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

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 dszlbndfn

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

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

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 PA Book about bandwidth (lumped, transmission line) Design requirements Impedance inversion Phase compensation

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

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

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%

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

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 Circuit area Quarter-wave lines

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 For example, Our GaAs MESFET-based Doherty amplifier has very high measured efficiency, but look at the gain and phase…… BTW,This is from a conventional Doherty with transmission lines

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 Previously,we have done some work on DSP controlled Doherty to improve linearity, it’s done on a conventional Doherty with transmission line, but should be also available to lumped Doherty “Doherty Amplifier with DSP Control to Improve Performance in CDMA Operation,” 2003 IEEE MTT-S Digest, p687-690

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

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