S-BAND MEDIUM-POWER MESFET AMPLIFIER WITH HIGH STABILITY

20.20-

- P D I

M. Samuel, A. Nawarange, and S. Mahapatra A.C.R.E. Indian Institute of Technology Bombay 400 076, India

K E Y TERMS FET amplifier, stabiliry. limit design

ABSTRACT I n this Letter a novel technique to design a FET amplifier with po- tentially unstable FETs is reported. A two-stage FET amplifer with small signal gain of 20.4 2 0.2 dB and power output of 23 dBm over the frequency band of 2.3-3.1 GHz is reported. Measured out- putiinput VSWR are also reported.

INTRODUCTION Advance communication systems require power amplifiers with characteristics such as high gain, high stability, wide bandwidth, and good input/output VSWR. A gallium arse- nide (GaAs) metal-semiconductor field effect transistor (MESFET) is an excellent device as a microwave amplifier. However, MESFETs have high inputloutput impedances and instability over a wide frequency range. Several techniques have been reported in the past to stabilize the MESFETs [ 1- 31. This Letter describes a method to improve the overall stability factor of the cascaded amplifier without great sacrifice to the individual stage gain. Results of a two-stage amplifier over the 2.9-3.1-GHz band are reported.

AMPLIFIER DESIGN AND FABRICATION A two-stage FET amplifier in which two MESFETs are cas- caded using a transmission line (TL) is shown in Figure 1. The major design difficulty is in selecting the appropriate length of transmission line. Using a computer program de- veloped for this purpose, the maximum available gain, sta- bility factor, and the resultant S parameters of the configu- ration were analyzed as a function of the electrical length of the TL. It was observed that the overall stability factor of the cascaded network is controlled by varying the electrical length of the TL. From the resultant S parameters of the cascaded network the inputioutput matching networks were synthe- sized for a conjugate match for minimum insertion loss and broad bandwidth. The design now is thus straightforward. The other advantage is that the intermediate TL used also serves as an interstage matching network.

The amplifier was constructed on 0.031-in-thick RT/Du- roid substrate (E, = 2.32). The MESFETs used are AT8250 in the first stage and AT8160 in the second stage. Chip ca- pacitors are used to provide the input and output DC blocks. Bias is supplied to both gate and drain via feed-through filter

53.73n 86.730

-0 ou t In

6 3.7 6 ,n L1 .05

F 2 U

3 G H z MEDIUM POWER A M P L I F I E R

Figure 1 Two-stage medium-power FET amplifier

capacitor and rf chokes on the center of the radial line stubs. The radial line stub in conjunction with the high-impedance stub provides rf to dc isolation. Blocking capacitors were sol- dered on the substrate and the FET device was mounted directly on the copper block.

RESULTS AND DISCUSSION A 5 0 4 TL having electrical length of 150 was chosen from the analyzed data, which gave an overall stability factor of k = 5.2 for the cascaded network, even though the stability factor of individual MESFETs are 0.36 for AT8250 and 0.767 for AT8160.

The testing of the amplifier was done on Wiltron network analyzer. The graph of the amplifier gain is shown in Figure 2. A gain of 20.4 ? 0.2 dB was observed from 2.9 to 3.1 GHz. The input/output return loss is better than 13.7 (VSWR = 1.52) in the band of interest, as shown in Figure 3. The am- plifier gave 23.0-dBm output power at 1-dB gain compression point.

CONCLUSION A hybrid S-band GaAs FET medium power amplifier on RT/ Duroid substrate with 20.4 2 0.2-dB gain and output power

20.601

E d 19.00

t 13.10

I I I I I L 2 . 1 2 . 8 3-2 3.6

Freq. ( G H 2 I

Flgure 2 Gain versus frequency

18.0 t

: I/ 10.0

I

L I 2.4 2 . 8 3 . 2 3.6

Freq. ( G H z 1

Figure 3 Return loss versus frequency

260 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 5, No. 6, June 5 1992

dB > 23 dBm has been developed. It shows that a two-stage cascaded amplifier can have an improved or even higher sta- bility factor by introducing a transmission line of appropriate length between the two stages, even though the individual MESFETs have stability factor less than 1. The sacrifice in individual stage gain is very small in comparison to the im- proved stability we obtained by this method.

ACKNOWLEDGMENT This work represents part of a project supported by DRDO DTSR, New Delhi, India.

REFERENCES 1. J. V. Dilorenzo and D. D. Kandelwal, GaAs FET Principles and

Technology, Artech House, 1982. 2. T. T. Ha, Solid State Microwave Amplifer Design, Wiley, New

York, 1981. 3. K. L. Kotzebue, “Microwave Amplifier Design with Potentially

Unstable FETs,” IEEE Trans. Microwave Theory Tech., Vol. MTT-27, NO. 1, pp. 1-3.

Received 1-3-92

Microwave and Optical Technology Letters, 516, 260-261 0 1992 John Wiley & Sons, Inc. CCC 0895-2477/92/$4.00

FD-TD ANALYSIS OF OPEN CYLINDRICAL DIELECTRIC RESONATORS Ying Shen, Zhiqiang Bi, Keii Wu, and John Litva Communications Research Laboratory McMaster University Hamilton, Ontario, Canada L8S 4K1

KEY TERMS Dielectric resonators, time domain, finite-difference method

ABSTRACT Diferent structures of open dielectric resonators (DR), which in- clude isolated DRs, tuning-screw DRs, and semiopen supported DRs on a microstrip substrate, are analyzed using the finite-differ- ence time-domain method with the eficient numerical integration fourier transform. The numerical results are in very good agreement with the available theoretical solutions and the experimental results.

1. ‘INTRODUCTION The use of dielectric resonators for applications in microwave circuits requires accurate and efficient methods for calculating the resonant frequencies and the spatial distributions of the field. Various methods have been developed to study the res- onant frequencies of DRs. Most of them, such as the mode- matching method, integral-equation method, and finite-ele- ment method, are carried out in the frequency domain [l].

The finite-difference-time-domain (FD-TD) method has been widely used for solving electromagnetic problems. Re- cently, some investigators have used the FD-TD method to solve some eigenvalue problems for closed resonant problems

[2 , 31. Up to now, none has addressed the open resonant problems using the FD-TD method.

In this article a numerical procedure based on the FD-TD method with efficient numerical integration of fourier trans- form and the first-order absorbing boundary condition [4] is used to calculate the resonant frequencies of an open dielectric resonator. The DRs discussed in the article consist of isolated DRs, tuning-screw DRs, and semiopen DRs which are sup- ported on a microstrip substrate. The resonant frequencies calculated for all of these structures are compared with either available published results or measured results. It has been shown that the FD-TD method with efficient numerical in- tegration of fourier transform is very suitable for solving open DR problems, especially for complicated structures. The com- parison of FD-TD results with experimental results further demonstrates that the FD-TD approach is an accurate and efficient tool for modeling arbitrary DR structures with ro- tational symmetry.

II. NUMERICAL PROCEDURE A generalized dielectric resonator is under consideration in this article, where the relative dielectric constant of DR is E , .

The DR is assumed to be in an open or unbounded environ- ment.

Dielectric resonators of cylindrical shape which were ex- cited in the TEols mode, were used in most of the DR ap- plications. For simplicity, in this article only the TEola mode is discussed. The relevant form of Maxwell’s equations are

l z

a 6H, a H , -(€I?,) = - - - at az ar ’

a 1 a r E , at r dr - (pH*) = - -

Observat ion point

Exc i ta t ion point

Figure 1 FD-TD grids of an isolated dielectric resonator

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 5, No. 6, June 5 1992 261