Transcript Junxiong Deng, Prasad Gudem, Lawrence

PA Workshop
High Efficiency SiGe BiCMOS WCDMA
Power Amplifiers
With Dynamic Biasing Techniques
by
Junxiong Deng1, Prasad Gudem2, Larry Larson1, Peter Asbeck1
1University
of California at San Diego
2Qualcomm,
Center for Wireless
Communications
San Diego, CA
©Deng, Larson and Gudem, May 16th, 2003
1
Presentation Outline
 Research Goals
 Proposals and Implementation



Standard Bias Techniques
Dynamic Current Biasing
Dynamic Voltage Biasing
 Status and Future Work
 Conclusion
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©Deng, Larson and Gudem, May 16th, 2003
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Research Goals
 SiGe BiCMOS Power Amplifiers for WCDMA applications
with far higher efficiency than existing approaches

Good Average Efficiency

Although peak efficiency is more than 50%
for class AB, but average efficiency is only 2%

High Output Power (+27dBm)

High Power Gain ( > 21dB)

Low Noise Figure (10dB)

Small Error Vector Magnitude (EVM<10%)

Adjacent Channel Leakage Ratio (ACLR>33dB for 5MHz)
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©Deng, Larson and Gudem, May 16th, 2003
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Standard Biasing Techniques
Vcc
Vcc
Vcc
x/10
10*Rbias
x/10
x
Rbias
x
A
RFin
RFin
Constant current biasing

The DC current does NOT change as RF power increases
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©Deng, Larson and Gudem, May 16th, 2003
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Standard Biasing of PA – Constant Current Biasing
IC
VBEQ
I CQ
0.8V
120mA
VBE
120mA
PS
RFin
Constant current biasing: Poor Linearity!
•
The key here is to realize that the average collector currents needs to
remain constant. Therefore the quiescent base-emitter voltage falls as
shown in the figure on the left. Note that the drop in quiescent baseemitter voltage results in a degradation of the linearity of the amplifier
because the current would clip at lower input power.
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©Deng, Larson and Gudem, May 16th, 2003
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Standard Biasing of PA – Constant Voltage Biasing
Vbb
IC
Llarge
x
450 mA
120 mA
VBE
RFin
RFin

Constant Voltage Biasing


V
BEQ
The advantage is that the DC current increases as the
RF power increases. Due to this increase in DC
current the linearity of the RF amplifier improves. At
0.8V
the same time, compared to a power amplifier
always burning large bias current, constant voltage
bias scheme also saves much power.
Good enough? No!
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©Deng, Larson and Gudem, May 16th, 2003
I CQ
120 mA
PS
6
Dynamic Current and Voltage Biasing Principle
Ic
Imax
Previous Researches:
DVB
Ic.bias

DCB: Conexant, RFMD

DVB: UCSD (Prof. P. Asbeck)
Motorola (J. Staudinger)
A
Challenges:
DCB+DVB
Vmin

DCB
Vce.bias

DCB: gain change

DVB: gain change, cost, chip
size (dc-dc converter)
Vmax Vce
Dynamic Biasing Strategies: DCB, DVB, and DCB+DVB
(DCB means Dynamic Current Biasing; DVB means Dynamic
voltage biasing.)
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©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Current Biasing – Gain and Phase Issue

How does gain and phase change?
Idc
PA
G

QPSK
(a)
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(b)
©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Current Biasing – Gain and Phase Issue
How do the gain and phase change?
Gain change:
• Constant # of devices: gain change > 8 dB;
• Dynamically switching devices: gain
change < 1 dB
constant #
of devices
> 8dB
switching devices
Gain (dB)

Ic (mA)
Phase change:
• Due to Cjc, Cpi and gm with bias current
C
B
Cjc
Rb
Cpi
ro
Rpi
gm*Vpi
E
Simplified BJT model
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©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Current Biasing - Implementation

Dynamically switch number of transistors’ fingers and change the bias current
 This keeps the gain more constant @ low bias current
NFET switch
NFET switch
NFET switch
Vcon
Vcon
Vcom
Vcom
Vcon
Vcom
Vcon
Vcon
Vcon
Circuit Schematic of Dynamic Current Biasing
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©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Current Biasing – Simulation Results
DC Current and Gain under different bias conditions
 constant bias’s average efficiency: 2.4%
 dynamic bias’s average efficiency: 5.8% (improved by 140% !!!)
 gain almost constant !!!
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©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Voltage Biasing - General Principle
OMN
High-power
S1
RFout
RFin
Output
Matching
Network
S2
RFout
S1
RFin
low-power
OMN
RFout
RFin
Schematic of Series Connected Voltage Reduction Topology
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©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Voltage Biasing - Implementation
Vcc
`
NFET switch
Group A
Vcon
Vcon
CVB
100xdevices
RFout
OMN
IMN
Vcc
RFin
Vcc/2
Vcon
CCB
Group B
- Schematic of Output Stage with
Vcon
20xdevices
hi-power and low-power groups
CVB
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©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Voltage Biasing – Simulation Results
Pe (%) and Idc (mA) vs. Pout (dBm)
5
600
Pe
4.5
500
4
3.5
300
Idc (mA)
Pe (%)
3
2.5
P

  
P

out
400
p( Pout )dPout
out
p( Pout )dPout
 ( Pout )
2
200
(Courtesy P. Asbeck)
1.5
ClassAB
1
100
0.5
1stepDCB
Ideal DCB
0
-24
-20
-16
-12
Center for Wireless
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-8
-4
0
4
Pout (dBm)
8
1stepDVB
0
12
16
20
24
©Deng, Larson and Gudem, May 16th, 2003
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Dynamic Voltage Biasing – Simulation Results

Average Efficiency Comparison
Bias Type
Average Efficiency
Class AB
2.44%
Class AB w/ 1 step DCB
5.88%
Class AB w/ Ideal DCB
7.36%
Class AB w/ 1 step DVB
7.75%
Class AB w/ 2 step DVB
8.94%
 This shows dramatic improvements compared to class AB
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©Deng, Larson and Gudem, May 16th, 2003
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Status and Future Work

Dynamic current biasing chips
(IBM 6HP) are being tested.

Send full dynamic voltage
biasing chips to foundry by
October.
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Communications
©Deng, Larson and Gudem, May 16th, 2003
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Conclusions

In summary, research has been carried out to improve average
power efficiency of WCDMA PA, by doing dynamic current and
voltage biasing.

The key point of our research is to keep power gain to be constant
while improving average power efficiency. Simulation results
have indicated the validity and effectiveness of the proposed
method. Measurement results will come out soon.

With our dynamic biasing techniques, average power efficiency
can be improved by more than 200%!!!
Center for Wireless
Communications
©Deng, Larson and Gudem, May 16th, 2003
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