POWER AMPLIFIERS AN INTRODUCTION ON
POWER AMPLIFIERS
Prof. Edgar Sánchez-Sinencio
Outline
Introduction
Power Amplifier Classes
Linear PAs
Switching PAs
Lineariziation techniques
Input
Output
Supply
2
Introduction
Performance Metrics
Why Power Amplifiers?
RF Power Amplifier’s vast applications
Wireless and wireline communications
Output transmitted power is relatively
large portion of the total power
consumption.
Power efficiency of PAs can greatly
influence overall power efficiency.
4
Power Amplifier performance metrics
Metrics defined in standards
Output Power
ACPR (Adjacent Channel Power Ratio)
Signal Modulation
Metrics not defined in standards
PAE (Power Added Efficiency)
Drain Efficiency
Power Gain
IIP3
P1-dB
5
Output Power
Power delivered to the load within the band
of interest.
Load is usually an antenna with Z0 of 50Ω
Doesn’t include power contributed by the
harmonics or any unwanted spurs
Sinusoidal
Modulated Signal
2
Vout
Pout =
2 RL
Pout / avg = ∫
∞
0
1
ϕ ( p ) dp =
T
Probability profile of Modulation: Prob (Pout=p)
∫
T
0
v(t ) dt
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Output Power
Maximum output power varies drastically
among different standards
Standard
Modulation
Max. Pout
AMPS
FM
31 dBm
GSM
GMSK
36 dBm
CDMA
O-QPSK
28 dBm
DECT
GFSK
27 dBm
PDC
π/4 DQPSK
30 dBm
Bluetooth
FSK
16 dBm
802.11a
OFDM
14-19 dBm
802.11b
PSK-CCK
16-20 dBm
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Efficiency
Efficiency Most common
efficiency metric
DC ⎯
⎯→ RF
Pout − Pin
× 100%
PAE =
PDC
Shows how efficiently supply DC power is
converted to RF power
Drain efficiency is often used to indicate the
efficiency of a single power amplifier stage
η drain
Pdelivered
=
×100%
PDC
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Linearity measures
Linearity Requirement can be different
based on modulation
Variable Envelope
Information is carried in the amplitude
π 4 DQPSK and OQPSK
Constant Envelope
Information is carried in the phase
GFSK and GMSK
AM-to-AM, AM-to-PM distortion and P1-dB
ACPR (Adjacent Channel Power Ratio)
IP3
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Linearity measures
Power mask is an indication of how much
Output Power [dBc]
spectrum regrowth is allowed
ZigBee
0
Bluetooth
-10
-20
-30
-40
-50
-5
-4
-3
-2
-1
0
+1
+2
+3
+4
+5
Frequency offset
[MHz]
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Introduction
Power Amplifier Class Types
PA Class types; Linear PAs
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Efficiency and conduction angle
To calculate power efficiency, power of main
harmony and DC current should be calculated
DC part of current
nth harmonic of current
cos(α / 2) = −
IQ
I pk
=−
IQ
I MAX − I Q
Conduction angle
13
Output voltage shape
If load tank filters out all harmonics,
output voltage is pure sinusoidal even
when there is current discontinuity
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Efficiency
P1 I1, rms .V1, rms
η=
=
Pdc
I dc .Vdc
I MAX
I MAX
⎧
⎪ I1 = 2 , I dc = 2
⎪
⎛ I MAX 2 ⎞⎛ VDD ⎞
⎪
⎟⎜
⎟
⎜
Class A: ⎨
2 ⎠⎝ 2 ⎠
⎪η = ⎝
= 50% max .
⎪
⎛ I MAX ⎞
⎟.VDD
⎜
⎪
⎝ 2 ⎠
⎩
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Efficiency
I MAX
I MAX
⎧
⎪ I1 = 2 , I dc = π
⎪
⎪
⎛ I MAX 2 ⎞⎛ VDD ⎞
Class B: ⎨
⎜
⎟⎜
⎟
2 ⎠⎝ 2 ⎠
⎪η = ⎝
= 78% max .
⎪
⎛ I MAX ⎞
⎜
⎟.VDD
⎪
⎝ π ⎠
⎩
Class C efficiency depends on
α
and
ideally can reach 100% but at that
point output power also reaches zero!
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Class E
so that before switch turns
on (Soft switching) :
•
•
Vs = 0
∂Vs
=0
∂t
ZVS ☺
Non-overlapping voltage and
current minimize switch power
consumption ☺
max(VD ) = 3.6 VDD
• So low-voltage operation is needed
for reliability
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Class F
By adding odd harmonics :
Drain voltage starts to
increasingly resemble square
wave
Decreasing the voltage across
transistor during conduction
time and hence increasing
efficiency
All-harmonics-tuned=>class D
max(VD ) = 2 VDD
Not ZVS operation
☺
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Linearization Techniques
How to linearize highly efficient PAs?
Linearization Techniques
Non-linear power amplifier can reach great efficiencies
But they lack linearity
Linearization techniques can be applied to non-linear PAs to get
a good linearity and a modest efficiency
Control is applied at
Input
Back-off
Pre-distortion
Cartesian feedback
Polar feedback
Output
Feed-forward
LINC (Linearization using Nonlinear Components)
Supply
EER (Envelope Elimination and Restoration)
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Input: Back-off
Simplest and most common linearization
Back - off
10
15 20
25
30 35
1-dB compression point
Target Output Power
5
Output Power (dBm)
PAE is greatly reduced
-20
-15
-10
-5
0
5
10
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Input: Pre-distortion
Tracking gain and change variations of amplifier is
very challenging using analog techniques
Digital Look-up tables often used
PA gain and phase response varies with bias,
temperature and supply changes
Predistortion
Modulator
PA
Gain
Phase
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Input: Cartesian Feedback
Feedback is used to increase linearity
Large loop gain is needed to improve linearity;
very difficult to achieve at RF frequencies
Down-converting alleviates this problem
Stability is a big challenge
I
LPF
Modulator
PA
Q
LPF
Phase
LO
Demodulator
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Input: Polar feedback
VCO
θ
PD
VGA
Filter
Filter
Two loop controls gain and phase
Gain loop
PLL
PA
r
Envelope
Detector
Doesn’t require up/down conversion
If AM/PM distortion of PA is not severe, phase feedback is not
needed
Stability challenging
Polar feedback loops should operate at wider bandwidth
compared to Cartesian feedback
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Output: Feed-forward
Distortion is calculated and subtracted from output:
Precise matching of A1, A2, A3 needed
Tracking over process, time and temp is tough
Constant analog Delay is challenging
Stability is not a problem
Operates at the bandwidth of carrier frequency rather than base
band hence applicable in multi-carrier systems
Vo
Vout
PA
Delay
1/A2
attenuator
A1
A3
Delay
vx
Auxiliary
amp
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Output: Feed-forward Analysis
Vo
Vout
PA
Delay
1/A2
Vo = AVin + Vd ,
⇒ Vout
Vx =
Vo
V
− Vin = d
A
A
V
= Vo − A( d ) = AVin
A
Gain and phase mismatch
can degrade linearity of
power amplifier
[*]:
attenuator
A1
A3
Delay
vx
Auxiliary
amp
⎛ ∆A ⎞
⎛ ∆A ⎞
∆IP3 = 1 − 2⎜1 +
⎟ cos ∆φ + ⎜1 +
⎟
A ⎠
A ⎠
⎝
⎝
[*] B. Razavi, RF Microelectronics
2
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Output: LINC
(Linear Amplification using Nonlinear Components)
Theoretically any non-constant envelope signal on a
carrier can be split into two constant-envelope
signals
A complex conversion at RF is very challenging task
Signal combination at output is practically
problematic
PA
Vin
Signal
Separator
PA
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Output: LINC Analysis
PA
Vin
Signal
Separator
vin = a (t ) cos[ωc t + ϕ (t )] = v1 (t ) + v2 (t )
1
v1 (t ) = V0 sin[ωc t + ϕ (t ) + θ (t )]
2
1
v2 (t ) = − V0 sin[ωc t + ϕ (t ) − θ (t )]
2
−1 ⎡ a (t ) ⎤
θ (t ) = sin ⎢
⎥
V
⎣ 0 ⎦
PA
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Supply: EER
(Envelope Elimination and Restoration)
Amplitude and phase are amplified separately
Amplitude information is fed at the output by supply
Substantial power could be dissipated in the supply
modulation circuitry providing the whole current of PA
Dc-to-dc can be used but still delivered current is quite large
Delay mismatch between two paths introduces distortion
Envelope
Detector
Supply
Modulator
Limiter
PA
Vin
Vo
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Power Amplifier notes of MIT OpenCourseWare
Steve C. Cripps, Advanced Techniques in RF
Power Amplifier Design, Artech House Publishers
Mohammed Ismail and Mona Hella, RF Cmos
Power Amplifiers: Theory, Design and
Implementation
Several Thesis on PAs
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Some Research Ideas
Design a non-linear Power Amplifier for output
power of 10 dBm delivered to the load of 50Ω
antenna at the operating frequency of 2.4 GHz.
Optimize the efficiency. Measure linearity (IIP3).
Then use one linearization technique to increase
IIP3 to 30 dBm. Efficiency will be decreased as a
result of overhead circuits. Can we come up with a
different kind of linearization technique to reduce
complexity and power consumption of overhead?
Design a signal separator at 2.4 GHz to be used in
LINC technique.
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