Implemention of InGaP/GaAs HBT MMIC PA with
gain-expansion drive-amplifier and Doherty structure
Jeong-Seok Jang
Kyoung-Hak Lee
Navaid Div.
NorthStar Systems
Anyang-si, Gyeonggi-do, South Korea
Chang1022@hotmail.com
Industry-Academic Cooperation Foundation
Namseoul University
Cheonan-si Chungcheongnam-do, South Korea
khlee@nsu.ac.kr
Abstract— In this paper, performance improvement of
MMIC Power Amplifier using InGaP/GaAs HBT process is
studied. To meet 3.9G and 4G mobile communications service
demand, a power amplifier with improved efficiency and
linearity that utilizes the Doherty structure and gain-expansion
drive-amplifier is designed and implemented. In the
measurement results, proposed amplifier has P1dB of 27.3dBm,
efficiency of 15.2%@10dB back-off and OIP3 of 48.5.
Keywords—Amplifier, Doherty, IMD, efficiency
I.
II.
IMPROVEMENT PA PERFORMANCE
A. Linearization technique using gain-expansion driveamplifier [3],[4], [7]
For two-tone signals ω1 and ω2 with the same amplitude A,
such that,
vi (t ) = A(cos ω1t + cos ω 2 t )
(1)
We can write equation for the nth-degree output voltage using
power series.
INTRODUCTION
3.9G and 4G mobile communication standard is intended for
global roaming and high data-stream. Applications based on
3.9G and 4G require PA with high linearity and high PAE over
a wide output range. Miniaturization and low cost are also
important requirements for PA. So HBT PA are very strong
candidates for 3.9G and 4G power devices because of their
high gain, high efficiency, high linearity and single supply
voltage operation[1-4].
The operation-class dependence of the PAE of HBT PA
makes near-class-B operation promising for obtaining high
PAE at low output power as well as at maximum output power.
PA under near-class-B operation, however, have strong gainexpansion characteristics that degrade linearity because AM-toAM distortion in the back-off region. So, PA suppresses gain
deviation by combining a gain-expansion and gaincompression. But this way does not have a high PAE at a low
output power [5-7].
Recently mobile communication services adopt OFDM
modulation providing high date rate and robustness to narrow
band fading and interference. However, OFDM signal, which
is multi-carrier modulation, has a large Peak-to-AveragePower-Ratio (PAPR). Due to the high PAPR of OFDM signal,
the PA have to operate at a large amount of back-off output
power from P1dB to achieve a linear operation, which further
degrades the overall efficiency. Therefore efficiency of the PA
has become an important issue for mobile communication
services because it determines the battery lifetime[2].
In this paper, we proposed that InGaP/GaAs MMIC PA
using gain-expansion drive amplifier and Doherty structure.
vo (t ) = k1vi (t ) + k3 vi3 (t ) + k5 vi5 (t ) + ⋅ ⋅ ⋅
(2)
where k1, k3, k5 are complex coefficient of power amplifier’s
gain and IMD. The output at a fundamental frequency ω1 and
ω2 is,
vo (t ) fund = (k1 A +
9
25
k3 A3 + k5 A5 )(cos ω1t + cos ω2t )
4
4
(3)
And the outputs at a third-order and fifth-order IMD frequency
are
3
25
vo (t ) IMD3 = ( k3 A3 +
k5 A5 ){cos(2ω1t − ω2 t ) + cos(2ω 2 t − ω1t )}
4
8
(4)
5
5
vo (t ) IMD5 = ( k 5 A ){cos( 3ω1t − 2ω 2 t ) + cos( 3ω 2 t − 2ω1t )}
8
(5)
K1A in eq. (3) is the linear amplitude and the other terms are
the gain deviation of the amplifier. In eq. (3), the sign for k3/k1,
k5/k1 should be positive when gain expansion occurs for any
value of A. So, the IMD and the carrier signal are in-phase. In
eq. (4), the signs for k3/k1, k5/k1 should be negative when gain
compression occurs for any value of A. Then the sign of the
coefficients of eq. (4) are the opposite of the sign of k1[7].
978-1-4799-0604-8/13/$31.00 ©2013 IEEE
TABLE I.
GAIN DEVAIATION AND IMD PHASE
Gain deviation
(Vout |w1)
IMD phase
(Vout|2w1-w2)
3rd and 5th degree
components
Expansion
In-phase
k3 , k 3
⟩0
k1 k 1
Compression
Out of phase
k3 , k 3
⟨0
k1 k 1
Fig.3 shows simulation results at small signal region. And
Fig. 4 is simulation results at large signal region.
B. Doherty amplifier
The basic operation principles of the Doherty Amplifier are
well described in many articles. From the paper, we have seen
that the advantages of the Doherty amplifier are the simple
circuit configuration and improved efficiency.
General devices for PA have reactive parasitic components.
Due to the components, the Doherty amplifier faced with some
serious problems when the conventional load network is used.
To solve the problems, we have used offset line. Fig. 1 shows
the 1 on 1 doherty amplifier structure.
Fig. 1. Structure of the doherty power amplifier
(a) Gain and MAG
(b) In/Out return loss
Fig. 3. Small signal characteristics (simulation results)
(a) In/Out characteristics
(b) efficiency
The amplifier can delivers a maximum power due to the
power matching circuits. For the peaking amplifier, the output
impedance seen at the output junction can be transformed close
to open by the proper length of offset line. Since the output
impedance of the power devices is low resistance and high
capacitive one, it can be rotated to a high resistance by the
offset line. Thus, the leakage power to the peaking amplifier at
a low-power operation can be substantially reduced and the
proper load modulation can be guaranteed.
III.
DESIGN AND MEASUREMENT
In this paper, MMIC PA was fabricated using InGaP/GaAs
HBT process from Win semiconductor Inc. Reducing the AMto-AM distortion by gain deviation, it designed drive PA using
gain expansion characteristic. Fig. 2 shows proposed PA block
diagram of two stages MMIC PA
(c) OIP3 characteristics
Fig. 4. Large signal characteristics
Fig. 5 shows the photography of test board for
measurement of PAs. Fig. 6 is measurement results and it
shows the comparison between conventional Doherty-structure
amplifier and proposed MMIC PA. Table Ⅱ presents the
comparison of measurement results.
Fig. 2. Structure of the doherty amplifier with gain expansion drive amplifier
44.5
38
48.5
NF
2.92
2.84
4.36
Proposed MMIC PA has high gain characteristic.
Conventional class AB PA and Doherty PA are the
characteristic of P1dB about 30dBm, but proposed PA shows
P1dB characteristic of 27.2dBm. It is analyzed that
mismatching between drive PA and final PA cause power loses.
In this case, conventional class AB PA shows the highest
efficiency characteristic but the Doherty PA has the highest
efficiency in the light of saturation point. It is found that using
drive PA causes efficiency performance degradation than
Doherty PA. The uniqueness of efficiency characteristic is that
measurement results of efficiency are higher than the
simulation results. The reason of this result is because the
actual current consumption is less than the one during the
simulation about 7mA. OIP3 which is linearity property shows
that proposed PA has the most outstanding performance.
Fig. 5. Test board for measurement
(a) Gain characteristics
OIP3
(b) In/Out characteristics
IV.
(c) Input return loss
(d) Output return loss
CONCLUSION
In this paper, InGaP/GaAs MMIC PA with gain expansion
of drive PA and Doherty structure is proposed. For
improvement efficiency degradation, Doherty structure was
added to final amplifier. In consequence, OIP has improved
about 4dB comparing to conventional class AB MMIC and
about 10dB comparing to Doherty MMIC. However, using
drive PA cause efficiency degradation. And mismatching
between drive PA and final cause significant power loss. If
matching circuits added, this problem would be resolved but
chip-size would be bigger. If this issue could be resolved with
further research, the development of PA with better
performance would be available.
REFERENCES
[1]
[2]
(e) Noise Figure
(f) PAE characteristics
[3]
Fig. 6. Measurement results
TABLE II.
COMPARISON OF MEASUREMENT RESULTS
Class AB
Doherty
proposed
Gain[dB]
22.9
17.8
30.9
P1dB[dBm]
30.3
30.77
27.3
S11/S22[dB]
-13.5 / -9.3
-8.1/-10.9
-14.2/-14.4
PAE@P1dB[%]
41.06
39.87
24.21
PAE@10dB
back-off[%]
11.4
27.6
15.21
[4]
[5]
[6]
[7]
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