Bias modulation memory effects handouts - lintech.com · 1 Bias induced memory effects in RF power amplifiers DR. MARC J. FRANCO LINEARIZER TECHNOLOGY INC. Hamilton, New Jersey, USA

1

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

2

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

3


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.)

4


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

5


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)

6



Magnitude and phase of the gain of an RF power amplifier at two different frequencies

7



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)

8



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

9


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

10


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)

11


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

12


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

13


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

14


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


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


C/I

(dB)

C/I3 UPPER

C/I3 LOWER

C/I5 UPPER


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


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


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


C/I

(dB)

C/I3 UPPER

C/I3 LOWER

C/I5 UPPER


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


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


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


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


C/I

(dB)

C/I3 UPPER

C/I3 LOWER

C/I5 UPPER


saturation

Lower C/IM 3

Upper C/IM 3

Lower C/IM 5

Upper C/IM 5

15




The intermodulation distortion can be asymmetrical

16


Asymmetric intermodulation distortion

Asymmetric intermodulation Asymmetric intermodulation distortiondistortion

Asymmetry of 3rd order intermodulation distortion products due to memory effects

17





The reduction of the intermodulation distortion with a predistortion linearizer is difficult when the distortion sidebands are asymmetrical

18


Predistortion of asymmetrical signalsPredistortion of asymmetrical signalsPredistortion of asymmetrical signals

A memoryless predistortion linearizer cannot completely cancel asymmetric intermodulation distortion

19





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!

20


Two tone signalTwo tone signalTwo tone signal

Representation of a two tone signal in the frequency and time domains

21


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

22


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

23


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

24


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

25


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

26


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

27


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

28


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 [%]

29


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

30


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

31


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

32


Frequency response of a bias network that minimizes memory effects and maximizes the efficiency by short-circuiting the second harmonic



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

33


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



34


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)



35


Spectrum of 4 WCDMA carriers with both drain bias networks



TRADITIONAL BIAS NETWORK

IMPROVED BIAS NETWORK

TRADITIONAL BIAS NETWORK

IMPROVED BIAS NETWORK

36


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

]

37


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

38


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)

Documents

Bias modulation memory effects handouts - lintech.com · 1 Bias induced memory effects in RF power amplifiers DR. MARC J. FRANCO LINEARIZER TECHNOLOGY INC. Hamilton, New Jersey, USA