![Page 1: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/1.jpg)
Introduction to RF PA design
Presented by Dr. Chris [email protected]
Copyright © 2018 Chris Potter, Cambridge RF, U.K. All rights reserved
Power Amplifier Techniques WorkshopCambridge Wireless
22nd May 2018
![Page 2: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/2.jpg)
Contents
◼ Generic PA transistor
◼ Achieving power amplification
◼ Application types
◼ Considerations of carrier frequency, power level
◼ Modulation waveform and how it affects PAPR
◼ Key specs
◼ Noise, distortion, efficiency
◼ Circuit and Topology choices
◼ Class A, AB, C, Balanced circuits
◼ Device Technology Overview
◼ Design methods
![Page 3: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/3.jpg)
Generic PA Transistor
◼ A control current or voltage causes a greater current swing at the output
◼ A bias circuit holds the operating point at a desired part of the linear range
◼ Optimisation of output current-voltage relationship (load line)
◼ Minimisation and control of parasitics
![Page 4: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/4.jpg)
Applications of PAs
Application Power Technology
TV Broadcast 100W x many in parallel
LDMOS, GaN
Cellular Base-station
10 to 50W LDMOS, GaN, GaAs FET
Cellular Handset
0.25 to 2W GaAs, Si, SiGe
WiFi 0.1W GaAs HBT, Si
Bluetooth, ZigBee, LoRa
0.001 – 0.1W Si
![Page 5: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/5.jpg)
Device Technology Overview
◼ Differing parasitics and thermal conductivity characteristics
0.1W
1W
10W
100W
1000W
1000W
1GHz 3GHz 10GHz 30GHz
SiC MESFET
GaAs HEMT
Si MOS, LDMOS
GaN
GaAs HBT
Tubes, TWT
![Page 6: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/6.jpg)
Modulation and PAPR
Technology Modulation PAPR
CW Sinewave 0dB
2-tone 2x sinewaves 3dB
GSM, DECT, Bluetooth 1.0
GMSK, GFSK 0dB
Bluetooth EDR, TETRA
QPSK, 8PSK 3.2dB
3GPP-UTRA WCDMA 11.2dB
3GPP-EUTRA LTE (OFDM) 11.5dB
802.11n OFDM 11.5dB
DVB-T OFDM 11.5dB
![Page 7: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/7.jpg)
PA Specs: Efficiency
◼ PAs are most efficient when operating near maximum output power
◼ Drain Efficiency is the ratio of RF output power over DC input power
◼ Power-Added Efficiency subtracts RF input power from DE calculation
◼ For signals with high crest factor, the PA is backed-off and operates at lower efficiency most of the time
%100,
%100,
−
=−
=
inDC
inRFoutRF
inDC
outRF
P
PPPAEEfficiencyAddedPower
P
PDEEfficiencyDrain
![Page 8: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/8.jpg)
PA Specs: Distortion
◼ Two input tones presented to a device with non-linearity will create many other frequency products
f1 f2
f1+f2
f2-f1
2f1+f2
2f1 2f2 3f1 3f2
2f1-f2 2f2-f1
2f2+f1
Third-order distortion products
Intermodulation distortion (IM3)
Third-order distortion products
Second-order distortion products
Frequency
Am
plitu
de
( ) ( )
( )( ) ( )( ) ......2cos2cos......
coscos
12
2
21
2
3210
21
++−+−+++=
+=
tABtBAaaaav
tBtAv
out
in
Input tones at f1, f2
Third-order terms
![Page 9: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/9.jpg)
PA Specs: Noise
=
output
output
input
input
N
S
N
S
FfactorNoise ,
Thermal noise, P = kTB
Equates to -174 dBm/Hz at room temperature
G1 G2 G3
...11
21
3
1
21 +
−+
−+=
GG
F
G
FFF
Cascaded noise figure (Friis’ equation)
=
output
output
input
input
N
S
N
S
dBNFfigureNoise log10)(,
![Page 10: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/10.jpg)
Circuit Topology - PA Classes
◼ Class A◼ Linear, simplest circuit,
◼ Transistor is conducting all the time
◼ 25 or 50% efficiency limit depending on circuit
◼ Class AB◼ Transistor is conducting most of the time
◼ Common compromise between efficiency and linearity
◼ Class B◼ Transistor is conducting half of the time
◼ 78.5% theoretical efficiency limit
◼ Class C◼ Transistor is conducting less than half of the time
◼ Non-linear amplification, high efficiency
![Page 11: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/11.jpg)
Class A
◼ Transistor output swings from near-supply to near-zero
◼ Half-power dissipated in transistor, half in load
◼ Not all of the power in the load is useful signal though
◼ Example circuit:
◼ PR1=132.46mW (AC+DC)
◼ PR1=27.646mW (AC)
◼ PV2=228.92mW (AC+DC)
◼ 12% efficiency
![Page 12: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/12.jpg)
Class B
◼ Transistor output swings from near-supply to near-zero
◼ Reduced input DC to cause transistor to spend 50% time off
◼ No dissipation while it is off
◼ Increased input swing
◼ Example Circuit:
◼ PR1=103.87mW (AC+DC)
◼ PR1=60.086mW (AC)
◼ PV2=147.97mW (AC+DC)
◼ 40.6% efficiency
50% 50%
![Page 13: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/13.jpg)
Class C
◼ LC added to load, Transistor output swings from above-supply to near-zero
◼ Low input DC bias to cause transistor to spend >50% time off
◼ Increased input swing (less gain)
◼ Example Circuit:
◼ PR1=161.56mW (AC)
◼ PV2=227.28mW (AC+DC)
◼ 71.1% efficiency
![Page 14: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/14.jpg)
Class AB◼ Linear amplification
with better than Class A efficiency
◼ Use two Class B stages
◼ Operate them in antiphase
◼ Combine outputs
◼ Adjust bias to minimise crossover distortion
◼ Example Circuit:
◼ PR1=206.31mW (AC)
◼ PV2=303.08mW (AC+DC)
◼ 68.1% efficiency
![Page 15: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/15.jpg)
Balanced Power Amplifier
◼ 90° Hybrids, parallel PA stages
[SA]
[SB]
a1
b1
a2
b2
a1A
b1Aa2A
b2A
a1B
b1B a2B
b2B
3dB 90° 3dB 90°
![Page 16: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/16.jpg)
PA Design Challenges
◼ Delivering RF power into 50 ohms requires a voltage swing 50x the load current (Amps) (from V = IR)
◼ Typical PA devices handle 5V to 28V supply, (GaN may be 55V)
◼ The load current for a typical PA device may be several Amps, with peaks of 10s of Amps
◼ “Matching” is required to transform the low output impedance up to 50 ohms
◼ Expect output device has parasitic series inductance and shunt capacitance
![Page 17: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/17.jpg)
Devices and Package Parasitics
◼ The diagram shows an internally matched LDMOS device. Drain voltage is provided via a ¼ wave feed line. RF and Modulation frequencies are terminated by bulk decoupling capacitors at the supply side of the ¼ wave feed.
1/4 λ
VDD
LDMOS Package
![Page 18: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/18.jpg)
Examples (Reference Designs)
![Page 19: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/19.jpg)
Design Approaches
◼ Small Signal approach
◼ Load line
◼ Conjugate match
◼ Nonlinear approach
◼ Harmonic Balance modelling and simulation
◼ Non-linear characterisation and simulation
◼ Load pull
![Page 20: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/20.jpg)
Load Pull System
Source Tuner Load Tuner
Signal Generator
Preamplifier
Coupler
Power meter
Sensor
Isolator
Coupler
Sensor
Spectrum Analyzer
Vector Signal
Analyzer
Input
Transformer
Output
TransformerDUT
Attenuator
Dual PSU
GPIBGPIB
![Page 21: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/21.jpg)
Load Pull System
![Page 22: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/22.jpg)
Load Plane Contours
Load match contours of ACLR,
gain and drain current for a 20
Watt 3GPP W-CDMA Device
1.40.5
0.1
0.2
0.3 0.4
0.1
0.2
0.3
0.4
0.5
0.6
0.7 0
.8 0.9 1.0
1. 2
1.4
1.6
1.8
3.0
2.0
4.0
5.0
10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 1.2
1.4
1.6
1.8
2.0
3.0
4.0
5.0
10
20
20
50
50
0.6
0.7
0.8
0.9
1.0
1.2
1.6
1.8 2.0
3.0 4.0
5.0
502010
Z0=3
BestEfficiency
MaximumGain
BestACLR
![Page 23: Introduction to RF PA design - Cambridge Wireless](https://reader031.vdocuments.site/reader031/viewer/2022012516/618fcd2de6316329a75d2386/html5/thumbnails/23.jpg)
Conclusions
◼ Generic PA transistor
◼ Achieving power amplification
◼ Application types
◼ Considerations of carrier frequency, power level
◼ Modulation waveform and how it affects PAPR
◼ Key specs
◼ Noise, distortion, efficiency
◼ Circuit and Topology choices
◼ Class A, AB, C, Balanced circuits
◼ Device Technology Overview
◼ Design methods