Center for Wireless Communications 1 ©Deng, Larson and Gudem, May 16th, 2003 PA Workshop High Efficiency SiGe BiCMOS WCDMA Power Amplifiers With Dynamic

Center for Wireless Communications 1©Deng, Larson and Gudem, May 16th, 2003

PA Workshop

High Efficiency SiGe BiCMOS WCDMA

Power Amplifiers

With Dynamic Biasing Techniquesby

Junxiong Deng1, Prasad Gudem2, Larry Larson1, Peter Asbeck1

1University of California at San Diego2Qualcomm, 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

10*Rbias Rbias

RFin

x/10 x

x/10

Vcc Vcc

Vcc

RFin

x

Constant current biasing

A

The DC current does NOT change as RF power increases


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.

CI

BEV

inRF

120 mA

BEQV

0.8V

SP

CQI

120 mA


Standard Biasing of PA – Constant Voltage Biasing

RFi n

x

Ll arge

Vbb

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!

CI

BEV

inRF

450

mA120 mA

BEQV

0.8V

SP

CQI

120 mA


Dynamic Current and Voltage Biasing Principle

Ic

VceVmax

Imax

DVB

DCBDCB+DVB

Ic.bias

Vce.bias

A

Vmin

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?

PA

Idc

QPSK

G

(a) (b)


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

Gai

n (

dB

)

Ic (mA)

> 8

dB

switching devices

constant #of devices

B

E

C

Rb

Cpi Rpi

Cjc

gm*Vpiro

Simplified BJT model

Dynamic Current Biasing – Gain and Phase Issue


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 - Implementation

NFET switchVcon

Vcom

Vcon

NFET switchVcon

Vcom

Vcon

NFET switchVcon

Vcom

Vcon


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

OutputMatchingNetwork

RFin

RFout

S1

S1

S2

RFin

RFoutOMN

RFin

RFoutOMN

High-power

low-power

Schematic of Series Connected Voltage Reduction Topology


Dynamic Voltage Biasing - Implementation

RFin

RFoutOMN

100xdevices

`Vcc

Vcc

Vcc/2

20xdevices

IMN

Group A

Group B

NFET switch

Vcon

Vcon

Vcon

Vcon

CVB

CVB

CCB

- Schematic of Output Stage with

hi-power and low-power groups


Dynamic Voltage Biasing – Simulation Results

(Courtesy P. Asbeck)

( )

( )

( )

out out out

out out out

out

P p P dP

P p P dP

P

Pe (%) and Idc (mA) vs. Pout (dBm)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24

Pout (dBm)

Pe

(%

)

0

100

200

300

400

500

600

Idc

(mA

)

Pe

ClassAB

Ideal DCB 1stepDVB1stepDCB


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


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%!!!

Documents

Center for Wireless Communications 1 ©Deng, Larson and Gudem, May 16th, 2003 PA Workshop High Efficiency SiGe BiCMOS WCDMA Power Amplifiers With Dynamic