A High-Efficiency, Small-Size GaN Doherty Amplifier for LTE Micro ...

A High-Efficiency, Small-Size GaN Doherty Amplifier
for LTE Micro-Cell and Active Antenna System Applications
Peter Xia, Milos Jankovic
TriQuint Semiconductor, 500 W Renner Road, Richardson, TX , 75080, USA
E-mail address: peter.xia@tqs.com
Abstract — In this paper a high-efficiency, small-size GaN
Doherty amplifier for LTE micro-cell base station and active
antenna systems base station application is presented. It is
implemented with a TriQuint Semiconductor wideband discrete
GaN RF power transistor, theT1G6001528-Q3. Doherty
amplifier performance is in the LTE standard frequency range
2.62 GHz ~2.69 GHz; average output power=38.5dBm; the peak
saturated output power is >46dBm; drain efficiency is >55%;
gain is >15dB; 2 carrier 2x10 MHz; 8dB PAR LTE signal
waveform with Netlogic standard DPD; ACPR is better than -
50dBc; Doherty amplifier size 30mm x 70mm.
Index Terms — GaN, Doherty Amplifier, LTE, Micro-Cell,
Active Antenna System, Base Station.
I. INTRODUCTION
In today’s communications networks, achieving higher data
rates and spectrum efficiency are always motivations for
developing new technology. In order to meet the increasingly
more stringent requirements of high data rate and high
spectrum efficiency demanded by wireless
telecommunications subscribers, the 4G wireless system
including Long Term Evolution (LTE) have been developed to
take advantage of some new technology. For example:
Orthogonal Frequency Division Multiplexing ( OFDM ) and
Multiple Input Multiple Output ( MIMO ) , which have
properties of higher data rates and higher spectral efficiency in
20 MHz signal bandwidth; downlink data rates of 100
megabits per second ( Mbps ) and an uplink data rate of 50
Mbps can be achieved. LTE modulation signal bandwidth is
10 MHz for one carrier and 20MHz for two carriers. In order
to provide a high data rate while consuming less power, small
size base stations such as a micro-cell base stations or an
active antenna system base station will be utilized more
frequently in LTE-based networks than in W-CDMA (3G)
networks. In this type of small-size wireless base station, highefficiency
and small-sized RF power amplifiers are necessary
to provide the performance required as cost-effectively as
possible.
Due to higher efficiency at a 6~10dB back-off range and
better linearity with a digital pre-distortion ( DPD ) system,
Doherty amplifier configurations are popularly used in
wireless base station RF power amplifiers [1]~[2]. Although
there are now some new advanced technology being
developed for wireless base-station RF power amplifiers
[3]~[4], high power and high efficiency RF amplifier
configuration utilizing a Doherty amplifier configuration still
is the most common wireless base station technology in mass
production.
A gallium nitride (GaN) RF power transistor, due to its high
efficiency and high power density, has characteristics
supporting the needs of next-generation RF power device
applications [5]~[7] and was therefore the technology of
choice for implementing this amplifier design.
II. RF GAN POWER DEVICE
The active device employed in the reported Doherty Power
Amplifier is TriQuint Semiconductor T1G6001528-Q3
packaged high electron mobility transistor (HEMT). It is a
wideband discrete GaN on SiC HEMT, which operates at 28V
and be able to support DC to 6 GHz frequency requirements.
The device is built with TriQuint’s production 0.25μm GaN
on SiC process, which features advances field plate techniques
to maximize power and efficiency at high drain bias operating
conditions. This optimization can potentially lower systems
costs in terms of simple amplifier line-ups and lower thermal
management costs.
The T1G6001528-Q3 uses a discrete die with 5 mm of total
gate periphery. It is constructed based on four, 1.25mm unit
HEMT cells, as shown in Fig. 1. The die placement and bond
wire profiles are optimized for broadband performance.
Fig. 1. T1G6001528-Q3 GaN transistor 1.25mm unit cell
The packaged device utilized in this amplifier typically
provides 18W of output power (P3dB) and linear gain higher
than 10dB at 6 GHz. Maximum PAE is greater than 50%
across the whole band. At 2.6 GHz frequency, its saturated

power is about 25W; gain is approximately 16dB; the
maximum saturated efficiency is approximately 75%.
Fig. 2 T1G6001528-Q3 package
T1G6001528-Q3 package is show as Fig.2, the device
dimension except input / output lead is 5mm x 6mm. The
performance of this small device comes from its high power
density. The small form factor of the transistor is a key factor
that enables development of a smaller-scale Doherty amplifier.
The T1G6001528-Q3 provided this performance: Vd=28V,
Idq=100mA, at pulse waveform PW=50uS, duty=10%, its
load-pull data at 2.65GHz is measured as following in Fig. 3.
The reported Doherty amplifier is designed based on this loadpull
data.
Fig. 3 T1G6001528-Q3 load-pull data
III. DOHERTY AMPLIFIER CONFIGURATION
A symmetric Doherty amplifier is a very popular RF high
power, high efficiency amplifier configuration for
contemporary wireless base stations. The amplifier
demonstrated in this paper is designed using two
T1G6001528-Q3 discrete packaged HEMTs, and the whole
Doherty amplifier size is 30mm x 70mm as shown as Fig. 4.
The small size is very necessary for micro-cell base stations or
active antenna systems base stations where space is at a
minimum.
RF in
Bias Vgs≈-­‐3.7V
Idq=100mA
Bias Vgs=-­‐5V
Carrier amplifier
T1G6001528-­‐Q3
Peakingamplifier
T1G6001528-­‐Q3
28V Vds
28V Vds
Fig. 4 30mm x 70mm Doherty Amplifier Board
RF out
The amplifier PCB material is Taconic RF35B, with a
thickness (H) of 16.6mm, and a dielectric constant (εr) of
3.66.
There is a 3dB hybrid at the input area, which is used to
split the input signal into a carrier amplifier (the up path) and a
peaking amplifier (the down path). The carrier amplifier is
biased in class AB at Idq=100mA; the peaking amplifier is
biased in class C mode. Since the carrier amplifier and
peaking amplifier operate in different modes, their output
impedances are not identical, so their output matching
circuitry is slightly different.
When designing a Doherty amplifier, the ideal situation is
to design load impedance at Zopt equal to the maximum Psat
point, and design load impedance at 2*Zopt equal to the
maximum efficiency point. But because the T1G6001528-Q3
is a wideband general purpose device, it is not specifically
designed for 2.65 GHz Doherty purposes; its maximum
efficiency is not at a 2:1 VSWR circle of the maximum
saturated power. When we designed this Doherty amplifier,
we had to compromise the load impedance at Zopt and 2 *
Zopt. This means the load impedance at Zopt is not at the
maximum saturated power point and the load impedance at 2 *
Zopt is not at the maximum efficiency point.
IV. DOHERTY AMPLIFIER PERFORMANCE
The Doherty amplifier utilizing the T1G6001528-Q3 (as
documented in Fig 3) has been measured with several kinds of
signal waveforms to characterize its performance for base
station applications.
Fig.5 shows its AM/AM and AM/PM curve, which is a
critical parameter for DPD correction performance. In a
typical normal LDMOS device Doherty amplifier, phase
always drops when input power increases, which will reduce
DPD correction performance. But in this amplifier utilizing
the T1G6001528-Q3, phase variation of input power is totally
different, which will be a benefit for DPD correction.

Fig.6 shows its PAR and efficiency variation with output
power, which is measured at 2.65GHz, the WCDMA signal
waveform with PAR @ 0. 01% CCDF = 10.2dB.
Fig. 7 shows its high power performance over the standard
LTE frequency range of approximately 2.62GHz to 2.69GHz,
which is measured with a WCDMA signal waveform, the
data is tested at 38dBm average output power, the saturated
power is calculated using an average output power plus PAR.
It can be seen from Fig.7 that in a standard LTE frequency
range (2620 MHz~2690 MHz), when average output power at
38dBm, its efficiency is higher than 55%.
In contemporary base station designs, the RF power
amplifier output need varies when the number of phones
calling users within a given geographic area varies. This
requirement is called “traffic control”. In order to let base
stations always operate at high efficiency, generally the
variation of output power is implemented through adjusting
RF power amplifier working voltages. So that the base station
Gain(dB)
17
16
15
14
13
12
11
10
Output_PAR(dB)
15
13
11
9
7
5
2.65GHz AM/AM & AM/PM
Gain
Phase
13 15 17 19 21 23 25 27 29 31 33
Pin( dBm)
Fig. 5 AM/AM and AM/PM
T1G6001528 Doherty Output PAR and Efficiency vs. Pout
1C-­‐WCDMA , 10.2dB PAR at 0.01%CCDF
30 31 32 33 34 35 36 37 38 39
Pout (dBm )
130
128
126
124
122
120
118
116
Phase(° )
AM/AM
AM/PM
Fig. 6 PAR and efficiency variation with output power
60
56
52
48
44
40
36
32
28
24
20
Efficiency ( % )
Psat ( dBm) & Eff ( % )
Pout (dBm)
60
58
56
54
52
50
48
46
44
42
40
39.5
39
38.5
38
37.5
37
36.5
36
35.5
35
WCDMA Performance
1C WCDMA , 10.2dB PAR at 0.01%CCDF
Eff
Psat
Psat
Eff
Gain
40
10
2620 2630 2640 2650 2660 2670 2680 2690
Frequency ( MHz )
Gain
Fig. 7 Psat , Eff, Gain variation over frequency range
T1G6001528 Doherty Eff and Pout @7.5dB OBO vs Drain Voltage
1C-­‐WCDMA , 10.2dB PAR at 0.01%CCDF
24 26 28
Drain Voltage (V)
30 32
Fig. 8 Eff and Pout variation with drain voltage
Always operates at the highest efficiency, base station RF
power amplifiers are required to provide stable efficiency
when working voltages change. Fig. 8 shows the drain
voltage range of 24V ~ 32V, with the same WCDMA
waveform, and the same PAR (7.5dB), the efficiency and
average output power varies with the voltage.
To satisfy the requirements of high-efficiency base station
operations, most RF power amplifiers need to operate with
digital pre-distortion (DPD) to obtain linear performance,
which is represent with ACPR in RF power amplifiers. So for
today’s base station RF power amplifiers, the performance of
DPD correction is very important. In order to verify this
Doherty amplifier’s DPD correction effect , the system
utilized in this paper was tested with a Netlogic standard DPD
system, in a two carrier LTE (20 MHz signal bandwidth)
environment, with PAR @ 0.01% CCDF = 8dB. The
measured ACPR performance at 2.65GHz is shown Fig 9, at
20
19
18
17
16
15
14
13
12
11
60
59
58
57
56
55
54
53
52
51
50
Gain ( dB )
Efficiency (%)

38.5dBm Pout. After DPD is applied, the ACPR is better than
-50dBc.
ACPR (dBc)
-­‐20
-­‐25
-­‐30
-­‐35
-­‐40
-­‐45
-­‐50
-­‐55
-­‐60
T1G6001528 Doherty Linearity
2C 0110 10MHz LTE, 8dB PAR at 0.01%CCDF, 2650MHz
Before DPD
After DPD
33 34 35 36 37 38 39
Pout (dBm)
Eff
Fig. 9 ACPR performance with DPD
V. CONCLUSION
A 2.6 GHz GaN Doherty amplifier has been demonstrated.
At 38.5dBm average output power, in a standard LTE
frequency range of 2.62 GHz ~ 2.69 GHz, drain efficiency is
greater than 55%; gain is greater than 15dB. For a two carrier
20 MHz bandwidth 8dB PAR LTE signal, ACPR after DPD is
better than -50dBc. The Doherty amplifier dimension is 30mm
x 70mm. The combination of high efficiency and small size
made possible utilizing gallium nitride transistors in a base
station amplifier is very useful for LTE micro-cell base
stations as well as active antenna array system designs.
65
60
55
50
45
40
35
30
25
Eff ( % )
ACKNOWLEDGEMENT
The authors would like to express their appreciation for the
assistance and support of TriQuint Semiconductor’s Defense
Products and Foundry Services business unit, which provided
load pull fixtures, gallium nitride transistors and additional
resources for the initial assessment of this project, as well as
Mr. Jeff Gengler who provided DPD testing.
REFERENCES
[1] Steve C Cripps, “RF Power Amplifier for Wireless
Communication”, Norwood, MA, Artech House, 1999.
[2] Frederick H. Raab, et al., “Power Amplifier and Transmitter for
RF and Microwave”, IEEE Trans. Microwave Theory Tech.,
Vol. 50 pp. 814-826, March 2002
[3] D. Kimball, et al., “High Efficiency WCDMA Envelope
Tracking Base-Station Amplifier Implemented with GaAs
HVHBTs”, 2008 IEEE MTT-S Int. Microwave Symposium
Digest.
[4] I, Kim, et al., “Envelope Injection Consideration of High Power
Hybrid EER Transmitter for IEEE 802.16 Mobile WiMAX
Application”, 2008 IEEE MTT-S Int. Microwave Symposium
Digest .
[5] H. Deguchi, et al., “A 33W GaN HEMT Doherty Amplifier with
55% Drain Efficiency for 2.6GHz Base Stations”, 2010 IEEE
MTT-S Int. Microwave Symposium Digest.
[6] H. Sano, et al., “A 40W GaN HEMT Doherty Power Amplifier
with 48% Efficiency for WiMAX Application”, 2007 IEEE
Compound Semiconductor Integrated Circuit Symposium
Digest.
[7] N. Yoshimura, et al., “A 2.5-2.7GHz Broadband 40W GaN
HEMT Doherty amplifier with higher than 45% drain efficiency
for multi-band applications”, 2012 IEEE Topical Conference on
Power Amplifiers for Wireless and Radio Applications.
.