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The Doherty Power Amplifier –1936 to the Present Day

Ray Pengelly,

Prism Consulting NC, LLC

Hillsborough, NC 27278 USA

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Summary

• Early History

• Broadcast Transmitters

• Handset Transmitters

• Cellular Infrastructure Transmitters

• Modern Trends

• Conclusions

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Employed by Western Electric and Bell Laboratories

Early Days of Doherty Amplifiers

AM Radio Transmitters• Invented in 1936• Patented in 1940 • Improved ConversionEfficiency by a factor of2 to over 60%Technique adopted byWestern Electric, RCA, Continental and Marconi

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Evolution of Tube-Based Doherty Amplifiers

1938 - 50 KWatts

1978 - 150 Kwatts (BBC – UK)

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Chronology of Tube-Based Doherty Amplifiers

Installation Year

Frequency Power Level Territory

1936 to 1940 AM, 30 KHz to 3 MHz

50 KW (500 KW total)

USA

1953 AM, 30 KHz to 300 KHz

500 KW (1 MW total)

Europe (Voiceof America)

1956 AM, 30 KHz to 300 KHz

1 MW East Asia (Voice of America)

1978 AM, 300 KHz to 3 MHz

150 KW (5 MW total)

UK (BBC)

1979 AM, 300 KHz to 3 MHz

2 MW (16 MW total)

Middle East

All Doherty amplifiers were water-cooled requiring major infrastructure builds in certain countries such as the Middle East

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Solid-State Doherty PA’s for Broadcast Transmitters

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Motivations for High Efficiency Solutions for Broadcast

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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

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Typical Performance of Si LDMOSFET Doherty Amplifier used for Broadcast Applications

Freescale MRF6VP3450 Transistor - Peak Power: 500 watts @ 720 MHz

Doherty PA Efficiencyis maintained at>53% over 3 dBback-off

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4 - Up MRF6VP3450H Doherty Modules for Broadcast

Courtesy of Freescale and Egatel

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Example of State-of-the-Art Wideband Broadcast Doherty PA

“ The key achievement and importance to industry lies in the fact that these results wereachieved using standard 50V LDMOS devices with an easy to implement passive inputsplitter. This approach allows low-cost energy-efficient high-power wideband amplifierimplementations and easy system integration. The realized wideband DPA demonstratorcovers the entire UHF TV band (470-803MHz) with an average efficiency of 43% whilemaintaining peak power capability greater than 700-W over the entire band. Thisdemonstrator offered 15-20% more efficiency than the currently used wideband class-ABpower amplifiers used in broadcast transmitter systems” (Courtesy - NXP and DelftUniversity of Technology)

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Solid-State Doherty PA’s for Handset Transmitters

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IMS2015L-Band Iridium Satellite Handset Doherty Power Amplifiers

Balanced Doherty PA produced7 watts peak power

Each Doherty PA provided> 48% drain efficiency

Hybrid Chip and WireConstruction – GaAs pHEMTTechnologyCirca 1995 - courtesy of

Mike Gaynor (Ex-Motorola)

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Challenges for and technologies used in Handset Doherty PA’s

• 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

Conventional DPA

Series Type DPA

e.g. 836.5 MHz MMIC DPA in 2mm InGaP/GaAsLow power mode: 40% efficiency at 23 dBm outHigh power mode: 38% efficiency at 28 dBm out Courtesy of

POSTECH, S. Korea

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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 quarter-

wave 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

Gain > 28 dBAverage Output Power – 27.5 dBmPAE – 36%10 MHz BW LTE signal with 7.5 dB PAREVM – 3.8%ACLR of -32 dBc

Courtesy of POSTECH and Samsung,South Korea

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Solid-State Doherty PA’s for CellularInfrastructure Transmitters

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IMS2015Cellular Remote Radio Head Doherty Power Amplifier Examples - the de-facto standard

Nokia - 2100 MHz150 Watts

Ericsson - 2100 MHz 80 Watts

Huawei - 850 MHz60 Watts

All units averaged > 40%overall efficiency

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Modern Trends in Doherty Power Amplifiers

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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

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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

Example of LDMOS FET asymmetric DPACarrier: 140 Watts2 Peakers: 280 Watts2.11 to 2.14 GHz 15 dB GainPave = 75 watts @8 dB back-off; 46% efficiency with 3G W-CDMACourtesy of Freescale, 2010

Example of GaN HEMTAsymmetric DPACarrier: 150 WattsPeaker: 300 Watts2.5 to 2.7 GHz14 dB GainPave = 80 watts @8 dB back-off 49% efficiency with 4G LTECourtesy of Cree, 2014

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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

Example of 4-Way DPA using 25 Watt Cree HEMTsOutput Power = 100 WattsPeak Efficiency = 77%

Single Carrier W-CDMA Signal at 2.14 GHz with a PAR of 6.5 dB

Drain Efficiency = 61%Average Power = 20 WattsACLR = -31 dBc

Courtesy of Bell Labs,Alcatel Lucent

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Multi-Band, Broadband and Re-Configurable DPA’s• Multi-band, broadband and re-

configurable 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

Example of Tri-Band DPA for 1.8 to 2.7 GHz

Power Gain > 11 dBPave = 38 dBmPAR = 6.5 dBEfficiencies:58% at 1.85 GHz50% at 2.15 GHz42% at 2.65 GHzCourtesy of Bell Labs, Alcatel Lucent

Example of Reconfigurable DPA for1.9 to 2.6 GHz using MEMS switches

(Mohamed, Boumaiza & Mansour, 2013)

Power Gain > 11 dBPave = 32 dBmI/P back-off = 9 dBEfficiencies:60% at 1.9 GHz61% at 2.14 GHz64% at 2.6 GHz

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GaN HEMT Doherty PA Efficiencies versus Time

0

10

20

30

40

50

60

70

80

1995 2000 2005 2010 2015 2020 2025

PA Drain Efficiency, %

Year

PA Drain Efficiency, % -- Assumes PAR of 7.5 dB and frequencies between 2.1 and 2.7 GHz

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Conclusions

• DPA’s are extensively used in broadcast transmitters and cellular base-stations – 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!

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Thank you

For further details contact raypengelly@gmail.com