Bias induced memory effects in RF power amplifiers
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Transcript of Bias induced memory effects in RF power amplifiers
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Bias induced memory effectsBias induced memory effectsin RF power amplifiersin RF power amplifiers
DR. MARC J. FRANCODR. MARC J. FRANCO
LINEARIZER TECHNOLOGY INC.LINEARIZER TECHNOLOGY INC.Hamilton, New Jersey, USAHamilton, New Jersey, USA
www.lintech.comwww.lintech.com
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Linearizer Technology, Inc.
IntroductionIntroductionIntroduction
freq.
Input Power
Output Power
AMP
freq.
NEW
FREQUENCIES
NONLINEAR TRANSFER FUNCTION
2f1-f2 2f2-f1
f2f1f2f1
In practice, the gain of an RF power amplifier is a nonlinear function
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Linearizer Technology, Inc.
IntroductionIntroductionIntroduction
In practice, the gain of an RF power amplifier is a nonlinear function
If the gain is ideally nonlinear, it should be only a function of the input signal level
Real RF power amplifiers have memory – their gain is a function of various parameters (frequency, temperature, etc.)
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Linearizer Technology, Inc.
OutlineOutlineOutline
RF power amplifier models
Generation of memory effects in RF power amplifiers
Typical memory effects
Why memory effects are a problem?
Minimization of memory effects
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Linearizer Technology, Inc.
RF power amplifier modelRF power amplifier modelRF power amplifier model
Memoryless RF power amplifier model and transfer function(ideal)
y(t)ysat
-ysat
zsat
-zsat
z(t)= f [y(t) ]
y(t) z(t)f [y(t)]
saturation
saturation
Memoryless RF Power Amplifier (ideal)
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Linearizer Technology, Inc.
RF power amplifier modelRF power amplifier modelRF power amplifier model
Magnitude and phase of the gain of an RF power amplifier at two different frequencies
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Linearizer Technology, Inc.
RF power amplifier modelRF power amplifier modelRF power amplifier model
RF power amplifier model (with memory)- the nonlinear gain depends on the frequency of the signal -
X(f)
NONLINEAR SUBSYSTEM LINEAR SUBSYSTEMLINEAR SUBSYSTEM
Y(f) y(t) z(t) Z(f) W(f)H1(f) H2(f)f [y(t)]
x(t) w(t)
X(f)
freq.fMfm f0
Z(f)
freqfMfm f0
W(f)
freq.fMfm f0
H1(f) H2(f)
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Linearizer Technology, Inc.
RF power amplifier modelRF power amplifier modelRF power amplifier model
Transfer function f [y(t)] of the nonlinear subsystem and generation of intermodulation distortion (IMD) and harmonics
Frequency
(ω1)
ω2 - ω1
2ω2 - ω12ω1 – ω2
ω2 + ω1
2ω1 2ω2
2ω2 + ω12ω1 + ω2
3ω23ω1
(ω2)
∑=
+=3
1
)()()(n
nnn tyjbatz
NONLINEAR SUBSYSTEM
y(t) z(t)f [y(t)]
Amplitude
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Envelope memory effectsEnvelope memory effectsEnvelope memory effects
Effect of the envelope frequency on the generation of third order intermodulation distortion products
(ω1)
ω2 - ω1
2ω2 - ω12ω1 – ω2 2ω1
(ω2)
Frequency
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Linearizer Technology, Inc.
Memory effects due to harmonicsMemory effects due to harmonicsMemory effects due to harmonics
Effect of the second harmonic on the generation of third order intermodulation distortion products
Frequency
(ω1)
ω2 - ω1
2ω2 - ω12ω1 – ω2
ω2 + ω1
2ω1 2ω2
(ω2)
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Linearizer Technology, Inc.
3rd order intermodulation distortion composition
33rdrd order intermodulation distortion order intermodulation distortion compositioncomposition
The 3rd order intermodulation
distortion (IMD) is caused by
IMD3 of the nonlinear active deviceIMD3 due to the envelope of the signalIMD3 due to the second harmonic
IMD3
IMD3 due to envelope
IMD3 due to 2nd
harmonic
Resultant IMD3
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Linearizer Technology, Inc.
Thermal memory effectsThermal memory effectsThermal memory effects
Variations in the envelope of the signal produce rapid changes in temperature in the active device of the amplifier
If the envelope frequency is high (>100 KHz), the thermal inertia is such that the device temperature will be constant
If the envelope frequency is low (<100 KHz), the temperature of the active device will vary as a function of the envelope
Changes in the temperature of the active device affect its nonlinear gain
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Problems due to memory effectsProblems due to memory effectsProblems due to memory effects
The intermodulation distortion can increase due to the contribution of the envelope and second harmonic
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Effects of the envelope frequency on the intermodulation distortion
Effects of the envelope frequency on the Effects of the envelope frequency on the intermodulation distortionintermodulation distortion
The carrier to intermodulation ratio (C/IM) usually decreasesfor a very widely separated two-tone signal
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
-4 dB of output power respect to
saturation
Lower C/IM 3
Upper C/IM 3
Lower C/IM 5
Upper C/IM 5
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
-4 dB of output power respect to
saturation
Lower C/IM 3
Upper C/IM 3
Lower C/IM 5
Upper C/IM 5
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
-4 dB of output power respect to
saturation
Lower C/IM 3
Upper C/IM 3
Lower C/IM 5
Upper C/IM 5
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100
C/I5 LOWER
4 dB OPBO
40
35
30
25
20
15
10
1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100CARRIER SPACING (MHz)
C/I
(dB)
C/I3 UPPER
C/I3 LOWER
C/I5 UPPER
-4 dB of output power respect to
saturation
Lower C/IM 3
Upper C/IM 3
Lower C/IM 5
Upper C/IM 5
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Linearizer Technology, Inc.
Problems due to memory effectsProblems due to memory effectsProblems due to memory effects
The intermodulation distortion can increase due to the contribution of the envelope and second harmonic
The intermodulation distortion can be asymmetrical
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Linearizer Technology, Inc.
Asymmetric intermodulation distortion
Asymmetric intermodulation Asymmetric intermodulation distortiondistortion
Asymmetry of 3rd order intermodulation distortion products due to memory effects
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Linearizer Technology, Inc.
Problems due to memory effectsProblems due to memory effectsProblems due to memory effects
The intermodulation distortion can increase due to the contribution of the envelope and second harmonic
The intermodulation distortion can be asymmetrical
The reduction of the intermodulation distortion with a predistortion linearizer is difficult when the distortion sidebands are asymmetrical
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Linearizer Technology, Inc.
Predistortion of asymmetrical signalsPredistortion of asymmetrical signalsPredistortion of asymmetrical signals
A memoryless predistortion linearizer cannot completely cancel asymmetric intermodulation distortion
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Linearizer Technology, Inc.
Problems due to memory effectsProblems due to memory effectsProblems due to memory effects
The intermodulation distortion can increase due to the contribution of the envelope and second harmonic
The intermodulation distortion can be asymmetrical
The reduction of the intermodulation distortion with a predistortion linearizer is difficult due to the asymmetry of the distortion sidebands
The observed distortion can be exclusively due to memory effects and not to a nonlinear effect!
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Two tone signalTwo tone signalTwo tone signal
Representation of a two tone signal in the frequency and time domains
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Linearizer Technology, Inc.
Typical RF power amplifierTypical RF power amplifierTypical RF power amplifier
1
2
3
FET
RS=RDS=CDS=CDC=CDG=
RI=GGS=CGS=
F=T=G=
ID=
0 Ohm100 Ohm0 pF0 pF0 pF1 Ohm1 S0 pF0 GHz0 ns0.1 SF1
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2
IND
L=ID=
1 nHL2 CAP
C=ID=
1 pFC3 CAP
C=ID=
1 pFC4
DCVS
V=ID=
1 VV1
DCVS
V=ID=
1 VV2
1 2
SUBCKT
NET=ID=
"F1001" S1
1 2
SUBCKT
NET=ID=
"F1001" S2 PORT
Z=P=
50 Ohm1
PORT
Z=P=
50 Ohm2
INPUT MATCHING
OUTPUT MATCHING
ACTIVE DEVICE
DRAIN BIASGATE BIAS
1
2
3
FET
RS=RDS=CDS=CDC=CDG=
RI=GGS=CGS=
F=T=G=
ID=
0 Ohm100 Ohm0 pF0 pF0 pF1 Ohm1 S0 pF0 GHz0 ns0.1 SF1
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2
IND
L=ID=
1 nHL2 CAP
C=ID=
1 pFC3 CAP
C=ID=
1 pFC4
DCVS
V=ID=
1 VV1
DCVS
V=ID=
1 VV2
1 2
SUBCKT
NET=ID=
"F1001" S1
1 2
SUBCKT
NET=ID=
"F1001" S2 PORT
Z=P=
50 Ohm1
PORT
Z=P=
50 Ohm2
INPUT MATCHING
OUTPUT MATCHING
ACTIVE DEVICE
DRAIN BIASGATE BIAS
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Linearizer Technology, Inc.
Typical RF power amplifier -Output circuit
Typical RF power amplifier Typical RF power amplifier --Output circuitOutput circuit
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2 DCVS
V=ID=
1 VV1 DCCS
I=ID=
10 mAI1
CAP
C=ID=
1 pFC3
1 2
SUBCKT
NET=ID=
"F1001" S1
PORT
Z=P=
50 Ohm2
OUTPUT MATCHING
ACTIVE DEVICE EQUIVALENT
MODEL
gmVgs Cds
Ids
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2 DCVS
V=ID=
1 VV1 DCCS
I=ID=
10 mAI1
CAP
C=ID=
1 pFC3
1 2
SUBCKT
NET=ID=
"F1001" S1
PORT
Z=P=
50 Ohm2
OUTPUT MATCHING
ACTIVE DEVICE EQUIVALENT
MODEL
gmVgs Cds
Ids
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Linearizer Technology, Inc.
Typical RF power amplifier -Output circuit
Typical RF power amplifier Typical RF power amplifier --Output circuitOutput circuit
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2 DCVS
V=ID=
1 VV1 DCCS
I=ID=
10 mAI1
CAP
C=ID=
1 pFC3
1 2
SUBCKT
NET=ID=
"F1001" S1
PORT
Z=P=
50 Ohm2
OUTPUT MATCHING
gmVgs Cds
RES
R=ID=
1 OhmR1
IND
L=ID=
1 nHL1
Capacitance function of the drain voltage
Real inductor
(lossy)
Ids
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2 DCVS
V=ID=
1 VV1 DCCS
I=ID=
10 mAI1
CAP
C=ID=
1 pFC3
1 2
SUBCKT
NET=ID=
"F1001" S1
PORT
Z=P=
50 Ohm2
OUTPUT MATCHING
gmVgs Cds
RES
R=ID=
1 OhmR1
IND
L=ID=
1 nHL1
Capacitance function of the drain voltage
Real inductor
(lossy)
Ids
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Linearizer Technology, Inc.
Typical RF power amplifier -AM/AM and AM/PM bias modulation
Typical RF power amplifier Typical RF power amplifier --AM/AM and AM/PM bias modulationAM/AM and AM/PM bias modulation
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2 DCVS
V=ID=
1 VV1 DCCS
I=ID=
10 mAI1
CAP
C=ID=
1 pFC3
1 2
SUBCKT
NET=ID=
"F1001" S1
PORT
Z=P=
50 Ohm2
OUTPUT MATCHING
gmVgs Cds
RES
R=ID=
1 OhmR1
IND
L=ID=
1 nHL1
AM/PM
AM/AMIds
IND
L=ID=
1 nHL1
CAP
C=ID=
1 pFC1
CAP
C=ID=
1 pFC2 DCVS
V=ID=
1 VV1 DCCS
I=ID=
10 mAI1
CAP
C=ID=
1 pFC3
1 2
SUBCKT
NET=ID=
"F1001" S1
PORT
Z=P=
50 Ohm2
OUTPUT MATCHING
gmVgs Cds
RES
R=ID=
1 OhmR1
IND
L=ID=
1 nHL1
AM/PM
AM/AMIds
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Linearizer Technology, Inc.
AM/AM and AM/PM generated by bias modulation
AM/AM and AM/PM generated by bias AM/AM and AM/PM generated by bias modulationmodulation
AM/AM is generated mainly by the voltage drop across the loss resistance of the drain inductor
AM/PM is mainly generated by the variations of the drain-source capacitance as a function of drain voltage
The sidebands due to the bias modulation have the same frequency as the intermodulation distortion products
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Linearizer Technology, Inc.
Two tone drain signal waveformTwo tone drain signal waveformTwo tone drain signal waveform
RF ENVELOPE (CURRENT)
DRAIN VOLTAGE
Drain voltage and current are out of phase due to the reactance in the biasing network
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Linearizer Technology, Inc.
Asymmetry in the bias induced modulation
Asymmetry in the bias induced Asymmetry in the bias induced modulationmodulation
Simultaneous amplitude and phase modulation does not generate asymmetric sidebands
Simultaneous amplitude and delayed phase modulation does generate asymmetric sidebands
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Linearizer Technology, Inc.
Drain bias modulation sensitivityDrain bias modulation sensitivityDrain bias modulation sensitivity
Carrier to intermodulation ratio (C/IM) as a function of drain amplitude modulation at constant output power
-50
-40
-30
-20
-10
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00
C/IM
ratio
[dB
]
C/I Ratio (insensitive to OPBO)Amplitude modulation percentage [%]
-50
-40
-30
-20
-10
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00
C/IM
ratio
[dB
]
C/I Ratio (insensitive to OPBO)Amplitude modulation percentage [%]
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Linearizer Technology, Inc.
Minimization of bias-inducedmemory effects
Minimization of biasMinimization of bias--inducedinducedmemory effectsmemory effects
Bias modulation effects can be reduced by terminating the activedevice in a very low impedance at the envelope frequency
Both drain and gate bias circuits can generate bias modulation
Thermal memory effects depend on the physical properties of the active device
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Linearizer Technology, Inc.
Minimization of memory effects -Typical drain bias network
Minimization of memory effects Minimization of memory effects --Typical drain bias networkTypical drain bias network
Typical drain biasing network in which the inductance has been reduced to minimize memory effects
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Linearizer Technology, Inc.
Minimization of memory effects -Improved drain bias network
Minimization of memory effects Minimization of memory effects --Improved drain bias networkImproved drain bias network
Improved drain biasing network in which the inductance has been reduced to minimize memory effects, and the envelope is terminated in a short circuit
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Linearizer Technology, Inc.
Frequency response of a bias network that minimizes memory effects and maximizes the efficiency by short-circuiting the second harmonic
Minimization of memory effects -Improved drain bias network
Minimization of memory effects Minimization of memory effects --Improved drain bias networkImproved drain bias network
VERY LOW IMPEDANCE AT THE ENVELOPE
FREQUENCY
HIGH IMPEDANCE AT THE CARRIER
FREQUENCY
LOW IMPEDANCE AT THE SECOND
HARMONIC
VERY LOW IMPEDANCE AT THE ENVELOPE
FREQUENCY
HIGH IMPEDANCE AT THE CARRIER
FREQUENCY
LOW IMPEDANCE AT THE SECOND
HARMONIC
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Linearizer Technology, Inc.
FET
d
s
g
PARALLEL RESONATORSAT CARRIER FREQUENCY VERY LOW IMPEDANCE
AT ENVELOPE FREQUENCY
SERIES RESONATORAT 2nd HARMONIC
Implementation of the improved drain bias network in a UHF power amplifier
Minimization of memory effects -Improved drain bias network
Minimization of memory effects Minimization of memory effects --Improved drain bias networkImproved drain bias network
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Linearizer Technology, Inc.
Measured ac component at the drain of the FETwith a 5 MHz two-tone signal at 1 dB output back-off
LEFT: Traditional bias networkRIGHT: Improved bias network
(20 mV/div, 50 nsec/div, 30 MHz low pass filter)
Minimization of memory effects -Improved drain bias network
Minimization of memory effects Minimization of memory effects --Improved drain bias networkImproved drain bias network
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Linearizer Technology, Inc.
Spectrum of 4 WCDMA carriers with both drain bias networks
Minimization of memory effects -Improved drain bias network
Minimization of memory effects Minimization of memory effects --Improved drain bias networkImproved drain bias network
TRADITIONAL BIAS NETWORK
IMPROVED BIAS NETWORK
TRADITIONAL BIAS NETWORK
IMPROVED BIAS NETWORK
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Linearizer Technology, Inc.
Gate modulation sensitivityGate modulation sensitivityGate modulation sensitivity
Carrier to intermodulation ratio (C/IM) as a function of gate amplitude modulation
-50
-40
-30
-20
-10
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00
Amplitude modulation [%]
C/IM
ratio
[dB
]
-50
-40
-30
-20
-10
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00
Amplitude modulation [%]
C/IM
ratio
[dB
]
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Linearizer Technology, Inc.
ConclusionConclusionConclusion
Memory effects in RF power amplifiers are variations of the nonlinear gain due to the frequency of the signal, the frequency of the envelope of the signal, or temperature
Memory effects generate asymmetry in the distortion sidebands – this effect can reduce or increase them
It is desirable to minimize memory effects during the design of the RF power amplifier
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Linearizer Technology, Inc.
ConclusionConclusionConclusion
Bias modulation of the amplifier is generated by terminating the active device on an impedance greater than zero at the signal envelope frequency
Bias modulation can be minimized by using biasing networks with minimum reactance at the envelope frequency
A worst case scenario will occur for high power, low drain voltage amplifiers, operating at low frequencies with very wide signal bandwidth (high envelope frequency)