Linearity and Efficiency Improvement Using …wseas.us/e-library/conferences/2013/Baltimore/TESIMI/...Linearity and Efficiency Improvement Using Harmonic Suppression Power Combiner

Linearity and Efficiency Improvement Using Harmonic Suppression Power Combiner in GaN S-band Power Amplifier Design

CAROLINE WAIYAKI1, MICHEL A. REECE1, EDWARD VIVERIOS2

1Center of Microwave, Satellite and RF Engineering(COMSARE), Morgan State University, 5200 Perring Parkway, Baltimore, MD, 21251

2 Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783 UNITED STATES OF AMERICA

[email protected] Abstract: - In this paper, a Gallium Nitride (GaN) power amplifier (PA) design implementing a novel power combiner with harmonic suppression is presented. This new combiner, denoted as ‘Wei-Chi’ after the author, is compared to the traditionally used Wilkinson combiner in a single-stage two-way configuration at S-band. The Wei-Chi PA demonstrates 7% output power (Pout) and 26% power added efficiency (PAE) improvement under single-tone CW measurements. Under two-tone measurements, the Wei-Chi PA delivers twice the output power and a 69.5% PAE improvement at the C/I ratio of 30 dBc in comparison to the Wilkinson PA. In addition, a 4-way Wei-Chi combiner is implemented within a 20W high power amplifier (HPA) microwave integrated circuit (MIC) design and is compared to a high power amplifier (HPA) that utilizes a 4-way Wilkinson power divider/combiner. The Wei-Chi HPA achieved 10% PAE improvement while maintaining comparable output power performance with the Wilkinson HPA. Key-Words: - Gallium Nitride, linearity, PAE, power amplifier, HPA, power combiner. 1 Introduction Power combining is frequently used to achieve high power levels in microwave and millimeter-wave Gallium Arsenide (GaAs) SSPA designs as they continue with the quest to replace the traveling wave tube (TWT) technologies for Satellite communication (SATCOM) applications. In the past, spatial combining had been favored over planar due to high losses when combining more than eight devices and/or power amplifiers [1] - [2]. Gallium Nitride device technology has advanced over the recent years producing greater than five times power densities compared to GaAs device technology. With these high power densities the GaN SSPAs only require about four devices at the power amplifying stage to achieve high output power levels especially at millimeter-wave frequencies [3], compared to GaAs SSPAs that can require sixteen and more devices for similar output power [4]. GaN technology has revived the need for planar power combining use in SSPA MMIC designs.

Wilkinson power combiner/divider has been widely used at microwave/millimeter-wave frequencies. Most recently, a novel power combiner with harmonic suppression was demonstrated in [5] at 1 GHz. This new combiner, the Wei-Chi

combiner, gave an advantage over the Wilkinson combiner by suppressing 2nd and 3rd harmonics while giving similar performance to the Wilkinson combiner at the fundamental frequency.

In this paper the Wei-Chi combiner was designed and implemented in a two-way, and four-way, GaN PA design and the results compared to a Wilkinson PA at 2.8 GHz. The two-way, Wei-Chi GaN PA demonstrated 7% output power (Pout) and 26% power added efficiency (PAE) improvement under single-tone CW measurements. Under two-tone measurements, the Wei-Chi PA delivers twice the output power and a 69.5% PAE improvement at the C/I ratio of 30 dBc in comparison to the Wilkinson PA. The four-way Wei-Chi GaN HPA demonstrated 10% PAE improvement compared to the 4-way Wilkinson GaN HPA without degrading power performance.

This is the first time to the author’s knowledge that the Wei-Chi combiner has been implemented in a GaN PA design. 2 Wei-Chi Combiner Design Fig. 1 shows the schematic diagrams for the Wei-Chi combiner and Wilkinson combiner. The theory

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ISBN: 978-960-474-330-8 152

α = 1 Values R = 100Ω ZA = 50Ω ZB = 50Ω ZC = 40.82Ω Θ = 90 °

α = 0.6 Values R = 60Ω ZA = 41.2Ω ZB = 45.8Ω ZC = 44.1Ω Θ = 110 °

behind this combiner is detailed in [5] and highlighted in this section. The Wei-Chi combiner follows the theory of Wilkinson as recorded by Pozar [6], and includes the additional microstrip ZB and ZC stubs. The extended line, ZB, (electrical length = λ /4) and the shunt stub, ZC, (electrical length = λ/6) are primarily responsible for the suppression of the second and third harmonic bands respectively by the creation of additional transmission zeros [5].

(a) (b) Fig. 1: Schematic diagrams: (a) Wei-Chi combiner

[5], (b) Wilkinson combiner [6]

Even-mode and odd-mode analyses were performed to determine the line impedances and electrical length (θ) according to [5]. (See equations 1 through 5). Fig. 2 displays the line impedances as a function of α.

(1)

(2) (3)

(4)

(5)

Fig.2: Line Impedance vs. α plot [5]

Based on preliminary simulations α = 0.6 and α = 1 were used to calculate the line impedances, which are displayed in Table 1 below. The 2-way configuration implemented α = 1 values while the 4-way configuration used α = 0.6 values.

Table 1: Impedance values for α=0.6 and α=1

2.1 Wilkinson and Wei-Chi combiners AWR’s Microwave Office (MWO) circuit simulator was used to design the 2-way and 4-way Wilkinson and Wei-Chi combiners. Ansoft’s High Frequency Structure Simulator (HFSS) was used to perform the electromagnetic simulations. Fig. 3 shows the two-way combiners’ photographs while Fig. 4 shows the four-way HFSS’ layout of the combiners.

(a) (b)

Fig. 3: Photographs of (a) Wilkinson & (b) Wei-Chi two-way divider/combiners

(a) (b)

Fig. 4: HFSS’ Four-way divider/combiner layouts: (a) Wei-Chi; (b) Wilkinson

( )

( )

251

2

11arctan

2

32

21

2

0

2

0

0

20

+<=

−+

−−=

−=

−=

−+−

=

ZR

ZZ

ZZ

ZZ

C

B

A

α

αααπθ

αα

αααααα


ISBN: 978-960-474-330-8 153

2.2 Combiner Experimental Results 2.1.1 2-way Combiner Results The scattering parameter measurements were taken using Agilent’s PNA Network analyzer over the 0.1 to 10 GHz. Fig. 5 and Fig. 6 show a comparison of the Wilkinson and Wei-Chi combiners’ insertion loss and return loss performance respectively. The Wei-Chi suppresses the 2nd harmonic by greater than 30dB and 3rd harmonic by greater than 20dB, while maintaining similar performance as Wilkinson at the fundamental frequency.

Fig. 5: Comparison of Combiners Insertion Loss

(Solid – Wei-Chi combiner; Dash – Wilkinson combiner; Triangle –S21, Square – S31)

Fig. 6: Comparison of Combiners Return Loss

(RL) (Solid – Wei-Chi combiner; Dash – Wilkinson combiner; Triangle –Input RL, circle – Output RL)

The combiner’s back-to-back configuration gives the overall loss of the divider/combiner as implemented in the HPA design. Fig. 7 and Fig. 8 below display the two-way, back-to-back Wilkinson and Wei-Chi combiners’ insertion loss performance.

Fig.7: Two-way, back-to-back, Wilkinson combiner insertion loss in dB (Solid - measured; Dash -

simulated)

Fig.8: Two-way, back-to-back, Wei-Chi combiner insertion loss in dB (Solid - measured; Dash -

simulated) Table 2 gives a summary of the tabulated results

at the fundamental frequency, 2.8 GHz and 2nd harmonic, 5.6 GHz, and 3rd harmonic, 8.4GHz, for the two-way, back-to-back combiners’ configurations.

Table 2: Tabulated results for two-way, back-to-back, S-band Wilkinson & Wei-Chi Combiners

Two-way Wilkinson Two-way Wei-Chi

Parameter Simulated Value

Measured Value

Simulated Value

Measured Value

S21 @ 2.8 GHz -0.113 dB -0.594 dB -0.134 dB -0.489 dB

S21 @ 5.6 GHz -1.36 dB -2.486 dB -9.185 dB -39.18 dB

S21 @ 8.4 GHz -1.106 dB -1.822dB -25.09 dB -25.23 dB

S11 @ 2.8 GHz -48.56 dB -21.29 dB -36.67 dB -18.51 dB

S22 @ 2.8 GHz -43.82 dB -22.05 dB -42.29 dB -18.51 dB


ISBN: 978-960-474-330-8 154

2.1.2 Four-way back-to-back Combiner Results The 4-way combiners were measured as combiner/spitter as well as back-to-back to show the overall loss associated with the combiner. Fig. 9 and Fig.10 show back-to-back insertion losses of Wei-chi configuration and the Wilkinson confuration respectively.

The insertion loss values are 0.9205 dB and 0.6225dB respectively at 2.8 GHz. The losses compare but the novel Wei-Chi network has added advantage of harmonic suppression. At 5.6 GHz the measured insertion loss is -37 dB and at 8.4GHz is -39dB (see Fig. 10).

Fig. 9: Four-way Wilkinson Combiner back-to-

back Measured vs. Simulated insertion loss [Solid-Simulated; Dashes-Measured]

Fig. 10: Four-way Wei-Chi Combiner back-to-back

Measured vs. Simulated insertion loss. [Solid-Simulated; Dashes-Measured]

3 High Power Amplifier Design In this section, a single-stage PA design is discussed. The main purpose of this PA is to be used with the combiners and benchmark their performance. In addition the 2-way and 4-way combined HPAs implemented in a single-stage configuration are discussed.

3.1 5-Watt Single-Stage PA Design A single-stage PA was designed using the Cree’s 10W CGH40010F GaN packaged device and was fabricated on 5870 duroid material. Fig. 11 shows the photograph of the prototyped single-stage PA.

Fig. 11: Photograph of 2.8GHz 5W single-stage PA

AWR’s Microwave Office software was used for

the PA simulations. The power performance of the PA was measured at 2.8GHz and the bias of 20V drain voltage and 200 mA quiescent current and the results are displayed in Fig. 12 below.

Fig. 12: 2.8GHz 5W-PA Measured vs. Simulated Power performance. [Solid-Simulated; Symbol-

Measured; Circle – Pout (dBm); Square– Gain (dB); Triangle – PAE (%)]

Power gain of 12.5 dB and the Pout at 1 dB

compression point of 37.65 dBm (5.8W) was measured at the above bias. Theoretically combining four PAs should yield 43.65 dBm (23.2 W) with no losses. This value will be used to gauge how well the 4-way combiners performed.

3.2 Four-way HPA Design The single-stage PA described in section 3.1 was used to implement the four-way HPA designs without any further optimization. Fig. 12 and Fig. 13 display photographs of the fabricated 2.8GHz


ISBN: 978-960-474-330-8 155

HPA implementing four-way Wilkinson and Wei-Chi combiner/divider networks respectively.

Fig. 11: 2.8 GHz Wilkinson HPA photograph

Fig. 12: 2.8 GHz Wei-Chi HPA photograph

The two HPAs were designed to fit 5X5 In2 heat

sink that was used for the single-stage PA. The same bias used in the single-stage PA was to characterize the HPAs. The measured vs. simulated results from the Wilkinson HPA and Wei-Chi HPA are shown in the following fig. 13 and fig. 14 respectively. Despite the Wilkinson combiner exhibiting 0.3 dB loss reduction, the power performances for the 2 designs were comparable. Table 3 shows a tabulated measured power results of the 2 PA configurations at 2.7 GHz and bias of Vds = 20V and Ids = 200mA. Table 3: Four-way HPA tabulated measured results

Fig. 13: 2.8GHz Wilkinson HPA power

performance [Diamond – Pout (dBm); Triangle – Gain (dB); Circle – PAE (%)]

Fig. 14: 2.8GHz Wei-Chi SSPA power performance

[Diamond – Pout (dBm); Triangle – Gain (dB); Circle – PAE (%)]

3.3 Two-way HPA Design Based on the analysis of the four-way HPA design performance, the single-stage design was re-designed for improved power performance. Fig. 15 shows a photograph of the re-designed single-stage PA.

Fig. 15: Photograph of re-designed single-stage PA

The single-stage design shifted in frequency slightly to peak at 2.7 GHz as shown in Fig. 16 below.

Parameter Wei-Chi SSPA Wilkinson SSPA Gain 12.58dB

11.83dB

Pout1dB 42.54dBm

41.91dBm

PAE1dB 36.47 %

35.5 %

PAEPeak 47.11 %

42.77 %


ISBN: 978-960-474-330-8 156

Fig. 16: Single-stage S-parameter results in dB at a

bias of Vds = 20V and Ids = 200mA. (Solid - measurements; Dash - simulated)

Although the combiners were optimized at 2.8

GHz, they still gave good performance at 2.7 GHz and therefore were not modified. Fig. 17 shows the power performance of the re-designed single-stage PA at 2.6-, 2.66-, 2.7-, 2.75-, and 2.8- GHz frequencies bias of Vds = 20V and Ids = 200mA.

Fig. 17: Pout (dBm - square), Gain (dB - diamond)

and PAE (%) @ 1dB - triangle and Peak - circle (Solid –Measured; Dash – Simulated)

Then two PA designs were combined using the

two-way Wilkinson and Wei-Chi combiners discussed in section2 as shown in Fig.18 below.

(a) (b)

Fig. 18: Photographs of (a) Wilkinson PA & (b) Wei-Chi PA

The s-parameter measurements of the two-way

PA designs were performed at class AB bias of Vds = 20V and Ids = 200mA and they are shown in Fig. 19 below.

Fig. 19: Two-way PA S-parameter results in dB at a bias of Vds = 20V and Ids = 200mA. (Solid – Wei-

Chi PA; Dash – Wilkinson PA; Square - S11; Triangle - S21; Circle - S22)

3.3.1 Single-Tone Measurements The power measurements were taken at 2.66-, 2.7-, 2.75-, and 2.8 GHz. Fig. 20 below shows the single-tone CW power measurements at class AB bias point of Vds = 20V and Ids = 200mA for both PA configurations. Figure 21 shows the power performance at 2.8 GHz, 5.6 GHz, 2nd harmonic, and 8.4 GHz, 3rd harmonic, at the same bias for both amplifier configurations.

Fig. 20: Two-way PA Results: Pout (dBm), Gain

(dB) and PAE (Solid –Wei-Chi PA; Dash – Wilkinson PA)

Fig. 21: Measured Pout in dBm at Fundamental, 2nd, and 3rd Harmonic frequencies (Solid – Wei-Chi PA;

Dash – Wilkinson PA)


ISBN: 978-960-474-330-8 157

Table 4 shows a tabulated measured power results of the single-stage PA and the two-way

configurations at 2.7 GHz and bias of Vds = 20V and Ids = 200mA.

Table 4: Single-tone Power performance for Single-stage PA, Wei-Chi HPA, & Wilkinson HPA Parameter Single-Stage PA Wei-Chi SSPA Wilkinson SSPA

Power Characteristics at 1dB of Compression:

Gain1dB 15.45 dB (@ PIN = 16.53dBm)

15.48 dB (@ PIN = 20.59dBm)

15.16 dB (@ PIN = 18.54dBm)

Pout1dB 31.98dBm (@ PIN = 16.53dBm)

36.07dBm (@ PIN = 20.59dBm)

33.7dBm (@ PIN = 18.54dBm)

PAE1dB 37.56 % (@ PIN = 16.53dBm)

35.28 % (@ PIN = 20.59dBm)

28.13 % (@ PIN = 18.54dBm)

Power Characteristics in Compression at Maximum PAE:

Gain 11.32 dB (@ PIN = 26.15dBm)

10.91 dB (@ PIN = 30.36dBm)

10.93 dB (@ PIN = 28.7dBm)

PoutSAT 37.47dBm (@ PIN = 26.15dBm)

41.27dBm (@ PIN = 30.36dBm)

39.63dBm (@ PIN = 28.7dBm)

PAEPeak 66.49 % (@ PIN = 26.15dBm)

65.48 % (@ PIN = 30.36dBm)

51.92 % (@ PIN = 28.7dBm)

3.3.2 Two-Tone Measurements The two-tone measurements were performed with two carriers spaced 5 MHz apart. Typical linearity measure used in industry is carrier‐to-intermodulation ratio (C/I) of 30 dB. C/I is based on the ratio expressed in dB between the amplitude of either carrier and the highest intermodulation product (IMD3).

Fig. 22 below shows that the Wei-Chi PA achieved 30 dBc at 18 dBm input drive which resulted in Pout of 33.54 dBm. The Wilkinson PA achieved 30 dBc at 15 dBm input drive producing 30.43 dBm Pout. It is noted that at 30 dBc linear condition, then the Wei-Chi would give twice the output power at the expense of being driven with a higher input power. The difference in 3 dBm output power between the Wei-Chi and Wilkinson PAs has a great impact on the PAE performance. The Wei-Chi PA yields 30.5% PAE while the Wilkinson PA yields 17.7 % PAE at the 30 dBc, giving a 69.5 % improvement under linear condition.

Fig. 22: Two-tone measured results for Wei-Chi PA

(Solid) & Wilkinson PA (Dash)

4 Conclusion In this paper, the novel harmonic suppression combiner, Wei-Chi combiner, was successfully implemented in GaN PA designs at S-band. Wilkinson combiner, mostly used in microwave application, was also designed to bench-mark the new combiner performance.

In the two-way and four-way PA design configurations, the Wei-Chi combiner exhibited comparable performance to Wilkinson combiner at the fundamental frequency, while demonstrating superior harmonic suppression of greater than 20 dB at the 2nd and 3rd harmonic frequencies. In addition, Wei-Chi PA exhibited significant power and efficiency improvements at S-band. However, the Wei-Chi combiner demonstrated a much narrow bandwidth performance compared to Wilkinson combiner.

The two-way Wei-Chi PA measured output power improvement of 7% over two-way Wilkinson PA was attained while the PAE was improved by 26% compared to the Wilkinson PA. The Wei-Chi PA also demonstrated great improvement under the two-tone conditions producing twice the output power and 69.5% PAE improvement at the C/I ratio of 30 dBc point.

The four-way Wei-Chi GaN HPA demonstrated 10% PAE improvement compared to the 4-way Wilkinson GaN HPA without degrading power performance.


ISBN: 978-960-474-330-8 158

5 Acknowledgement The authors wish to thank the COMSARE (Center of Microwave, Satellite, and RF Engineering) located within the Department of Electrical and Computer Engineering at Morgan State University. The authors also would like to thank Army Research Laboratory (ARL) for offering facilities to characterize the devices used in this paper. Finally thanks to Cree Inc. for providing the devices. References: [1] P. Khan, L. Epp and A. Silva, "A. Ka Band

Wideband Gap SSPA: General architecture considerations," IPN Progress Report 42-162, August 2005, pp. 1-19.

[2] R. York, "Some Considerations for Optimal Efficiency and Low Noise in Large Power Combiners," IEEE Trans. Microwave Theory & Tech., August 2001, vol. 49, no. 8, pp. 1477-1482.

[3] C. Campbell, M.-Y. Kao and S. Nayak, “High Efficiency Ka-band Power Amplifier MMICs Fabricated with a 0.15μm GaN on SiC HEMT Process," IEEE MTT-S Int. Microwave Symp. Dig., June 2012, pp. 1-3.

[4] K. Kong et al., "Ka-Band MMIC High Power Amplifier (4W at 30 GHz) with Record Compact Size," IEEE Compound Semiconductor Integrated Circuit Dig., October 2005.

[5] W.-C. IP and K.-K. M. Cheng, "A Novel Power Divider Design with Enhanced Harmonic Suppression and Simpler Layout," IEEE MTT-S Int. Microwave Symp. Dig., May 2010, pp. 125-128.

[6] D. Pozar, Microwave Engineering, Wiley, John & Sons, 2nd ed. 1997.


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Linearity and Efficiency Improvement Using …wseas.us/e-library/conferences/2013/Baltimore/TESIMI/...Linearity and Efficiency Improvement Using Harmonic Suppression Power Combiner