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
Presentation Outline • Research Goals • Proposals and Implementation • Standard Bias Techniques • Dynamic Current Biasing • Dynamic Voltage Biasing • Status and Future Work • Conclusion
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)
Standard Biasing Techniques Constant current biasing A • The DC current does NOT change as RF power increases
120 mA 120 mA 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.
120 mA 450 mA 120 mA 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!
Dynamic Current and Voltage Biasing Principle • Dynamic Biasing Strategies: DCB, DVB, and DCB+DVB (DCB means Dynamic Current Biasing; DVB means Dynamic voltage biasing.) 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 Current Biasing – Gain and Phase Issue • How does gain and phase change? Idc PA QPSK (a) (b)
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 Phase change: • Due to Cjc, Cpi and gm with bias current Simplified BJT model
Dynamic Current Biasing - Implementation • Dynamically switch number of transistors’ fingers and change the bias current This keeps the gain more constant @ low bias current Circuit Schematic of Dynamic Current Biasing
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 !!!
Dynamic Voltage Biasing - General Principle High-power low-power Schematic of Series Connected Voltage Reduction Topology
Dynamic Voltage Biasing - Implementation - Schematic of Output Stage with hi-power and low-power groups
Dynamic Voltage Biasing – Simulation Results (Courtesy P. Asbeck) Pe ClassAB 1stepDCB 1stepDVB Ideal DCB
Dynamic Voltage Biasing – Simulation Results • Average Efficiency Comparison This shows dramatic improvements compared to class AB
Status and Future Work • Dynamic current biasing chips (IBM 6HP) are being tested. • Send full dynamic voltage biasing chips to foundry by October.
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%!!!