A 900MHz Doherty Amplifier Implemented with Lumped Elements

1. A 900MHz Doherty Amplifier Implemented with Lumped Elements Y. Zhao, M. Iwamoto, D. Kimball, L. Larson, P. Asbeck University of California, San Diego

2. Introduction • To increase overall efficiency of RF PA • Use Doherty structure • To shrink circuit size for integration • Use lumped elements to replace transmission lines • To achieve good linearity • Use DSP to control Doherty Amplifier

3. Agenda • Doherty amplifier architecture • Doherty with lumped elements • Design issues • Simulations • Measurements • DSP application in Doherty amplifier • DSP control mechanism • Measurement Results • Conclusions

4. Iout Load-line of main amplifier High power operation Vout Low power operation Doherty Architecture • Low power range • only Main-PA operates • High power range • Main-PA goes into saturation • Aux-PA turns on

5. Lumped Doherty --- Design issues • Size of 90° transmission line • 900M --- 48mm • 2.4G --- 19mm • 5.2G --- 8.7mm • Size of a general PA package 4mm x 4mm Er = 3.48 • Design requirements • Impedance inversion • Phase compensation

6. Lumped Doherty --- Design issues • C-L-C (9.1pF, 2.7nH) • Impedance inverter • 90 degrees delay • Provide VDD to Aux-PA • Harmonic trap • L-C-L (1.8nH, 15pF, 3.3nH) • 90-degree phase shifter • part of the matching • DC block • L-match (13pF, 2.2nH) • Output impedance match • DC block

7. Lumped Doherty --- Measurement • A GaAs MESFET-based hybrid Doherty • amplifier was built with lumped elements Drain efficiency, PAE and Gain Bandwidth S21 1dB/div 1dB Bandwidth ~ 110MHz Frequency 50MHz/Div • CDMA needs 25MHz/836MHz • 802.11b,g need 83MHz/2.44GHz

8. Lumped Doherty --- Measurement PAE vs. Pout Lumped Doherty vs. a simulated Class AB IS-95 CDMA PDF Lumped Doherty – 14.1% > 3X Class AB – 4.4%

9. Comparison ---Lumped and distributed Similar PAE performance Slightly narrower bandwidth for lumped Doherty • ADS simulation • Same devices and design • Check 1dB bandwidth/900MHz • the lumped Doherty 130MHz • the distributed Doherty 150MHz

10. Circuit area Quarter-wave lines Comparison ---Lumped and distributed • Size reduction by using lumped elements is dramatic • Connection lines and components in hybrid circuit can be further shrunk in a IC module schematic

11. Potential problem --- Linearity Non-ideal gain and phase in high power region can cause a linearity problem For CDMA signal, ACPR may not be good enough

12. DSP Linearization Strategy • Gain control • Change Vgg2 according to the instantaneous envelope of the input CDMA signal • Phase control • Baseband phase predistortion • Open loop control with lookup table System block diagram “Doherty Amplifier with DSP Control to Improve Performance in CDMA Operation,” 2003 IEEE MTT-S Digest, p687-690

13. Measurement --- linearized IS-95 signal -42dBc 10dB/Div • High efficiency is maintained • ACPR improvement of up to 9dB is achieved; CDMA ACPR specifications are met • Peak power is limited by the device size uncorrected DSP corrected 0.5MHz/Div

14. Conclusions Conclusions • Doherty amplifier helps achieve high efficiency over wide output power range • Doherty amplifier with lumped elements shrinks circuit size while maintaining good performance • Linearity of the Doherty amplifier can be substantially improved by DSP to meet the CDMA ACPR specification • A promising approach for CDMA handset and other wireless applications such as 802.11