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Class-G Headphone Driver in 65nm CMOS Technology
Alex Lollio1, Giacomino Bollati2 and Rinaldo Castello1
1Università degli studi di Pavia, Pavia, Italy
2Marvell Italia, Pavia, Italy
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Headphone audio amplifiersTarget application
Typical operating conditions
VIN
VHV
-VHV
Key objectives:
• Low distortion
• Low noise
• High efficiency
• Single ended• RL = 32/16 Ω• BW = 20Hz–20kHz• PO,MAX = 40mW (on 16 Ω)
Modern cellular phones incorporates MP3 music playback and users may wish to use this feature for many hours
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Outline
• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)
• Class-G headphone driver (architecture, switching principle, distortion analysis)
• Prototype in 65nm CMOS technology(implementation, results, comparison)
• Conclusions3
Outline
• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)
• Class-G headphone driver (architecture, switching principle, distortion analysis)
• Prototype in 65nm CMOS technology(implementation, results, comparison)
• Conclusions4
Class AB (Linear amplifier)
PROs: Best linearity
No EMI problems
CONs: Low efficiency
Typically the preferred solution in headphone application
Class D (Switching amplifier)
PROs: Best efficiency
CONs: Less linearity than class AB
EMI problems
Emerging solution in headphone application
Headphone audio amplifiersAlternative topologies
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Headphone audio amplifiersAlternative topologies
Class G: It is a linear amplifier which uses two voltage supply rails which switches to the appropriate voltage as required by the instantaneous output voltage
PROs: High efficiency but less than class D
High linearity but less than class AB
No EMI problems
CONs: It needs two voltage supply rails
VIN
VLV
VHV
-VLV-VHV
VHV
-VHV
VLV
-VLV
VOUT VOUT
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Class Galternative topologies
Series topology (classical)
Parallel topology
• Only one output stage
• Switches are in series with the power transistors
• Two output stages work in parallel
• No switches in series with the power transistors
• It needs a careful switching circuit design
VHV
-VHV
VLV
-VLV
VHV
VLV
-VHV
-VLV
RL
RL
This is the adopted solution7
Class G: working principle
For Vout below the switching point the low voltage stage is active. For Vout above the switching point both the low voltage and high voltage stages drive the load (in different moments).
VHV
VLV
-VHV
-VLV
LV stageHV stage
iHV
iLV
iLV
iHV
iLV
iHV
Iout[A]Iout[A]
iLVt t
Switching point
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Class G: switching distortion
Distortion caused by the switching
Up to the switching point the class G linearity is the same as a class AB
Compared to class AB, class G has an additional source of distortion.
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Switching point
Class G: critical design choices
The implemented current based switching enables low distortion and high efficiency
• Switching point level: To achieve high efficiency, it must be as close as possible to the low voltage supply
Switching point close to low voltage supply
Switching point far from the low voltage supply
• Switching strategy: to minimize the distortion, switching must be as smooth as possible
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Outline
• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)
• Class-G headphone driver (architecture, switching principle, distortion analysis)
• Prototype in 65nm CMOS technology(implementation, results, comparison)
• Conclusions11
Overall amplifier architecture
• Three stage opamp with differential input and single ended output.
• The two identical second stages, gm2, and the third stages, gm3L and gm3H, work in parallel.
• Only the low voltage stage gm3L is supplied by the low voltage rail ±VLV. The rest of the circuit is supplied by the high voltage rail ±VHV
gm2
gm2
gm1
-gm3L
-gm3H
Switching stage
R2
R1
R1R2 RL
CM2
CM2CM1
VOUT
Main path
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Amplifier architecture: main path
First stage
Input pairs
gm1
1313
VO
VLV
-VLV
VHV-VHV
Floating battery
VHV
VHV
-VHV
RL
Second stage
Amplifier architecture: main path
gm2
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Floating battery ref: Renirie, Langen, Huijsing, 1995
VO
VLV
-VLV
VHV-VHV
Floating battery
VHV
VHV
-VHV
RL
Amplifier architecture: main path
Third stage
LV stage
gm3L
HV stage
gm3H15
RL
VO
VLV
-VLV
VHV
-VHV
-VHV
Floating battery
VHV
VHV
Amplifier architecture: switching stageconceptual schematic
PMOS switching
stage
NMOS switching
stage
RL
VO
VOVLV - VTH
-VLV + VTHVO
VLV
-VLV
VHV
-VHV
-VHV
Floating battery
VHV
VHV
Amplifier architecture: switching stageconceptual schematic
PMOS switching
stage
RL
VO
VOVLV - VTH
-VLV + VTHVO
VLV
-VLV
VHV
-VHV
-VHV
Floating battery
VHV
VHV
Switching principle details
• Switching point sensing is in voltage domain. A differential pair compares the output voltage to the switching point voltage VLV-VTH
• The switching between the high voltage and low voltage output stage is current based.The switching circuit injects all its bias current into the gate of the MOS to be switched off.
VOUT
LV stage
HV stage
iJH
iJL
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VOUT VLV - VTH
VHV
-VHV
-VLV
VLV
VHV
VHV
IBIAS
PMOS switching stage
Output currents during switching
t
Iout[A]
Ou
tpu
t c
urr
en
ts
iLV
iHV
t
VLV -VTH
VLV
Vout[V]
• When VOUT is lower than the switching point (VLV-VTH) the switching circuit enables the LV stage and disables the HV stage
• When VOUT is higher than the low voltage supply VLV only the HV stage drives the load
• When VOUT is between VLV-VTH and VLV both stages drive the load
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Switching distortion:Amplifier model during the switching
• We use a simplified model of the amplifier during the switching.
This current is used to represent the disturbance generated by the switching stage.
gm1 gm2 -gm3
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RL
VOUT
R1
R1
R2CM1
CM2
IJ
gm2f
Cm2s
f
s1
/fs
gm2
1iVout
T
2
T
TJ
Design criteria for distortion reduction
• From the model of previous slide we obtain the equation:
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Where21
1T
RR
R
Cm1
gm1f
time
ΔVOUT
VOUT
195u 200u
300m
305m
To minimize the distortion at the output we have to minimize ΔVout.
• Lower iJ value means better linearity and lower switching speed
• Higher amplifier bandwidth, fT, means higher linearity
Switching point
Outline
• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)
• Class-G headphone driver (architecture, switching principle, distortion analysis)
• Prototype in 65nm CMOS technology(implementation, results, comparison)
• Conclusions22
Chip micrograph
• 65nm CMOS process
• 0.14mm2 active area per channel
• Voltage supplies:
High voltage rail ±1.4V
Low voltage rail ±0.35V
• Switching point 50mV under the low voltage supply
• Max load capacitance 1nF
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Measurement results:Power dissipation versus output power
Fin=1kHzRL=32Ω
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Measurement results:THD+N and efficiency versus output power
• Sinusoidal input signal (fin=1kHz)• About 6dB extra distortion due to switching
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Measurement results:THD+N versus frequency
RL=32ΩBW= 20Hz – 20 kHz
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Measurement results: Spectrum at different output power
PO=20mWFin=1kHz
PO=1mWFin=1kHz
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Performance summary and comparison
Parameter This workJSSC 09
[1]
ESSCIRC 06
[2]
Technology 65nm 130nm 65nm
Supply voltage±1.4V
±0.35V
±1V
±0.6V2.5V
Quiescent power (per
channel)0.41mW 1.2mW 12.5mW
Peak load power (16Ω) 90mW 40mW 53.5mW
THD+N @ PRMS (32Ω)-80dB @
16mW
-84dB @
10mW
-68dB @
27mW (16Ω)
SNR A-weighted 101dB92dB (un-
weighted)-
FOM=
Peak load power
Quiescent power
219.5 33.3 4.328
Performance summary and comparison
Parameter This workMAX9725
[3]
TPA6141
[4]
LM48824
[5]
Supply voltage
1.4V with two
charge pumps + 1
buck
1.5V with a
charge pump
3.6V with 1
charge pump +
1 buck
3.6V with 1
charge pump +
1 buck
Quiescent power (per
channel)
0.41mW + 0.5mW
(2 CPs + 1 buck)1.57mW 2.16mW 1.62mW
PSUP @ PL=0.1mW 0.87mW + 0.6mW - 4.5mW 3.24mW
PSUP @ PL=0.5mW 1.63mW + 0.8mW - 7.2mW 5.58mW
Peak load power
(16Ω)90mW 50mW 50mW 74mW
THD+N @ PRMS
(32Ω)-80dB @ 16mW -84dB @12mW -80dB @20mW -69dB@20mW
SNR A-weighted 101dB 92dB 105dB 102dB
FOM=
Peak load power
Quiescent power
90 31.8 23.2 45.629
Outline
• Headphone amplifier(Class-AB, Class-D, Class-G PROs and CONs)
• Class-G headphone driver (architecture, switching principle, distortion analysis)
• Prototype in 65nm CMOS technology(implementation, results, comparison)
• Conclusions30
Conclusions
• A class-G headphone driver has been presented. It shows 50% less power consumption than the competitors.
• The class-G amplifier is very suitable in low voltage systems which require high efficiency and low distortion.
• A class-G headphone prototype with charge pumps and a buck converter is in progress
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References
[1] Vijay Dhanasekaran; Jose Silva-Martinez; Edgar Sanchez-Sinencio, "Design ofThree-Stage Class-AB 16Ohm Headphone Driver Capable of Handling Wide Range of Load Capacitance," Solid-State Circuits, IEEE Journal of , vol.44, no.6, pp.1734-1744, Jun 2009.
[2] P. Bogner, H. Habibovic and T. Hartig, ‘‘A High Signal Swing Class AB EarpieceAmplifier in 65nm CMOS Technology,’’ Proc. ESSCIRC, pp.372-375, 2006.
[3] Maxim, ‘‘1V, Low-Power, DirectDrive, Stereo Headphone Amplifier with Shutdown,’’ Rev. 3; 8/08, accessed on Jul. 7, 2009 < http://datasheets.maximic.com/en/ds/MAX9725.pdf>
[4] Texas Instrument, ‘‘Class-G Directpath Stereo Headphone Amplifier,’’ 3/09, accessed on Jul. 7, 2009 < http://focus.ti.com/lit/ds/symlink/tpa6141a2.pdf>
[5] National Semiconductor ”Class G Headphone Amplifier with I2C Volume Control,” August 31,2009, accessed on Jan. 25, 2010 < http://www.national.com/ds/LM/LM48824.pdf >
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