PA%20Workshop

1
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, San Diego, CA

2
Presentation Outline

• Research Goals
• Proposals and Implementation
• Standard Bias Techniques
• Dynamic Current Biasing
• Dynamic Voltage Biasing
• Status and Future Work
• Conclusion

3
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 ( gt 21dB)
• Low Noise Figure (10dB)
• Small Error Vector Magnitude (EVMlt10)
• Adjacent Channel Leakage Ratio (ACLRgt33dB for
  5MHz)

4
Standard Biasing Techniques
A

• Constant current biasing

• The DC current does NOT change as RF power
  increases

5
Standard Biasing of PA Constant Current Biasing

• 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 base-emitter voltage
  results in a degradation of the linearity of the
  amplifier because the current would clip at lower
  input power.

6
Standard Biasing of PA Constant Voltage Biasing

• Constant Voltage Biasing
• 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 the same time, compared to a power
  amplifier always burning large bias current,
  constant voltage bias scheme also saves much
  power.
• Good enough? No!

7
Dynamic Current and Voltage Biasing Principle

• Previous Researches
• DCB Conexant, RFMD
• DVB UCSD (Prof. P. Asbeck)
• Motorola (J. Staudinger)
• Challenges
• DCB gain change
• DVB gain change, cost, chip size (dc-dc
  converter)

• Dynamic Biasing Strategies DCB, DVB, and DCBDVB
• (DCB means Dynamic Current Biasing DVB means
  Dynamic voltage biasing.)

8
Dynamic Current Biasing Gain and Phase Issue

• How does gain and phase change?

Idc
PA
QPSK
(a)
(b)
9
Dynamic Current Biasing Gain and Phase Issue

• How do the gain and phase change?
• Gain change
• Constant of devices gain change gt 8 dB
• Dynamically switching devices gain change lt 1 dB
• Phase change
• Due to Cjc, Cpi and gm with bias current

Simplified BJT model
10
Dynamic Current Biasing - Implementation

• Dynamically switch number of transistors fingers
  and change the bias current
• ? This keeps the gain more constant _at_ low bias
  current

Circuit Schematic of Dynamic Current Biasing
11
Dynamic Current Biasing Simulation Results
DC Current and Gain under different bias
conditions

• constant biass average efficiency 2.4
• dynamic biass average efficiency 5.8
  (improved by 140 !!!)
• gain almost constant !!!

12
Dynamic Voltage Biasing - General Principle
High-power
low-power
Schematic of Series Connected Voltage Reduction
Topology
13
Dynamic Voltage Biasing - Implementation
- Schematic of Output Stage with hi-power and
low-power groups
14
Dynamic Voltage Biasing Simulation Results
Pe

• (Courtesy P. Asbeck)

ClassAB
1stepDCB
1stepDVB
Ideal DCB
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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
16
Status and Future Work

• Dynamic current biasing chips (IBM 6HP) are being
  tested.
• Send full dynamic voltage biasing chips to
  foundry by October.

17
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!!!