TH1-E1 The Doherty Power Amplifier 1936 to the Present Day Ray Pengelly, Prism Consulting NC, LLC Hillsborough, NC 27278 USA 1
Summary Early History Broadcast Transmitters Handset Transmitters Cellular Infrastructure Transmitters Modern Trends Conclusions 2
Early Days of Doherty Amplifiers IMS2015 AM Radio Transmitters Invented in 1936 Patented in 1940 Improved Conversion Efficiency by a factor of 2 to over 60% Technique adopted by Western Electric, RCA, Continental and Marconi Employed by Western Electric and Bell Laboratories 3
Evolution of Tube-Based Doherty Amplifiers IMS2015 1938-50 KWatts 1978-150 Kwatts (BBC UK) 4
Chronology of Tube-Based Doherty Amplifiers Installation Year 1936 to 1940 AM, 30 KHz to 3 MHz 1953 AM, 30 KHz to 300 KHz 1956 AM, 30 KHz to 300 KHz 1978 AM, 300 KHz to 3 MHz 1979 AM, 300 KHz to 3 MHz Frequency Power Level Territory 50 KW (500 KW total) 500 KW (1 MW total) USA Europe (Voice of America) 1 MW East Asia (Voice of America) 150 KW (5 MW total) 2 MW (16 MW total) UK (BBC) Middle East All Doherty amplifiers were water-cooled requiring major infrastructure builds in certain countries such as the Middle East 5
Solid-State Doherty PA s for Broadcast Transmitters 6
Motivations for High Efficiency Solutions for Broadcast IMS2015 7
Solid-State Doherty PA s for Broadcast Transmitters Replacing IOTs (Inductive Output Tubes) at power levels up to 50 KW Solid-state transistors used for both Digital Radio and Digital TV Transmitters mainly in UHF range Amplifiers dominated by Silicon LDMOSFET output stages today For digital TV in particular The characteristics of the carrier and peaking amplifiers change as the RF input changes, so pre-correction has to compensate for this to avoid creating large amounts of distortion and spurious spectral emissions Picture courtesy of Rohde and Schwarz 8
Typical Performance of Si LDMOSFET Doherty Amplifier used for Broadcast Applications Doherty PA Efficiency is maintained at >53% over 3 db back-off Freescale MRF6VP3450 Transistor - Peak Power: 500 watts @ 720 MHz 9
4 - Up MRF6VP3450H Doherty Modules for Broadcast IMS2015 Courtesy of Freescale and Egatel 10
Example of State-of-the-Art Wideband Broadcast Doherty PA IMS2015 The key achievement and importance to industry lies in the fact that these results were achieved using standard 50V LDMOS devices with an easy to implement passive input splitter. This approach allows low-cost energy-efficient high-power wideband amplifier implementations and easy system integration. The realized wideband DPA demonstrator covers the entire UHF TV band (470-803MHz) with an average efficiency of 43% while maintaining peak power capability greater than 700-W over the entire band. This demonstrator offered 15-20% more efficiency than the currently used wideband class-ab power amplifiers used in broadcast transmitter systems (Courtesy - NXP and Delft University of Technology) 11
Solid-State Doherty PA s for Handset Transmitters 12
L-Band Iridium Satellite Handset Doherty Power Amplifiers Balanced Doherty PA produced 7 watts peak power IMS2015 Circa 1995 - courtesy of Mike Gaynor (Ex-Motorola) Each Doherty PA provided > 48% drain efficiency Hybrid Chip and Wire Construction GaAs phemt Technology 13
Challenges for and technologies used in Handset Doherty PA s IMS2015 MMIC DPA s using 0.25 and 0.15mm GaAs phemt processes have been deployed at 17 and 20 GHz respectively for digital satellite systems MMIC DPA s using 0.13mm RF CMOS process have been deployed at 60 GHz for wireless personal network transceivers But MMIC DPA s for 900 MHz, 1800 MHz, 2140 MHz etc.. handset applications are more difficult to design because of low cost and chip size constraints CMOS and HBT processes are popular semiconductor technologies Lumped elements to reduce size Novel circuit approaches to reduce size e.g. 836.5 MHz MMIC DPA in 2mm InGaP/GaAs Low power mode: 40% efficiency at 23 dbm out High power mode: 38% efficiency at 28 dbm out Conventional DPA Series Type DPA Courtesy of POSTECH, S. Korea 14
Latest MMIC DPA s for LTE Handsets Novel circuit approaches used to produce small handset DPA s have been recently extended to provide solutions for LTE applications across bandwidths of 1.6 to 2.1 GHz MMICs use low Q quarterwave transformers; lumped element phase compensation networks on the input and incorporation of transistor output capacitances into phase compensation networks on the output InGaP/GaAs 2mm HBT process Off-chip bond wires for critical inductors IMS2015 Gain > 28 db Average Output Power 27.5 dbm PAE 36% 10 MHz BW LTE signal with 7.5 db PAR EVM 3.8% ACLR of -32 dbc Courtesy of POSTECH and Samsung, South Korea 15
Solid-State Doherty PA s for Cellular Infrastructure Transmitters 16
Cellular Remote Radio Head Doherty Power Amplifier Examples - the de-facto standard IMS2015 Nokia - 2100 MHz 150 Watts Huawei - 850 MHz 60 Watts All units averaged > 40% overall efficiency Ericsson - 2100 MHz 80 Watts 17
Modern Trends in Doherty Power Amplifiers 18
Modern Trends - Summary Wealth of semiconductor technologies being used RF CMOS, GaAs phemt, InGaP/GaAs HBT, Si LDMOS and GaN HEMT Range of frequencies and power levels being covered UHF to 60 GHz, 100 s of mw s to KW s Variety of circuit techniques have been adopted Classical Inverted Asymmetric unequal power division to carrier and peaker; different sized transistors for carrier and peaker and combinations of both N-Way to increase backed-off (linear) power range for high efficiency Use of different classes of amplifiers in carrier and peaker Multi-band (dual and tri-band) Broadband Reconfigurable using switched sections or varactor tuning 19
Asymmetric DPA s For power back-off levels greater than 6 db, further improvements in efficiency are possible with asymmetric Doherty topologies where the Carrier and Peaking amplifiers exhibit unequal power capability Implementation requires unequal device geometries for the carrier and peaking amplifiers This asymmetry introduces difficulties in designing proper input/output matching structures for the peaking/carrier amplifiers IMS2015 Example of LDMOS FET asymmetric DPA Carrier: 140 Watts 2 Peakers: 280 Watts 2.11 to 2.14 GHz 15 db Gain Pave = 75 watts @ 8 db back-off; 46% efficiency with 3G W-CDMA Courtesy of Freescale, 2010 Example of GaN HEMT Asymmetric DPA Carrier: 150 Watts Peaker: 300 Watts 2.5 to 2.7 GHz 14 db Gain Pave = 80 watts @ 8 db back-off 49% efficiency with 4G LTE Courtesy of Cree, 2014 20
N-Way DPA s N-Way DPA s have more than one peaking amplifier Number of efficiency peaking points at backed-off power is directly proportional to the number of peaking stages Optimized transistor sizes to maximize backed-off efficiency e.g. 1:3:4 for a 3- way DPA for 12 db back-off Disadvantage of N-Way is lower gain because of N-Way input splitter IMS2015 Example of 4-Way DPA using 25 Watt Cree HEMTs Output Power = 100 Watts Peak Efficiency = 77% Single Carrier W-CDMA Signal at 2.14 GHz with a PAR of 6.5 db Drain Efficiency = 61% Average Power = 20 Watts ACLR = -31 dbc Courtesy of Bell Labs, Alcatel Lucent 21
Multi-Band, Broadband and Re-Configurable DPA s Multi-band, broadband and reconfigurable DPA s provide solutions for next generation small-cell through macro-cell base-station transmitters Need to be able to cover multiple carrier frequencies in the 800 to 3500 MHz bands Need to be able to cover different signal standards with multiple PAR s IMS2015 Example of Reconfigurable DPA for 1.9 to 2.6 GHz using MEMS switches (Mohamed, Boumaiza & Mansour, 2013) Power Gain > 11 db Pave = 32 dbm I/P back-off = 9 db Efficiencies: 60% at 1.9 GHz 61% at 2.14 GHz 64% at 2.6 GHz Example of Tri-Band DPA for 1.8 to 2.7 GHz Power Gain > 11 db Pave = 38 dbm PAR = 6.5 db Efficiencies: 58% at 1.85 GHz 50% at 2.15 GHz 42% at 2.65 GHz Courtesy of Bell Labs, Alcatel Lucent 22
GaN HEMT Doherty PA Efficiencies versus Time PA PA Drain Efficiency, % -- % Assumes PAR of 7.5 db and frequencies between 2.1 and 2.7 GHz 80 70 60 50 40 30 20 10 0 1995 2000 2005 2010 2015 2020 2025 Year 23
Conclusions DPA s are extensively used in broadcast transmitters and cellular basestations not so much in handsets Innovative circuit designs to improve efficiencies at high backed-off power levels are becoming more common Broadband and multi-band DPA s have been successfully demonstrated for the latest 5G wireless requirements MMIC DPA s are today deployed from < 30 MHz to > 60 GHz! Doherty Power Amplifiers, after 80 years, are alive and well! 24
Thank you For further details contact raypengelly@gmail.com 25