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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 9, SEPTEMBER 2009 2139

Group Delay Equalized UWB InGaP/GaAsHBT MMIC Amplifier Using Negative

Group Delay CircuitsKyoung-Pyo Ahn, Student Member, IEEE, Ryo Ishikawa, Member, IEEE, and Kazuhiko Honjo, Fellow, IEEE

Abstract—A negative group delay (NGD) circuit has been em-ployed to equalize a group delay variation in a broadband ultra-wideband (UWB) InGaP/GaAs heterojunction bipolar transistor(HBT) monolithic microwave integrated circuit (MMIC) amplifier.Using the NGD circuit, a part of a salient group delay characteristicin the operation band of broadband amplifiers can be suppressedwithout an increase of the entire group delay. The MMIC amplifierhas a steep group delay increase in the lower frequency region ofthe full-band UWB band (3.1–10.6 GHz) due to the sum of phasevariations near the cutoff frequencies of the HBTs. The NGD cir-cuit has been inserted to reduce this increase of the group delayin the UWB band. By adding a three-cell NGD circuit while con-sidering input and output matching at the input side of the MMICamplifier, the group delay variation is decreased by 78%. However,gain was also decreased by insertion of the multistage NGD circuit.In an attempt to avoid this decrease in gain, a one-cell NGD circuitwas inserted into the feedback loop of the MMIC amplifier, andas a result, we were able to decrease the group delay variation by79%, with minimal gain deterioration.

Index Terms—Broadband amplifier, group delay, heterojunc-tion bipolar transistor (HBT), negative group delay (NGD) circuit,ultra-wideband (UWB).

I. INTRODUCTION

U LTRA-WIDEBAND (UWB) radio systems are the mostpromising candidates for short-range wireless personal

area networks [1] given their potential for offering a transmis-sion data rate much greater than several hundreds of megabitsper second with low microwave power radiations. Improving thetotal performance of group delay characteristics for complicatedRF modules is one of the major issues in UWB module designs,along with satisfying their own specifications. Studies of groupdelay characteristics in the designing of UWB modules weremainly performed in the field of filters. A few papers have re-ported on active circuits such as the UWB low-noise amplifier(LNA) [2]. Group delay compensation for a broadband ampli-fier using a composite right/left-handed (CRLH) circuit has alsobeen reported [3], and a different approach has been proposed tocompensate for dispersion in UWB applications using photonicphase filters [4].

Manuscript received December 23, 2008; revised June 03, 2009. First pub-lished August 11, 2009; current version published September 04, 2009.

The authors are with the Information and Communication Engineering De-partment, University of Electro-Communications, Chofu-shi 182-8585, Japan(e-mail: [email protected]; [email protected]; [email protected]).

Digital Object Identifier 10.1109/TMTT.2009.2027082

Fig. 1. Conceptual drawing of group delay equalization for a broadband feed-back amplifier using an NGD circuit.

Recently, negative group delay (NGD) circuits have openedup prospects for potential applications [5]–[8]. One intuitive ap-plication of the NGD circuit is as a group delay compensator.Compensation using NGD circuits for an InGaP/GaAs hetero-junction bipolar transistor (HBT) was studied by the authors [9].The first goal of [9] was to find an optimum electrode struc-ture—such as a single or double emitter—of HBTs to minimizegroup delay variation. For the same collector current density,group delay characteristics of the HBTs are different. However,for the same current driving capacity , their characteristicsare almost the same. The study concluded that only the total areaof emitter mesa—related to the base–emitter capacitancein the small-signal equivalent circuit of an HBT, irrespective ofthe electrode structures—affects the circuit characteristics (in-cluding group delay). The group delay variation can be com-pensated for by insertion of an NGD circuit. The group delaycharacteristic of the HBT has a steep increase at less than about3 GHz due to a phase variation near the cutoff frequency inthe low-pass characteristic of the HBT. The low-pass charac-teristic is mainly caused by base resistance and includingthe Miller effect in the HBT.

To demonstrate the group delay compensation for an actualactive circuit, an NGD circuit was inserted into a UWB mono-lithic microwave integrated circuit (MMIC) amplifier with anactive balun [10] constructed with the same InGaP/GaAs HBTsin order to equalize the group delay characteristic in the full-band UWB band (3.1–10.6 GHz). Fig. 1 shows a conceptual

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2140 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 9, SEPTEMBER 2009

Fig. 2. NGD Circuits applied in this paper.

drawing of group delay equalization for the UWB MMIC am-plifier using the NGD circuit. This amplifier also has a steepgroup delay increase in the low-frequency region of the UWBband due to the sum of the group delay variations of the HBTs.In addition, group delay variations in the low-frequency regionsof the UWB band can be seen in some other reports [11], [12].Using the NGD circuit, the group delay increase can be sup-pressed without a group delay increase at the middle- and high-frequency regions of the UWB band. Insertion of this NGD cir-cuit, however, causes a decrease in gain. The NGD circuit wasconnected at the input port or before an HBT in the feedbackloop. For the design of the group delay equalized amplifier, theamplifier circuit was first readjusted to reduce group delay varia-tion at the middle and high frequency regions of the UWB band[2] and the effect of insertion loss of the NGD circuit. Threetypes of NGD circuits were then used to compensate for the re-maining group delay variation in the low-frequency region ofthe UWB band.

II. BEHAVIOR OF NGD CIRCUITS

Fig. 2 shows NGD circuits taken up in this paper. CircuitsA and B were introduced in [13], and circuit C was proposedby the authors [9]. NGD characteristics of these circuits can beverified by using vector analysis. Impedance and admittance foreach circuit are expressed as

(1)

(2)

(3)

Using the impedance and admittance , forward transmis-sion scattering parameters and for a two-port net-work are derived as

(4)

Fig. 3. � vector loci for each NGD circuit shown in Fig. 2. (a) � .(b) � . (c) � .

(5)

where

In these equations, and are port impedance and port ad-mittance, respectively. Transition of the phase shift andfor increasing the angular frequency is as follows:

where

(6)


AHN et al.: GROUP DELAY EQUALIZED UWB InGaP/GaAs HBT MMIC AMPLIFIER 2141

Fig. 4. Equivalent circuit of the developed group delay equalized InGaP/GaAs HBT MMIC amplifier with an active balun.

Vector loci of and in a polar coordinate are obtainedto subtract the second terms of (4) and (5) from , as shown inFig. 3(a) and (b), respectively.

Similarly, using the NGD circuit is derived as (7),shown at the bottom of this page, where

An angular frequency at the phase shift of , except for, is similarly defined as as follows:

(8)

Equation (8) is defined in the case of . In this case,the transition of the phase shift for increasing the angularfrequency is as follows:

In the case of , the transition of the phase shiftfor increasing the angular frequency is as follows:

From (7), the vector loci of for the two cases are shown inFig. 3(c). Since the group delay is expressed as ,

the region where the vector rotates in a positive directionexpresses the NGD region. As shown in Fig. 3(c), the proposedNGD circuit can possess the NGD region from a low fre-quency with a resistive loss.

III. GROUP DELAY EQUALIZATION BY CONNECTING NGDCIRCUIT AT INPUT PORT

A three-cell NGD circuit [6] was added at the input port ofthe UWB MMIC amplifier [10] to equalize the group delaycharacteristic of the amplifier. Fig. 4 shows an equivalent cir-cuit of the UWB MMIC amplifier with the three-cell NGD cir-cuit. The HBTs – have a single-emitter structure whoseemitter mesa area is 2 20 m . has GHz,

GHz, pF, pF, and mSat a bias of V and V. The ampli-fier circuit has an active balun circuit, which is used to drivea UWB self-complimentary antenna in differential mode [14].Thus, the amplifier was evaluated as a single-ended input to adifferential-mode output system.

Fig. 5 shows simulated results of the gain characteristicwithout the NGD circuit in which the value of a peakinginductor was varied. The value of the peaking inductorcorresponds to the intrinsic part of an equivalent circuit of theinductor in this simulation. The peaking inductor was insertedto compensate for the gain in the high-frequency region in theUWB band. Generally, maximally flat gain and flat group delayvariation cannot be achieved at the same time. Considerationof both gain flatness and flat group delay variation is requiredfor the UWB applications. When the gain characteristic is flat,the group delay characteristic has a convex characteristic in thehigh-frequency region due to the peaking inductor. When the

(7)



Fig. 5. Effects of � on the simulated gain and the group delay before com-pensation using an NGD circuit.

group delay characteristic is adjusted so that the convex charac-teristic becomes flat, the gain decreases in the high-frequencyregion. This method is expedient for connecting an NGDcircuit, which induces gain degradation in the low-frequencyregion of the UWB band. As shown in Fig. 5, there is a peakpoint at less than 2 GHz in the group delay characteristic due toa resonance point by the inductance of the bias wiring line inthe MMIC and the bypass capacitance with the value of 20 pF.Such a large capacitance was used in the MMIC to avoid theresonance in the UWB band.

The three-cell NGD circuit was composed as a -type circuitusing two series circuits and one parallel circuit.Using this circuit structure, the bandwidth of the negative groupdelay can be expanded. In addition, the impedance-matchingcondition can be adjusted. For the two-stage cascade connectionof two-port networks (denoted as A and B), total transmissioncoefficient can be expressed using each -parameter asfollows:

(9)

If each reflection coefficient at the adjacent ports is enoughsmall , then

(10)

(11)

where denotes group delay. According to these equations,when the output impedance of the NGD circuit is matched to theinput of the UWB MMIC amplifier, i.e., 50 , the total groupdelay can be expressed as the sum of each group delay. In thiscase, the total gain can be expressed as the product of each gain.To compensate for group delay variation, the NGD area of NGDvalue NGD bandwidth has to be comparable to the excessivearea that has to be reduced, such as the 3.1–5-GHz band shownin Fig. 5. However, one-cell NGD circuits have a confined NGD

Fig. 6. Chip photograph: measured and simulated results of the three-cell NGDcircuit with small input and output return losses.

Fig. 7. (a) Chip photograph of the group delay equalized MMIC using thethree-cell NGD circuit. (b) Its measured and simulated results.

area, and a multistage configuration is required to increase thearea. Moreover, the matching condition is required.

Fig. 6 shows a photograph of a fabricated three-cell NGDMMIC along with the measured and simulated results. Fab-ricated MMICs shown in this paper were fabricated by the



Fig. 8. Equivalent circuit of the developed group delay equalized MMIC with the one-cell NGD circuits inserted into the feedback loop.

foundry service Win Semiconductor Company, Tao YuanShien, Taiwan. The parameter values were adjusted so that thegroup delay characteristic of the UWB MMIC amplifier wasequalized. For the simulation, the MMIC substrate specificationand circuit models including parasitic elements were suppliedby the foundry service company. As shown in Fig. 6, the NGDcharacteristic was confirmed at 1.5–5 GHz. In addition, inputand output return losses were more than 15 dB. However, abouta 5-dB insertion loss was also observed in a region of the gainband of the UWB MMIC amplifier. From (10), this insertionloss will distort the gain flatness of the UWB MMIC amplifierincluding the NGD circuit. Therefore, the amplifier circuit haveto be designed so that the total gain characteristic becomes flatin the UWB band.

Fig. 7 shows a photograph of a fabricated group delay equal-ized InGaP/GaAs MMIC amplifier along with the measured andsimulated results. Considering practical linewidth and resultantchip area, the characteristic impedance of the lines in this paperis about 70 in our InGaP/GaAs MMIC layer . Itwas reported that the sinusoidal group delay variation is causedby the transmission line impedance mismatch [15]. However thegroup delay variation caused by the mismatch is negligible be-cause the line lengths are not so long. The MMIC was mea-sured as a three-port system using an Agilent PNA-X seriesfour-port network analyzer. The output was transformed fromtwo 50- single-ended ports to one 100- differential port usingthe measured three-port -parameter. In the measurement, eachport was directly contacted by using 150- m pitch air-coplanarground–signal–ground (G–S–G) probes. For the bias lines onthe MMIC, external bypass chip capacitors with the capacitanceof 390 pF were connected through bonding wires.

As shown in Fig. 7(b), the group delay characteristic wasequalized from 3.1 to 5 GHz using the NGD circuit. As a re-sult, the measured group delay variation in the UWB band wasreduced from 38.5 to 8.6 ps. At the same time, however, abouta 5-dB gain degradation occurred from 3 to 4 GHz and abouta 3.2-dB gain degradation occurred on average in the UWBband. The differences between the simulated and measured re-sults were mainly caused by the deviations between the modelparameters and fabricated components.

Consequently, the group delay equalization method based onthe cascade connection using the NGD circuit is simple becausethe total group delay is expressed as the sum of each groupdelay. However, impedance matching has to be adjusted as wellas group delay in the NGD circuit. In addition, large gain degra-dation occurs due to the multistage structure of the NGD circuit.Though the higher band gain is intentionally lowered, the inser-tion loss still disturbs gain flatness of the equalized MMIC am-plifier. Therefore, the NGD compensator have to be redesignedso that the total gain characteristic of the equalized MMIC am-plifier becomes flat in the gain band.

IV. GROUP DELAY EQUALIZATION BY CONNECTING

NGD CIRCUIT INTO FEEDBACK LOOP

In order to avoid the large gain degradation by increasing thestage of the NGD circuit, a one-cell NGD circuit was integratedinto the UWB MMIC amplifier [10]. However, the amplifierhas a relatively large and wideband group delay variation thatcannot be compensated for by the one-cell NGD circuit whenthe one-cell NGD circuit is connected at the input port of theamplifier under the 50- condition. Hence, the one-cell NGDcircuit was inserted into the feedback loop where the impedancefor the one-cell NGD circuit is not 50 and has reactance. Sinceall measured results were in agreement with the simulated re-sults, as shown in Fig. 7(b), the appropriate insertion point inthe feedback loop was determined by the simulation.

Fig. 8 shows an equivalent circuit of the UWB MMIC ampli-fier with two types of one-cell NGD circuits. The one-cell NGDcircuit was inserted before the first transistor as prescribedby the simulation results. The circuit parameters of the amplifierwere the same as those for the amplifier circuit shown in Fig. 4.The one-cell NGD circuit (b) in Fig. 8 is the proposed NGDcircuit in which the transmission is suppressed in the low-fre-quency region by a series circuit. Fig. 9 shows photographsof fabricated one-cell NGD MMICs circuits in Fig. 8 along withthe measured and simulated results. Both NGD circuits showsimilar group delay performance and lower loss characteristicsthan the three-cell NGD circuit. The one-cell NGD circuit (b)



Fig. 9. (a) and (b) Fabricated NGD circuit. (c) Their measured and simulatedresults.

in Fig. 8 retains about 2.5-dB insertion loss below about 3 GHz,as described in Section II.

The one-cell compensation technique takes advantage of in-ternal capacitance of existing in the circuit. The group delayof an HBT is mainly characterized as an input part of the small-signal equivalent circuit for an HBT. The input part is simplifiedas a low-pass network that consists of a base resistance and

, by applying the Miller effect, and is expressed as

(12)

where is a load impedance, and of is 2.25 pF.Fig. 10(a) and (b) shows two schematics of the one-cell NGDcircuit [see (a) in Fig. 8] with and , respectively. andbase–emitter conductance are omitted to simplify the anal-ysis. The voltage transfer function of the former is expressed as

(13)

where . The analytical results of the two cases are com-pared in Fig. 10(c) with pF and , andthe component values in Fig. 8. Compared with the 50- case,the NGD area is expanded with the help of . The total groupdelay of (13) is obtained by subtracting the group delay of the

Fig. 10. (a) and (b) Schematics of the one-cell NGD circuit (a) in Fig. 8 with� and � , respectively. (c) Analytical results of the two cases with � �

�� pF and � � �� . (d) Impact of � � on the group delay in the case of� � �� pF.

numerator from that of the denominator. Therefore, the selec-tion of , , and controls the NGD characteristic at the givencondition of , and -parameter simulations are required inpractical designs.

The feedback desensitizes the group delay variation of theNGD circuit with . Generally, feedback gain is givenby

(14)

where is the gain of the amplifier, and feed-back factor is assumed to be constant. As the lower band gainis mainly determined by and the operating band of the NGDcircuit is the lower band, (13) is substituted for in (14) to



Fig. 11. (a) Fabricated group delay equalized MMIC amplifier using the NGDcircuit (a) shown in Fig. 8. (b) and (c) Its measured and simulated results.

analyze the impact of on the NGD characteristic. The an-alytical results in Fig. 10(d) show that the NGD area decreasesas increases.

Fig. 11 shows a photograph of a fabricated group delay equal-ized InGaP/GaAs MMIC amplifier using the NGD circuit (a)

Fig. 12. (a) Fabricated group delay equalized MMIC amplifier using the NGDcircuit (b) shown in Fig. 8. (b) and (c) Its measured and simulated results.

shown in Fig. 8 along with the measured and simulated re-sults. The NGD circuit was inserted so that the length of thewiring line in the feedback loop was unchanged. In Fig. 11(b),the group delay characteristic was equalized from 3.1 to 5 GHzusing the one-cell NGD circuit whose parameters were adjusted



TABLE IMEASURED GROUP DELAY AND GAIN IN FULL-BAND UWB BAND

(3.1–10.6 GHz); MERITS AND DEMERITS OF THE

TWO COMPENSATION TECHNIQUES

in simulation, though the shape of the group delay characteristicof the NGD circuit itself is different from that of the three-cellNGD circuit shown in Fig. 6. The measured group delay varia-tion in the UWB band was reduced from 38.5 to 9.1 ps. The gaincharacteristic in the low-frequency region of the UWB band wasimproved by about 2.9 dB, and the gain characteristic in theUWB band was improved by about 2.2 dB (in comparison withthat of the case using the three-cell NGD circuit). However, aredundant gain peak was observed at 2 GHz. In Fig. 11(c), theinput return loss in the low-frequency region of the UWB bandwas also improved since it was sensitive point where the NGDcircuit with loss was inserted in the feedback amplifier circuit.

Fig. 12 shows a photograph of a fabricated group delay equal-ized InGaP/GaAs MMIC amplifier using the NGD circuit (b)shown in Fig. 8 along with the measured and simulated results.In Fig. 12(b), the group delay characteristic was also equalizedfrom 3.1 to 5 GHz using the one-cell NGD circuit. The measuredgroup delay variation in the UWB band was reduced from 38.5to 7.9 ps. The gain characteristic in the low-frequency region ofthe UWB band was improved by about 2.2 dB in comparisonwith that of the case using the three-cell NGD circuit, thoughthe gain degradation was a little larger than the case using theNGD circuit (a) shown in Fig. 8. In addition, the gain at less than3 GHz was suppressed by using the proposed NGD circuit (b)shown in Fig. 8.

The numerical results of the fabricated group delay equalizedMMICs are listed in Table I. Using the NGD circuits, the groupdelay fluctuation can be suppressed by about 80%. In addition,the gain degradation can be prevented by effectively using theone-cell NGD circuits. The merits and demerits of the two com-pensation techniques are summarized in Table I.

V. CONCLUSION

NGD circuits were employed to equalize the group delay vari-ation in the broadband UWB InGaP/GaAs HBT MMIC ampli-fier. The MMIC amplifier has a steep group delay increase in thelower frequency region of the full-band UWB band (3.1–10.6GHz) due to the sum of phase variations near the cutoff fre-quencies of the HBTs. The NGD circuit was inserted to reducethis increase of the group delay in the UWB band. By addinga three-cell NGD circuit while considering input and outputmatching at the input side of the MMIC amplifier, the groupdelay variation was decreased from 38.5 to 8.6 ps. However, the

gain was also decreased by about 3.2 dB. In another approach,a one-cell NGD circuit was inserted into the feedback loop ofthe MMIC amplifier. Two types of one-cell NGD circuits weretested. For the reported NGD circuit, the group delay variationdecreased to 9.1 ps in the UWB band with a reduced gain de-terioration of 1.0 dB. However, a redundant gain peak was ob-served at 2 GHz. For the proposed NGD circuit, the group delayvariation decreased to 7.9 ps in the UWB band with low gain de-terioration and effective gain suppression at less than 3.1 GHz.

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[4] E. Hamidi and A. M. Weiner, “Post-compensation of ultra-widebandantenna dispersion using microwave photonic phase filters and its ap-plications to UWB systems,” IEEE Trans. Microw. Theory Tech., vol.57, no. 4, pp. 890–898, Apr. 2009.

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Kyoung-Pyo Ahn (S’06) received the B.E. and theM.E. degrees in radio sciences and engineering fromChungnam National University, Daejeon, Korea, in2003 and 2005, respectively, and is currently workingtoward the D.E. degree in information and commu-nication engineering from the University of Electro-Communications, Tokyo, Japan.

His research interests focus on the developmentof MMIC amplifiers, filters, and antennas for UWBapplications.



Ryo Ishikawa (M’07) received the B.E., M.E., andD.E. degrees in electronic engineering from TohokuUniversity, Sendai, Japan, in 1996, 1998, and 2001,respectively.

In 2001, he joined the Research Institute ofElectrical Communication, Tohoku University,Sendai, Japan. In 2003, he joined the Universityof Electro-Communications, Tokyo, Japan. Hisresearch interest is the development of microwavecompound semiconductor devices and relatedtechniques.

Dr. Ishikawa is a member of the Institute of Electrical, Information and Com-munication Engineers (IEICE), Japan, and the Japan Society of Applied Physics.He was the recipient of the 1999 Young Scientist Award for the Presentation ofan Excellent Paper of the Tohoku Chapter, Japan Society of Applied Physics.

Kazuhiko Honjo (M’82–SM’88–F’97) received theB.E. degree from the University of Electro-Com-munications, Tokyo, Japan, in 1974, and the M.E.and D.E. degrees in electronic engineering fromthe Tokyo Institute of Technology, Tokyo, Japan, in1976 and 1983, respectively.

From 1976 to 2001, he was with the NECCorporation, Kawasaki, Japan. In 2001, he joinedthe University of Electro-Communications, as aProfessor with the Information and CommunicationEngineering Department. He has been involved

in research and development of high-power/ broadband/low-distortion mi-crowave amplifiers, MMICs, HBT device and processing technology, miniaturebroadband microwave antennas and FDTD electromagnetic wave and deviceco-analysis.

Prof. Honjo is a Fellow of the Institute of Electrical, Information and Commu-nication Engineers (IEICE), Japan. He was the recipient of the 1983 MicrowavePrize and the 1988 Microwave Prize presented by the IEEE Microwave Theoryand Techniques Society (IEEE MTT-S). He was also the recipient of the 1980Young Engineer Award and the 1999 Electronics Award presented by the IEICE.


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