Converter Output
Power Savings in Class G
Power Dissipated in Class AB
Power Dissipated in Class G
Figure 4. Music Output Example
Class G Efficiency
Class G efficiency depends largely on the source
material (music or voice) and the characteristics
of the signal. With pure sine waves, depending on
signal amplitude, there is no efficiency gain compared to a Class A, B, or AB amplifiers under the
same conditions. If the amplitude remains at a level
where the Class G device operates from its lower
supply rail, then power dissipation does decrease
compared to the other architectures that can only
operate from a fixed, higher voltage supply.
SIGNAL PATH designer
For real-world Class G amplifiers, the maximum
efficiency occurs when operating under the lowest supply rails as opposed to operating at peak
output power on the higher rails due to biasing
conditions, current x voltage (I x V) loss, and IR
losses in the FETs. Peak efficiency depends on the
supply voltage switchover threshold. A design that
switches at a lower level improves efficiency when
operating from the lower supply but causes the
device to operate from the higher supply for a
longer period. Ideally, a Class G device operates for
as long as possible from the lower supply, minimizing I x V losses incurred when operating at higher
voltages. The power considerations must be balanced
with maintaining proper headroom, so distortion
due to clipping does not become an issue.
National Class G Technology
The LM48824 headphone amplifier integrates a
high-efficiency step-down (buck) DC-DC switching regulator with a ground-referenced headphone
amplifier. The switching regulator delivers a constant
voltage from an input voltage ranging from 2.4V to
5.5V. The switching regulator uses a voltage-mode
architecture with synchronous rectification, improving efficiency and reducing component count. The
LM48824 amplifier features National’s groundreferenced architecture that eliminates the large
DC-blocking capacitors required at the outputs
of traditional single-ended headphone amplifiers.
A low-noise inverting charge pump creates a
negative supply (HPVSS) from the positive supply
voltage (VDD). The headphone amplifiers operate
from these bipolar supplies, with the amplifier outputs biased about GND. Because there is no DC
component on the output signals, the large DCblocking, AC-coupling capacitors (typically 220 µF)
are not necessary, thereby conserving board space,
reducing system cost, and improving frequency
The LM48824 amplifier takes advantage of
National’s proprietary headphone architecture that
incorporates Class G amplifiers offering power
savings compared to traditional Class AB headphone
amplifiers. Additionally, output noise is improved
by common-mode sensing that corrects for any
differences between the amplifier ground and the
potential at the headphone return terminal, minimizing noise created by any ground mismatches. A
high-output impedance mode allows the LM48824
amplifier's outputs to be driven by an external source
without degrading the signal. Other features include
flexible power supply requirements, differential
inputs for improved noise rejection, a low-power
(2.5 µA) shutdown mode, and a 32-step I2Ccompatible volume control with mute function.
The LM48824 amplifier's click-and-pop suppression eliminates audible transients on powerup/down
and during shutdown. The LM48824 amplifier is
available in an ultra-small 16-bump, 0.4 mm pitch
micro SMD package (1.7 x 1.7 mm).
Design Tools
WEBENCH® Signal Path Designer® Tools
Design, simulate, and optimize amplifier circuits in this FREE online
design and prototyping environment allowing you to:
■ Synthesize an anti-alias filter
■ Select the best amplifier/ADC combo for your system specs
■ Make trade-offs based on SNR, SFDR, supply voltage
■ Simulate real-world operating conditions using SPICE
■ Receive samples in 24 hours
WaveVision 4.1 Evaluation Board
Test and evaluate A/D converters with National’s easy-to use
WaveVision 4.1 evaluation board. Each evaluation board comes
complete with USB cable and support software.
Features and benefits:
• Plug-n-play ADC evaluation board
• USB 2.0 interface to PC
• PC-based data capture
• Easy data capture and evaluation
• Highlighted harmonic and SFDR frequencies
• Easy waveform examination
• Produces and displays FFT plots
• Dynamic performance parameter readout with FFT
• Produces and displays histograms
National Semiconductor
2900 Semiconductor Drive
Santa Clara, CA 95051
1 800 272 9959
Mailing address:
PO Box 58090
Santa Clara, CA 95052
Visit our website at:
For more information,
send email to:
[email protected]
Don’t miss a single issue!
Subscribe now to receive email alertss
when new issues of
Signal Path Designer® are available:
Also, be sure to check out our
Power Designer! View online today at:
©2009, National Semiconductor Corporation. National Semiconductor, , WEBENCH, and Signal Path Designer are registered trademarks of National Semiconductor.
All other brand or product names are trademarks or registered trademarks of their respective holders. All rights reserved.
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Communications and Telecom
Computers and Peripherals
Data Converters
Consumer Electronics
DLP® Products
Energy and Lighting
Clocks and Timers
Space, Avionics and Defense
Power Mgmt
Transportation and Automotive
Video and Imaging
OMAP Mobile Processors
Wireless Connectivity
TI E2E Community Home Page
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2011, Texas Instruments Incorporated