the isolation characteristic, as shown in Figure 6(b). Tables 1
and 2 show the measured antenna efficiencies t various gap dis-
tances and interconnection positions. As the maximum value
available for h is limited to 3 mm, we chose the gap distance
between two boards to be 3 mm. The results reveal that the effi-
ciency of the proposed MIMO antenna is dependent upon the
isolation between the two radiating elements.
Figures 7(a)–7(f) show three-dimensional (3D) radiation pat-
terns of the designed two radiating elements at 750, 880, and
1920 MHz, respectively. Although the laptop structure affects
the radiation pattern, especially in low band, the measured radia-
tion patterns are nearly omnidirectional over most of the fre-
quency band. Figure 8 shows the measured antenna gain and the
envelope correlation coefficient (ECC) as a function of fre-
quency. The average gains of the two antenna elements are
shown as short dashed and dashed-dotted lines, and the ECC is
drawn as a solid line in Figure 8. The gain varies from ~�6.7
dBi to �1.7 dBi, and the ECC is mostly lower than 0.35 across
the entire bandwidth. Although the MIMO antenna is installed
within an electrically small USB dongle device, the proposed in-
ternal MIMO antenna can achieve good gain and ECC perform-
ances using the two-stage ground. The mean effective gain
(MEG) values vary from 0.28 to 0.47 over the entire bandwidth,
as shown in Figure 9.
4. CONCLUSIONS
A compact internal MIMO antenna with a separated two-stage
ground is proposed. The two radiating elements in the MIMO
antenna have good performance and a wide bandwidth for wireless
communications (LTE, DCN, and PCS-1900). Although the two
elements are embedded in a USB dongle and operate at low fre-
quency (LTE band), good isolation and relatively high antenna ef-
ficiency were achieved by using a two-stage ground structure. The
proposed antenna is a superior candidate for future wireless appli-
cations due to its capability to fit within a USB dongle terminal.
REFERENCES
1. Available at: http://en.wikipedia.org/wiki/4G.
2. Yong-Sun Shin and Seong-Ook Park, Spatial diversity antenna for
WLAN application, Microwave Opt Technol Lett 49 (2007),
1290–1294.
3. X. Wang, Z. Du, and K. Gong, A compact dual-element antenna
array for adaptive MIMO system, Microwave Opt Technol Lett 51
(2009), 348–351.
4. S. Hong, K. Chung, J. Lee, S. Jung, S.-S. Lee, and J. Choi, Design
of a diversity antenna with stubs for UWB applications, Microwave
Opt Technol Lett 50 (2008), 1352–1356.
5. Y. Kim, J. Itoh, and H. Morishita, Study on the reduction of the mu-
tual coupling between two L-shaped folded monopole antennas for
handset, Antennas Propag Soc Int Symp, San Diego, CA (2008), 1–5.
6. A.C.K. Mak, C.R. Rowell, and R.D. Murch, Isolation enhancement
between two closely packed antennas, IEEE Trans Antennas
Propag, 56 (2008), 3411–3419.
7. K.-J. Kim and K.-H. Park, The high isolation dual-band inverted F
antenna diversity system with the small N-section resonators on the
ground plane, Microwave Opt Technol Lett 49 (2007), 731–733.
8. Computer Simulation Technology (CST) and Acceleware Corpora-
tion, CST microwave studio 2008, Computer Simulation Technol-
ogy (CST) and Acceleware Corporation, 2008. Available at: http://
www.cst.com.
VC 2010 Wiley Periodicals, Inc.
A COMPACT BROADBANDTRANSFORMER-BASED LINEAR CMOSPOWER AMPLIFIER DESIGN
Jin Boshi, Zhao Chenxi, and Kim BummanDepartment of Electronics and Electrical Engineering, PohangUniversity of Science and Technology, Pohang, Kyoungbuk 790-784, Republic of Korea; Corresponding author:[email protected] or [email protected]
Received 3 May 2010
ABSTRACT: A novel broadband transformer-based CMOS poweramplifier (PA) design method is studied in this article. To obtain abroadband PA, the parasitic parameters of the transformer are absorbed
into the PAs load match and their impacts on bandwidth are studied.The fully-integrated PA combined with an 8-shaped transformer is
implemented in 0.13 lm CMOS process with only 1.2 � 1.2 mm2 chipsize and operates at Class AB mode. The single-stage PA delivers 27.36dBm output power with 27% efficiency and has 10.5 dB gain. It has 500
MHz bandwidth (1 dB degeneration) in the large and small signalmeasurements. IMD3 and IMD5 are also lower than �25 dBc at 19
dBm across the bandwidth. The spectrum of PA can meet the m-WiMAXspectrum mask at 19 dBm average power level. VC 2010 Wiley
Periodicals, Inc. Microwave Opt Technol Lett 53:422–425, 2011; View
this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25731
Key words: linear; CMOS; power amplifier; broadband; transformer
1. INTRODUCTION
Recently, with the demand of a single-chip transceiver and cost
pressure in volume production, CMOS power amplifier (PA)has
been studied intensively [1–3]. A transformer is an essential ele-
ment for implementation of a fully integrated CMOS PA
because it can successfully solve the source inductance problem
by offering an RF virtual ground and boost the load impedance
level [4]. To make the PA more compatible, the PA also
inclines to have multimode and multiband operation, thus, a
broadband operation becomes favored for PA [5]. Previously,
Figure 9 Measured MEGs for radiator 1 and 2
Figure 8 Measured average gains and ECCs of two antenna elements.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
422 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 DOI 10.1002/mop
the designs of transformer-based CMOS PA are heavily depend-
ent on EM tools optimization, and there are few works to give
the broadband design guidelines. The purpose of this article is
to demonstrate the method of a broadband transformer-based
CMOS PA design. Firstly, a novel broadband transformer design
method by absorbing the parasitic parameters of transformer
into the load match for PA is explained. Secondly, to verify the
broadband performance, a linear PA using 0.13 lm standard
CMOS process is implemented, and the measured results prove
that the PA has over 20% bandwidth and good linearity.
2. BROADBAND TRANSFORMER DESIGN
In the transformer-based PA, the transformer functions not only
for the power combination but also for the load match for PA.
Any of load mismatches will cause the output power degrading
across the bandwidth even though the transformer itself has a
broad bandwidth. Thus, the parasitic parameters of the trans-
former have to be absorbed into the power matching circuit
across the desired bandwidth.
The model and equivalent circuit of the transformer are
given in Figure 1. CIN and COUT are the input and output
capacitances, Cds is the output capacitance of the power cell and
is a very frequency-dependent variable. Lp, Ls, and Rp, Rs, are
the equivalent inductances and resistances of the primary and
secondary loops. k is the coupling factor between two loops. ZTis the transformed impedance. ZLoad is the load impedance seen
by PA and should follow the power match, and its variation
with frequency should be minimized to obtain constant output
power over the desired bandwidth. Because Rp, Rs, and Ls havelittle impacts on the load impedance transformation, they are
ignored. The variation of ZLoad with the different Lp and k are
depicted in Figures 2(a) and 2(b). From the above analysis, we
can find that to reduce the variation of ZLoad across a broad
bandwidth, Lp should be reduced and k should be increased,
accordingly.
To verify the theory work, an 8-shaped broadband trans-
former is proposed in this article as shown in Figure 3. It is
composed of two rings and combines four ways differential sig-
nals. The secondary loop is placed between the dual primary
loops to enhance the coupling factor k and reduce the primary
inductance Lp. Both of the loops use the top layer (3.3 lm thick-
ness copper metal), and dual primary loops are connected with
lower layers. The width is 12 lm for each loop and the spacing
is 3 lm. Because each magnetic loop has the same current
Figure 1 The model and equivalent circuit of the transformer. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
Figure 2 The ZLoad as a function of frequency with the different Lp
and Ls. (B) The ZLoad as a function of frequency with the different k.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
Figure 3 An 8-shaped transformer and the calculated transformed load
impedance. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com]
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 423
direction, they are separated by some distance to make it
immune to common mode oscillation. The total size of the
transformer is only 720 � 220 lm2, which is the most compact
one reported at the frequency of interest.
The extracted inductances and resistances of loops with
ADS2008TM MOM simulator are summarized in Table 1. The
value of Lp has only about one-fourth of Lss, and the coupling
factor k is 0.74. CIN and COUT are selected with 5.4 pF and 1.2
pF, respectively, to make the transformer resonant at 2.5 GHz.
The transformed impedance is calculated and depicted in Figure
3. The real part of ZLoad has less 10% variation from 2.0 to 2.9
GHz. The imaginary of ZLoad is minimized to reduce PAs power
loss.
The test pattern of transformer is shown in Figure 4. To mea-
sure the three-port transformer using two-port network analyzer,
the same two transformers are layout in back-to-back configura-
tion. The differential ports of one of the transformers are
directly connected to the other transformer. The insertion loss of
single transformer is half of total loss in dB unit. The measure-
ment and simulation are compared in Figure 4. The loss of
transformer is 0.96 dB (efficiency is 80%) at 2.5 GHz
3. IMPLEMENTATION OF THE BROADBANDLINEAR CMOS PA
A fully-integrated linear CMOS PA is designed and combined
with the proposed transformer. The schematic and chip photo-
graph are shown in Figures 5(a) and 5(b). All the matching
components are fully integrated, and some large inductances are
replaced by bonding wires to save area, enabling a small chip
size of 1.2 � 1.2 mm2. The on-chip input balun converts the
single ended signal into the four ways differential signals. Each
power stage uses the cascode structure to obtain a high gain. M1
is selected with 0.25 lm (thick type) to enhance the reliability,
TABLE 1 Extracted Parameters of 8-Shaped Transformer
Parasitic parameters Values
Primary inductance Lp 0.59 nH
Secondary inductance Ls 2.19 nH
Primary resistance Rp 0.8 XSecondary resistance Rs 4.3 XCoupling factor k 0.74
Figure 4 The layout of test pattern and measured loss of transformer.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
Figure 5 Fully-integrated linear CMOS PA (a) and fully-integrated
linear CMOS PA (b). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com]
Figure 6 Measured output power, gain, efficiency, and PAE. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
424 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 DOI 10.1002/mop
and M2 is selected with 0.13 lm (thin type) to enhance the
gain. The total gate width of four ways is 7680 lm. To improve
the linearity, the gate bias of M2 is selected with 0.55 V where
gm3 is zero-cross-point, and the second harmonic circuit is
attached at the drain and source terminals. The gate bias of M1
is 1.9 V to relieve the voltage stress and the drain supply is 2.4
V.
The single tone test at 2.5 GHz results are shown in Figure
6. The single stage PA can offer 10.8 dB power gain, and the
output power is 24.8 dBm at P1dB and saturated power is 27.36
dBm with 27% drain efficiency. The measured power gain at
P1dB point and S parameters are shown in Figure 7. From 2.2 to
2.7 GHz, the PA has less than 1 dB variation, and the bandwidth
can be over 20%. Even this PA is not designed for any specific
standard, however, it can be compatible with WLAN and
WiMAX bands. In the small signal test, S21 also presents the
similar performance with power gain. Both of S11 and S22 are
lower than �10 dB from 2.2 to 2.7 GHz. IMD3 and IMD5 are
measured with 5 MHz two-tone signal across the desired band-
width shown in Figure 8, and they are lower than �25 dBc at
19 dBm power level. To examine the outer band linearity of
PA, the PA is measured with 16QAM m-WiMAX modulation
signal (9.6 dB PAPR and 8.75 MHz signal bandwidth). It can
meet the m-WiMAX spectrum mask at 19 dBm average power
at 2.5 GHz.
4. CONCLUSION
In this article, a very compact broadband transformer-based
CMOS PA design method is explained, and the fully-integrated
CMOS PA using the proposed 8-shaped transformer is imple-
mented with a small chip size of 1.2 � 1.2 mm2. The PA can
deliver 27 dBm saturated power with a 27% PAE and offers
10.8 dB gain. Both of the power gain and S21 have 500 MHz
bandwidth. The PA also has the acceptable linearity across the
bandwidth of interest.
ACKNOWLEDGMENTS
This research was supported by WCU (World Class University)
program through the Korea Science and Engineering Foundation
funded by the Ministry of Education, Science and Technology
(Project No.R31-2008-000-10100-0).
REFERENCES
1. J. Kang, J. Yoon, K. Min, D. Yu, J. Nam, Y. Yang, and B. Kim, A
highly linear and efficient differential CMOS power amplifier with
harmonic control, IEEE J Solid-State Circuits 41 (2006),
1314–1332.
2. O. Degani, F. Cossoy, S. Shahaf, D. Chowdhury, D. Hull, C. Ema-
nue, and R. Shmuel, A 90nm CMOS power amplifier for 802.16e
(WiMAX) applications, In: Proceedings of IEEE radio frequency
integrated circuits (RFIC) symposium, Boston, 2009, pp.373–376.
3. B. Jin, K. Han, J. Choi, D. Kang, and B. Kim, The fully-integrated
CMOS RF power amplifier using the semilumped transformer,
Microwave Opt Technol Lett 50 (2008)2857–2860.
4. I. Aoki, S.D. Kee, D.B. Rutledge, and A. Hajimiri, Distributed
active transformer a new power combining and impedance transfor-
mation technique, IEEE Trans Microwave Theory Tech 50 (2002),
316–331.
5. C. Park, Y, Kim, H. Kim, and S. Hong, A 1.9-GHz triple-mode
class-E power amplifier for a polar transmitter, IEEE Microwave
Wirel Compon Lett 17 (2007), 148–150.
VC 2010 Wiley Periodicals, Inc.
Figure 7 Measured power gain at P1dB and S parameters. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
Figure 8 The measured linearity with 5 MHz two-tone signal. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com]
Figure 9 Measured spectrum with 16 QAM m-WiMAX modulation
signal. [Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com]
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 425