404IEICE TRANS. ELECTRON., VOL.E103–C, NO.10 OCTOBER 2020

INVITED PAPER Special Section on Microwave and Millimeter-Wave Technologies

Recent Progress on Design Method of Microwave Power Amplifierand Applications for Microwave Heating

Toshio ISHIZAKI†a), Senior Member and Takayuki MATSUMURO†, Member

SUMMARY Recently, GaN devices are often adopted in microwavepower amplifiers to improve the performances. And many new designmethods of microwave power amplifier were proposed. As a result, a high-efficiency and super compact microwave signal source has become easilyavailable. It opens up the way for new microwave heating systems. In thispaper, the recent progress on design methods of microwave power ampli-fier and the applications for microwave heating are described. In the first, adevice model of GaN transistor is explained. An equivalent thermal modelis introduced into the electrical non-linear equivalent device model. In thesecond, an active load-pull (ALP) measurement system to design a high-efficiency power amplifier is explained. The principle of the conventionalclosed-loop ALP system is explained. To avoid the risk of oscillation forthe closed-loop ALP system, novel ALP systems are proposed. In the third,a microwave heating system is explained. The heating system monitors thereflection wave. Then, the frequency of the signal source and the phase dif-ference between antennas are controlled to minimize the reflection wave.Absorption efficiency of more than 90% was obtained by the control of fre-quency and phase. In the last part, applications for a medical instrument isdescribed.key words: GaN power amplifier, pulse-mode operation, active load-pull,microwave heating, frequency and phase control, medical instrument

1. Introduction

In our daily life, microwave ovens became one of the essen-tial instruments for the modern people. To cook and warmfoods, we often use a microwave oven. Not only for dailylife, but also for industry, microwave heating is the mostpromising key technology to reduce the greenhouse gases,which is the cause of global warming. Although microwaveheating is very effective, there is a room for improving theheating equipment in the sense of technology. Because vac-uum tubes, such as a Magnetron, are still used in manycases. Vacuum tubes are not suit for precise heat control andthe reliability is not high. Recently, GaN transistor, whichis one of the wide-bandgap devices, becomes available inthe market. It has a great advantage over the conventionalSilicon devices and GaAs devices.

The authors studied microwave heating systems anddesign methods of high-efficiency GaN power amplifier forlong time. In this paper, the recent progress on designmethods of microwave power amplifier and the applica-tions for microwave heating is described. So far, there isno literature, which describes the technology from a power

Manuscript received January 26, 2020.Manuscript revised February 28, 2020.Manuscript publicized March 19, 2020.†The authors are with Ryukoku University, Otsu-shi, 520–

2194 Japan.a) E-mail: ishizaki@rins.ryukoku.ac.jp

DOI: 10.1587/transele.2020MMI0002

amplifier design to the heating system. This is the objectiveof this paper. Some contents of this paper might contain theresult of the published paper. However, the mutual relationsare explained systematically, first time in this paper. More-over, some contents are quite new achievements, which aredisclosed first time in this paper.

For power amplifiers used in microwave heating sys-tems, maximum output power and efficiency at high outputoperation are the most important. For power amplifiers usedin communication systems, linearity (or low distortion) andefficiency at the back-off operating point are important. Inthis respect, they are quite different. The average power andthe peak power are the same for microwave heating becauseof CW-operation. On the other hand, for communication,the difference between them is often 10 dB or more. There-fore, the important factors in the device model, the load-pullmeasurement systems, the amplifier designs, and the ther-mal designs are quite different. The main focus of this re-search is to provide tools for easy development specializingin amplifiers for microwave heating systems. Hereinafter,in this research, the operating frequency is set to 2.45GHz,which is commonly used for microwave heating.

In Sect. 2, a non-linear device model of GaN transis-tor is described. A thermal model is newly incorporatedto design a power amplifier with pulse-mode operation. InSect. 3, active load-pull (ALP) measurement systems are de-scribed. To prevent an oscillation of the closed-loop system,a novel composite-loop ALP system and a novel PLL-loopALP system are proposed. In Sect. 4, a microwave heat-ing system is described. The effectiveness of frequency andphase control is explained with the new experimental re-sults. In Sect. 5, an application of microwave heating systemfor medical instrument is introduced. A miniaturized GaNpower amplifier module, which is designed to equip in themedical instrument, is introduced.

2. Device Model of GaN Transistor

2.1 Non-Linear Device Model

The proposed transistor model incorporates nonlinear ele-ments into a typical small-signal equivalent circuit based onthe formula of Angelov model [1], [2]. The extrinsic parthas its own elements to model the packaged transistor. Adisadvantage of Angelov model is that the number of modelparameters is about 80 in order to increase the model ac-curacy. On the other hand, the proposed model expresses

Copyright c© 2020 The Institute of Electronics, Information and Communication Engineers

ISHIZAKI and MATSUMURO: RECENT PROGRESS ON DESIGN METHOD OF MICROWAVE POWER AMPLIFIER AND APPLICATIONS FOR MICROWAVE HEATING405

Fig. 1 I-V characteristics obtained by model

Fig. 2 Comparisons between model and measured values for non-linearcapacitance

the drain current model using the same formula as theAngelov model, but has only 38 parameters [3], [4]. Also, anew function formula was introduced for the nonlinear ca-pacitance model. Its aim is to concentrate on the design ofhigh power amplifier and the expression of precise predic-tions of output power and efficiency.

Drain current is expressed by Eq. (1). An example ofI-V curve of a GaN HEMT is shown in Fig. 1.

Ids = Ipk{1 + tanh(ψ)}(1 + λVds) tanh(αVds) (1)

where,

ψ = P1(Vgs−Vpk)+P2(Vgs−Vpk)2+P3(Vgs−Vpk)3+ · · ·Vpk = Vpks − ΔVpks + ΔVpks tanh(αsVds)

ΔVpks = Vpks − Vpko

Non-linear elements, Cgs, Cgd, are expressed byEqs. (2), (3). Examples of non-linear capacitances areshown in Fig. 2.

Cgs = ag(Vds)bg (2)

Cgd = ad(Vgd)bd (3)

The comparison of the measured and the simulated per-formances of an amplifier is shown in Fig. 3. As you cansee, the both performances are well matched in the tendency.The small difference in the efficiency is due to a loss of trans-mission line formed on an evaluation circuit board. Thus, itcan be said the obtained non-linear device model is very ef-fective for power amplifier design.

Fig. 3 Large signal performances of amplifier

Fig. 4 Measurement set-up for drain current transition of GaN transistor

Fig. 5 Measured waveform of drain current

2.2 Equivalent Thermal Model

The device model obtained in the previous section does notinclude the dependences on temperature. In the actual sit-uation, output power and efficiency vary according to thechannel temperature [5]. Thus, the authors tried to incorpo-rate a thermal model into the non-linear device model.

First, the variations of drain current in terms ofthe package temperature was measured. Figure 4shows the measurement set-up and 20W GaN HEMT(EGN21C020MK made by Sumitomo Electric Device In-novations, INC) was used for the experiment. The transis-tor package is heated by a digitally-controlled hot plate andthe package temperature keeps constant. The current wasmeasured in a pulse operation mode with duty ratio of 10%,pulse frequency of 500Hz (time period of 2ms), thus pulsewidth of 200μs. Figure 5 shows the measured waveform ofdrain current. The current starts to flow, when the gate volt-age is applied. Then, it decreases due to the temperature

406IEICE TRANS. ELECTRON., VOL.E103–C, NO.10 OCTOBER 2020

rise of channel. A thermal equivalent circuit is derived fromthe transition of drain current caused by self-heating of thetransistor. The equivalent circuit might be expressed by a3-stage ladder circuit.

Figure 6 shows an equivalent circuit model of a GaNtransistor with the thermal model. Although CW-mode op-eration is commonly used in microwave heating systems,pulse-mode operation might be effective for performanceimprovement. In that case, parameters, such as pulse width,must be adjusted in a trade-off between the average powerand the temperature characteristics. Figure 7 shows the sim-ulation results of channel temperature using the equivalentcircuit. In the figure, the voltage Vtp can be interpreted as thechannel temperature expressed in Celsius degree. The chan-nel temperature apparently decreased according to the dutyratio. Of course, the average output power also decreases ac-cording to the duty ratio. However, pulse operation can uti-lize the part of excellent performance of the transistor. Thus,the substantial efficiency can be improved. Figure 8 shows

Fig. 6 Equivalent circuit model of GaN transistor with thermal model

Fig. 7 Cannel temperature variations for pulse-operation power ampli-fier

Fig. 8 Performances of power amplifier with different operation modes

the measured performances of a power amplifier with dif-ferent operations. The initial bias conditions are Vds of 50V,Vgs of −1.4V and Ids of 100mA. Maximum efficiency was58% for CW operation, whereas 67% for pulse-mode oper-ation with duty ratio of 10%. The difference in efficiency by9% is great for the system. It can be said that the superiorityof pulse-mode operation is demonstrated successfully.

3. Active Load-Pull Measurement System

To design a microwave power amplifier, a passive load-pullmethod was often used so far [6]. In a passive load-pull mea-surement, the load impedance is changed by using a me-chanical impedance tuner so that the efficiency and/or out-put power is maximized. However, the passive-type tunercannot realize a large reflection coefficient due to the tunerloss. ALP (Active Load Pull) system is a remedy for thisproblem [7]–[9].

A performance simulation using a conventional devicemodel, in which small cells are synthesized on circuit sim-ulation, is not accurate enough for power amplifier designused in a heating system. Thus, the load-pull measure-ment at the actual operating power is needed. In particular,since the output impedance of a high-output device at a highfrequency becomes extremely low, the actual measurementnear the outer circle of the Smith chart is inevitable. For thisreason, active load-pull is required.

ALP has two basic configurations, open-loop type andclosed-loop type [6]. Commonly-used system is closed-looptype. However, closed-loop type has a problem, which isa risk of oscillation, because a feedback-loop exists in thesystem. The cause of oscillation is the un-flatness of thefrequency characteristic of the feedback-loop.

To solve this problem, a novel composite-loop activeload-pull system and a novel PLL-loop active load-pull sys-tem are proposed [10]–[12]. The gain margin of the sys-tem can be improved by the composite-loop system. Andthe PLL-loop system acts as an ultra-narrow band-pass fil-ter. Thus, the oscillation can be avoided by these proposedsystems.

First, the principles of the closed-loop ALP system andthe composite-loop ALP system are explained. Figure 9shows the block diagram of a closed-loop ALP system. Fig-ure 10 shows that of a composite-loop ALP system.

For the closed-loop system, ΓL is expressed as

Fig. 9 Closed-loop ALP system

ISHIZAKI and MATSUMURO: RECENT PROGRESS ON DESIGN METHOD OF MICROWAVE POWER AMPLIFIER AND APPLICATIONS FOR MICROWAVE HEATING407

Fig. 10 Composite-loop ALP system

Fig. 11 PLL-loop ALP system

ΓL =G · b2

b2= G (4)

The reflection coefficient is equal to the loop gain G. Theloop gain margin decreases according to increase of ΓL,and the possibility of oscillation becomes higher. Even ifthe loop gain is less than unity at the measurement fre-quency, it might exceed unity at a different frequency dueto un-flatness of the frequency characteristics. So, the oscil-lation frequency might be different from the measurementfrequency.

For the composite-loop system, G and PA should in-clude the combining loss.

G = G′ · Sc1 (5)

PA = |G′|2 · |b2|2 (6)

Here, G′ is the loop gain where the combining loss is notconsidered. Since the output voltage a2 is a combination ofthe closed-loop output ac and the open-loop output ao.

a2 = aC · SC1 + aO · SO1

b2 · ΓL = b2 ·G′ · SC1 + aO · SO1 (7)

In this case, the reflection coefficient ΓL and the loop gain Gare expressed as

ΓL = G +aO · SO1

b2= Γclosed + Γopen (8)

G = ΓL − aO · SO1

b2= ΓL − Γopen (9)

Here, Γclosed and Γopen can be realized by the closed-loop andthe open-loop, respectively. In the composite-loop system,the loop gain required for ΓL is decreased. Thus, the gainmargin is increased [12].

Another approach to reduce the risk of oscillation isPLL-loop ALP system. Figure 11 shows the block diagram

Fig. 12 Measurement points of PLL-loop ALP system

of PLL-loop ALP system. The reason why oscillation issuppressed by PLL-loop is as follows. In order to oscillate,it is necessary to satisfy both of amplitude condition andphase condition. The output signal of PLL follows only thephase change of the input reference signal. It does not referto the amplitude. Also, it can be equivalently treated as anultra-narrow band-pass filter that transmits only the oscilla-tion frequency of the VCO, so that loop does not oscillate.The amplitude of the injection signal is controlled by VGAto follow the amplitude of the output signal.

Figure 12 shows the measurement points of the devel-oped PLL-loop ALP system. The fringe area of smith chartis fully covered, and the trace of measurement points drawsa co-axial trajectory.

4. Microwave Heating System

To improve absorption efficiency of a microwave heatingsystem, frequency and phase difference between antennasare controlled [4]. This is the largest advantage of a solid-state heating system. A GaN power amplifier can provide ahigh output power with high efficiency. The 2.4GHz GaNpower amplifier was designed by using a non-linear devicemodel. And the load-pull was performed on a simulator.The optimum load impedances were obtained for the funda-mental frequency and the second harmonic frequency. Fig-ure 13 shows the performance of the developed 100W-GaNpower amplifier [4] and the performances are expressed forCW-mode, here. PAE of 64% was obtained at output powerof 110W. The performance is compared with those of thepreceding literatures [13]–[17]. The summary is listed inTable 1. The outputs of the two 100W-GaN power ampli-fiers were combined by a Wilkinson power combiner andinstalled in one module. The 200W power modules wereused in the microwave heating system.

Figure 14 shows the configuration of the solid-state mi-crowave heating system. The two 200W GaN power ampli-fier modules were used in this system. In the microwavechamber, two patch antennas are installed on the top and thebottom plates, respectively. Total 400W power is combinedin space. The heating frequency is determined by a signalgenerator. The spectrum is a line spectrum with low phasenoise, contrary to Magnetron, of which spectrum spreadsand cannot keep the phase relation. The output signal fromthe signal generator is divided and the phase is controlled

408IEICE TRANS. ELECTRON., VOL.E103–C, NO.10 OCTOBER 2020

Fig. 13 Performance of designed GaN power amplifier

Table 1 Performance comparison of GaN power amplifiers

Fig. 14 Configuration of solid-state microwave heating system

Fig. 15 Photograph of developed microwave heating system

by a phase shifter. Then, the signals are amplified by theGaN power modules. Frequency and phase difference arecontrolled by a PC through USB cables. Reflection waves

Table 2 Standard loads for microwave heating system

Fig. 16 Absorption efficiencies for frequency and phase control

are monitored by the PC. So, frequency and phase differenceare controlled so that the reflection waves are minimized.

The absorption efficiency was confirmed by experi-ments. Table 2 indicates the standard loads for microwaveheating system to evaluate the heating efficiency.

Figure 16 shows the experimental results for the re-spective water loads. Absorption efficiency is defined byEq. (10) and was measured by using directional couplers.Here, the loss of the microwave chamber is neglected.

(Absorption efficiency) = 1 − (Reflected power)(Input power)

(10)

Absorption efficiency of more than 90% was obtained.It is much higher than that of Magnetron, whose typicalvalue is from 50% to 70%. The measurements were car-ried out at a small power condition. As well-known, theelectric characteristics of water, such as dielectric constantand loss tangent, vary depending on temperature. Thus, anautomatic control system is very effective to keep the bestcondition for absorption efficiency while heating water.

5. Applications for Medical Instrument

A microwave medical forceps was developed by Dr. TohruTani, et al. [18], [19]. The system was already intro-duced in medical market by NIKKISO. Figure 17 showsthe photograph of the microwave medical forceps, named“Acrosurg.”. It is an innovative medical instrument, whichcan cut off blood vessels with hemostasis by sintering. Thefirst generation of the system used a Magnetron. And, thesecond generation uses a semiconductor power amplifier.The microwave signal source and the forceps are connected

ISHIZAKI and MATSUMURO: RECENT PROGRESS ON DESIGN METHOD OF MICROWAVE POWER AMPLIFIER AND APPLICATIONS FOR MICROWAVE HEATING409

Fig. 17 Microwave medical forceps system (“Acrosurg.” by NIKKISO)

(a) Whole structure (b) Corresponding patterns

Fig. 18 Structure of matching circuit fabricated in LTCC substrate

Fig. 19 Performance of 1-stage miniaturized GaN power amplifier

by a thin co-axial cable.The next target is to eliminate the co-axial cable, be-

cause the cable prevents free movement of the surgeon’sarm, and energy loss is occurred by the thin cable. Toachieve this, the microwave module is to be installed insidethe grip of the forceps, and DC power is supplied by a verythin DC-cable or supplied by an internal battery.

Therefore, a miniaturized GaN power module isstrongly expected. The matching circuit of the amplifier isminiaturized by adopting an LTCC (Low Temperature Co-fired Ceramic) technology. Figure 18 shows the structure ofa matching circuit fabricated in an LTCC substrate. Eachpattern on a layer corresponds to the original pattern formedon a resin substrate, as shown in Fig. 18 (b). Distributed cir-cuit elements using in the matching circuit were convertedinto lumped elements and fabricated in the multi-layered ce-ramic substrate. The measured performance of a 1-stageminiaturized GaN power amplifier is shown in Fig. 19. Agood performance with PAE of 40% at output power of

Fig. 20 Miniaturized GaN power amplifier module for medical forceps

20W was obtained. The width of this module is less than20mm [20]. Thus, the possibility of co-axial cable-free for-ceps was confirmed.

6. Conclusions

To realize a miniaturized microwave power source withhigh-efficiency, high-performance and high-reliability, newdesign methods of GaN power amplifier were investigated.A thermal model was incorporated into a non-linear devicemodel. Performance of a power amplifier with pulse-modeoperation was studied. As a result of deriving the non-lineardevice model with the thermal model, a precise design ofpulse parameters, which is suitable for heating application,became possible. An experimental power amplifier withpulse-mode operation showed higher efficiency than CW-mode operation.

Novel active load-pull systems were proposed.Their principles were explained and the stability wasdemonstrated.

A microwave heating system using the developed GaNpower amplifiers was explained. The experimental resultsshowed that frequency and phase control was very effective.Absorption efficiency of more than 90% was obtained.

Finally, a microwave medical instrument was intro-duced. To improve the operability of microwave forceps,miniaturized GaN power amplifier module was developed.The width of the module was less than 20mm. PAE of 40%at output power of 20W was obtained. It showed the possi-bility of co-axial cable-free forceps.

Acknowledgments

The authors wish to thank all our students involved in theseresearches. Also, the authors wish to thank Ms. AsakoSuzuki for contribution on the miniaturized GaN power am-plifier module using LTCC substrate. The authors wish tothank Dr. Tohru Tani and Dr. Shigeyuki Naka, Shiga Uni-versity of Medical Science, for their collaboration and sup-port to develop the GaN power amplifier for the medicalinstrument.

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Toshio Ishizaki received the B.S., M.S.,and doctorate of engineering degrees fromKyoto University, Kyoto, Japan, in 1981, 1983,and 1998, respectively. In 1983, he joinedPanasonic Corporation, Osaka, Japan, where hewas involved in research and development onmicrowave circuitry and components, especiallyon microwave dielectric filters and power ampli-fiers for cellular radio communications. In 2010,he became a professor at Ryukoku University,and this is his current title. He received the 1998

OHM Technology Award from the Promotion Foundation for ElectricalScience and Engineering, Japan. He also received the 2003 best paperaward from IEEJ, Japan, and received the 2015 best paper award fromIEICE, Japan. His current research subjects are in the area of wirelesspower transfer systems, microwave meta-material devices, microwave fil-ters, and high-efficiency power amplifiers. Dr. Ishizaki is a senior memberof IEEE and a senior member of IEICE. He served as chair of the IEEEMTT-S Kansai Chapter, from 2014 to 2016. Also, he served as steeringcommittee chair of APMC 2018.

Takayuki Matsumuro received the B.E.,M.E., and Ph.D. (Eng.) degrees in electrical en-gineering from Kyoto University, Kyoto, Japan,in 2012, 2014 and 2017 respectively. He hasbeen an assistant professor in the Departmentof Electronics and Informatics, Ryukoku Uni-versity, since 2017. He has been engaged in re-search on microwave power transmission. Hewas awarded the Best Presentation Award inThailand-Japan Microwave Conference 2013.He is a member of the IEEE and the Institute

of Electronics, Information and Communication Engineers (IEICE).