AN ANALOG PREDISTORTION LINEARIZER DESIGN

AN ANALOG PREDISTORTION
LINEARIZER DESIGN
In this article, a new type of analog predistortion linearizer is proposed to generate and control
predistorted third and higher order intermodulation signals (IM) separately. By using predistorted
signals, the intermodulation distortion signals generated in power amplifiers can be suppressed
effectively. In order order to validate the proposed approach, a high power amplifier (HPA), using the
proposed predistortion linearizer, has been fabricated to operate in the Korean PCS base station
transmitter band (1840 to 1870 MHz). The test results show that the IMD 3 and IMD 5 of the power
amplifier are improved by more than 40 and 23 dB for CW two-tone signals, respectively. The
predistorter also improves the adjacent channel power ratio (ACPR) by more than 10 dB for CDMA
(IS-95) 4FA signals.
In the past, voice transmission was the
main issue in communications. However,
new requirements have evolved from
voice to data and mobile communications,
which demand a larger amount of information
and better quality. Because of these requirements,
more complex modulation and demodulation
schemes and broadband channel bandwidths
are necessary in modern mobile communications
systems. In general, complex
modulation and demodulation schemes require
linear transmitters and receivers. In
power amplifier design, high linearity, in addition
to high efficiency, is a critical issue.
When signals are amplified in an HPA, unwanted
harmonics and intermodulation distortion
signals are generated, in addition to the
amplified desired signals, by the nonlinear
characteristics of the power amplifier. These
IM signals increase the bit error rate (BER) of
the data and the adjacent channel interferences,
and decrease the power amplifier efficiency.
Predistortion is a linearization tech-
nique that connects a circuit having inverse
distortion characteristics at the input of the
amplifier. Because of the inverse distortion
characteristics, the predistortion linearizer can
reduce the IM signals generated by the HPA. 1
If the third, fifth and higher order IM products
can be generated and controlled separately
in the predistorter, the IM products of
the power amplifier can be reduced more effectively.
2 This method presents a problem
caused by signal interference in small size circuits
and is difficult to implement. In this article,
a new type of analog predistortion lin-
YOUNG KIM
Kumoh National Institute of Technology
Gyungbuk, Korea
IK-SOO CHANG
Sogang University, Seoul, Korea
YONG-CHAE JEONG
Chonbuk National University
Chonju, Korea
Reprinted with permission of MICROWAVE JOURNAL ® from the February 2005 issue.
©
2005 Horizon House Publications, Inc.

TECHNICAL FEATURE
4f 1 −3f 2
2f 1 −f 2 2f 2 −f 1
4f 2 −3f 1
2f 1 −f 2 f 1 f 2 2f 2 −f 1
2f 1 −f 2 f 1 f 2 2f 2 −f 1 3f 1 −2f 2 f 1 f 2 3f 2 −2f 1
RF IN
RF OUT
f 1 f 2
V CC
RF OUT
RF IN
TR
R
C
▲ Fig. 1 Block diagram of the third-order intermodulation signal
generator (IMG 3 ).
earizer is proposed to reduce HPA
nonlinearity by generating and controlling
individual order IM signals.
PREDISTORTER
OPERATING THEORY
The nonlinearity of an amplifier can
be expressed in terms of a power series
v o = k 1 v i + k 2 v i + k 3 v i + k 4 v i … (1)
where
v o = output signal
v i = input signal
If the input signal consists of two
signals of equal amplitude, as
v i = A[cos(ω 1 t) + cos(ω 2 t)] (2)
then the DC, the intermodulation distortion
components (ω 1 ±ω 2 , 2ω 1 –ω 2 ,
2ω 2 –ω 1 , …) and the harmonic components
(2ω 1 , 2ω 2 , 3ω 1 , 3ω 2 ), besides the
desired amplified input signals (ω 1 , ω 2 )
appear at the output port because of
the HPA nonlinear characteristics. The
harmonic generator of the proposed
distorter has a simple structure. Figure
1 shows the block diagram of the thirdorder
intermodulation signal generator
(IMG 3 ). Its output signals obtained
from the IMG 3 are the main and thirdorder
intermodulation signals, given by
H 13 = k 1 v 1 + k 3 v 3 i
H 13 = a 1 cos(ω 1 t) + a 2 cos(ω 2 t)
+ a 3 cos(2ω 1 –ω 2 )t + a 4 cos(2ω 2 –ω 1 )t
+ a 5 cos(3ω 1 t) + a 6 cos(3ω 2 t) (3)
The desired signals (ω 1 , ω 2 , 2ω 1 –ω 2 ,
2ω 2 –ω1) are elements of H 13 as
shown by Equations 1, 2 and 3. The
bias voltage of the small signal transistor
is controlled to generate a
third-order IM signal as large as possible.
An MSA-0386 transistor from
HP is used in the IMG 3 generator.
Figure 2 shows
the block diagram
of the higher order intermodulation
signals generator (IMG h ), which uses
as input the output signals from the
IMG 3 . They are given by the expression
H h = k1H 13 + k 3 H 13 3 + k 5 H 13
5
DIODE
H h = b 1 cos(ω 1 t) + b 2 cos(ω 2 t)
+ b 3 cos(2ω 1 –ω 2 )t + b 4 cos(2ω 2 –ω 1 )t
+ b 5 cos(3ω 1 –2ω 2 )t
+ b 6 cos(3ω 2 –2ω 1 )t
+ b 7 cos(4ω 1 –3ω 2 )t
+ b 8 cos(4ω 2 –3ω 1 )t + … (4)
IMG h consists of a 3 dB hybrid coupler,
two anti-paralleled Shottky diodes
for high order intermodulation signal
AMPLITUDE (10 dB/div)
1.851 1.855 1.859
FREQUENCY (GHz)
▲ Fig. 3 IMG 3 output for CW two-tone
input signals.
AMPLITUDE (10 dB/div)
1.849 1.855 1.863
FREQUENCY (GHz)
▲ Fig. 4 IMG 3 output for CDMA 2FA
input signals.
▲ Fig. 2 Block diagram of the high order intermodulation signal
generator (IMG h ).
generation, and a resistor and capacitor
to control amplitude and phase of the
input signal to generate only the desired
high order IM terms. 3
Figure 3 shows the output of the
IMG 3 for the case of CW two-tone input
signals at 1.854 and 1.855 GHz.
Figure 4 shows the output of the
IMG 3 for the case of CDMA 2FA input
signals at 1853.75 and 1.856.25
MHz. Figures 5 and 6 show the IMG h
output in the case of CW two-tone and
CDMA 2FA signals, respectively.
A power amplifier, using the new
type of analog predistorter to control
the individual order intermodulations,
AMPLITUDE (10 dB/div)
1.851 1.855 1.859
FREQUENCY (GHz)
▲ Fig. 5 IMG h output for CW two-tone
input signals.
AMPLITUDE (10 dB/div)
1.845 1.855 1.865
FREQUENCY (GHz)
▲ Fig. 6 IMG h output for CDMA 2FA
input signals.

TECHNICAL FEATURE
RF IN
ALC
CKT
IMG 3
DELAY
IMG 3
CONTROL
BLOCK
IMG H
Figure 8 shows the ACPR improvements
as a function of input
power level for a CDMA 1FA signal.
Figure 9 compares the carrier to intermodulation
distortion (C/I) ratio of
the HPA with and without the prowas
designed, as shown in Figure 7. It
consists of an automatic level controller
(ALC) circuit, an IMG 3 and
IMG 3 control block, an IMG h and
IMG h control block, and a high power
amplifier (HPA). The input signals are
divided into the HPA and the ALC
paths by a directional coupler. Since
the HPA is usually operated over a
wide dynamic power range, the ALC
generates a constant signal level for effective
predistortion signal generation.
The ALC circuit, at the input port, stabilizes
the device, resulting in a constant
IM signal in spite of the changes
in input power level. Using this constant
ALC output level, IMG 3 and
IMG h generate the third and higher
order IM signals, respectively.
The IMG 3 and IMG h control blocks
consist of a variable attenuator and a
variable phase shifter to control the
magnitude and phase of the third and
higher order inverse IM signals to the
HPA. The delay circuit in the power
amplifier path compensates for the delay
in the IM generators and IM control
circuits. Adaptive control of the
magnitude and phase of the third and
higher IM signals is necessary to match
those of the intermodulation signals
generated by the HPA over the dynamic
power range. 4–6
EXPERIMENTAL RESULTS
In order to show the validity of the
proposed predistorter, a power amplifier
(STA1800-37 from Sewon Teletec
Inc.), employed in base stations and repeaters
of the Korean Personal Communications
Service (KPCS), was used
in the experiment. Its gain and 1 dB
compression power are 50 dB and 37
dBm, respectively. The delay circuit is
realized with a coaxial delay line of 14
ns. The variable attenuators and variable
phase shifters are of the reflection
IMG H
CONTROL
BLOCK
▲ Fig. 7 Block diagram of the new type of analog predistortion power amplifier.
HPA
RF OUT
type in order to obtain good reflection
characteristics. The variable phase
shifter uses a varactor diode 1T362
from Sony and a variable attenuator, a
PIN diode HSMP-4810 from HP.
AMPLITUDE (10 dB/div)
1.85061 1.85461 1.85861
(a)
FREQUENCY (GHz)
AMPLITUDE (10 dB/div)
IMPROVEMENT (dB)
16
14
12
10
8
6
4
2
0
1.25 MHz
2.25 MHz
885 kHz
−6 −7 −8 −9 −10 −11
INPUT POWER (dBm)
▲ Fig. 8 ACPR improvements versus input
power for a CDMA 1FA signal.
1.85061 1.85461 1.85861
(b)
FREQUENCY (GHz)
▲ Fig. 9 C/I characteristics of HPA (a) without and b) with the proposed predistortion
linearizer in the case of a two-tone signal (P O =37.7 dBm/tone).
AMPLITUDE (10 dB/div)
1.8512 1.8550 1.8578 1.8512 1.8550 1.8578
(a) FREQUENCY (GHz) (b) FREQUENCY (GHz)
▲ Fig. 10 ACPR characteristics of the HPA (a) without and b) with the proposed predistortion
linearizer in the case of a CDMA 1FA signal (P O =37.0 dBm).
AMPLITUDE (10 dB/div)
AMPLITUDE (10 dB/div)
1.8490 1.8550 1.8610 1.8490 1.8550 1.8610
(a) FREQUENCY (GHz) (b) FREQUENCY (GHz)
▲ Fig. 11 ACPR characteristics of the HPA (a) without and b) with the proposed predistortion
linearizer in the case of a CDMA 4FA signal (P O =30.0 dBm/FA).
AMPLITUDE (10 dB/div)

posed predistortion circuit for the case
of CW two-tone signals, where the
output power is 37.7 dBm/tone. The
test frequencies are 1.854 and 1.855
GHz. The improvements of IMD 3 and
IMD 5 are 42.87 and 23.95 dB, respectively.
Figure 10 shows the CDMA
1FA adjacent power ratio (ACPR) of
the HPA with and without the proposed
predistortion circuit. The output
power is 37 dBm and the test frequency
is 1.855 GHz. Figure 11 compares
the CDMA 4FA ACPR of the
HPA with and without the proposed
predistortion circuit, where the output
power is 30 dBm/FA and the carrier
frequencies are 1.8525, 1.85375,
1.85625 and 1.8575 GHz, respectively.
The improvements in ACPR are
10.25, 8.6 and 9.37 dB at f 0 ±885 kHz,
f 0 ±1.25 MHz and f 0 ±2.25 MHz, respectively.
The test signal generator is
ESG4433B from Agilent Technologies.
A signal similar to a carrier leakage
was observed at the center frequency
with this equipment.
CONCLUSION
In this article, a new type of analog
predistortion linearizer for controlling
individual intermodulation distortion
signals is proposed. The signal generators
generate and control predistorted
third and higher IM component signals
separately. The high order IM signals
are generated by using the main and
third-order IM signals at the input of
the generator. The proposed predistorter
structure offers the advantage of
little signal interference in small size
circuits and easy implementation. Using
the predistorted signals, the HPA’s
intermodulation distortion signals are
suppressed effectively. The test results
show that IMD 3 and IMD 5 are reduced
by more than 40 and 23 dB for
CW two-tone signals, respectively. The
predistorter improves the ACPR by
more than 10 dB for CDMA
(IS-95) 4FA signals. ■
ACKNOWLEDGMENT
This work was supported by the Research
Fund, Kumoh National Institute
of Technology, Gyungbuk, Korea.
References
1. P.B. Kenington, High Linearity RF Amplifier
Design, Artech House Inc., Norwood,
MA, 2000, pp. 341–420.
2. Y.C. Jeong, “A Design of Predistortion Linearizer
by Individual Order Control of Intermodulation
Distortion Signals,” PhD
Dissertation, Sogang University, 1996.
3. T. Nojima and T. Konno, “Cuber Predistortion
Linearizer for Relay Equipment in
800 MHz Band Land Mobile Telephone
System,” IEEE Transactions on Vehicular
Technology, Vol. 34, No. 4, Nov.1985.
4. S.A. Maas, Nonlinear Microwave Circuits,
Artech House Inc., Norwood, MA, 1988.
5. T.T. Ha, Solid-state Microwave Amplifier
Design, John Wiley & Sons Inc., Somerset,
NJ, 1981.
6. S.C. Cripps, RF Power Amplifiers for
Wireless Communications, Artech House
Inc., Norwood, MA, 1999.
TECHNICAL FEATURE
Young Kim received
his BSEE, MSEE and
PhD degrees in
electronics engineering
from Sogang
University, Seoul,
Korea, in 1986, 1988
and 2002, respectively.
In 2003, he joined the
school of electronics
engineering, Kumoh
National Institute of
Technology, Gumi, Korea. His interests include
the design of high power amplifiers and
linearization techniques, and RF and
microwave circuit analysis and design. He can
be reached via e-mail at youngk@kumoh.ac.kr.
Ik-Soo Chang
received his PhD
degree from Seoul
National University,
Seoul, Korea, in 1982.
He is currently a
professor at Sogang
University. He has
more than 20 years of
experience in RF and
microwave circuit
design.
Yong-Chae Jeong
received his BSEE,
MSEE and PhD degrees
in electronics
engineering from
Sogang University,
Seoul, Korea, in 1989,
1991 and 1996,
respectively. In 1998, he
joined the division of
electronics and
information engineering
at the Institute of Information and
Communication at Chonbuk National University,
Chonju, Korea. He is currently an associate
professor and chair of the Chonbuk University
IC Design Education Center (IDEC) WG, where
he teaches and conducts research in the areas of
microwave devices, base station amplifiers,
nonlinear devices and system linearizing
technology, and RFIC design.