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.

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of a diversity antenna with stubs for UWB applications, Microwave

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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

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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.

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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:bsjin@postech.ac.kr or boshi.J@gmail.com

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

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3. B. Jin, K. Han, J. Choi, D. Kang, and B. Kim, The fully-integrated

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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