0.5 μm GaAs PHEMT Medium Power Amplifier Design 功率放大器

0.5 μm GaAs PHEMT Medium Power Amplifier Design using Simple RC

Feedback Amplifier for Wireless LAN Applications

1A. Rasmi, 2A. Marzuki, 1M. Azmi Ismail, 1A. I. Abdul Rahim,

1M. Razman Yahya and 1A. Fatah Awang Mat

1Telekom Research & Development Sdn Bhd,

UPM-MTDC, Lebuh Silikon,

43400 Serdang Selangor, Malaysia.

2RMIC Group, School of Electrical Engineering,

Universiti Sains Malaysia (USM),

14300 Nibong Tebal, Pulau Pinang, Malaysia.

1amiza@https://www.360docs.net/doc/df8237527.html,.my

Introduction

Local Area Network (LAN) applications have driven the demand for personal wireless communications terminals, and these items need to operate at low voltages and be small in size [1]. Power amplifiers play a very important role in these systems in order to generate and transmit sufficient power signals. A start-of-the-art power amplifier design has to meet the system requirements for high gain, high efficiency, and meet the desired output power while the device and process technology of choice plays a crucial role in realizing a working system.

In this paper, a medium power amplifier (MPA) designed using 0.5μm GaAs PHEMT technology targeted for a wireless LAN application is presented. The MPA is a two-stage single-ended design which employs a simple RC feedback amplifier. F eedback is an important concept in circuit design, where a signal or voltage derived from the output is superimposed on the input. This output-to-input path can be used for several purposes such as to control output voltage, control gain, reduce distortion, improve stability, or create instability, as in an oscillator [2]. The RC feedback that was used in this MPA design is to linearize the stages as well as to improve the circuit stability and increases the bandwidth. The performance of the proposed amplifier in this paper will be discussed in terms of stability, gain, power-added efficiency (PAE), and output power.

Circuit Design

One of the design constraints in the power amplifier design is stability. The capacitor, C F and resistor, R F are added between the drain and gate of transistor is shown in Figure 1 to provide an RC feedback or commonly known as negative feedback [3] in order to produce broadband match and also to increase the stability of the device; so that the amplifier is always unconditionally stable. Amplifiers without RC feedback have a tendency to be rather unstable. The effect of unstable amplifier is the output becomes distorted in an unpredictable and random way. Another advantage of employing this feedback is the properties of the circuit can be made to depend primarily on the values of the feedback circuit elements and to be more-or-less independent of the particular characteristics of the active device (such as transistor ?, bias current, etc.).

As shown in Figure 1, resistor R F forms the feedback and capacitor C F is added to allow for independent biasing of the gate and drain of the transistor. C F can normally be chosen so that it is large enough to be a short circuit over the frequency of interest. In addition, the effect of feedback is to make the input and the output impedances more convenient for matching.

978-1-4244-2642-3/08/$25.00 ?2008 IEEE

Power amplifiers utilizing resistor as a feedback is discussed in [4] and [5]. The input and output impedances of the amplifier with feedback (the closed-loop amplifier) become sensitive to the gain of the amplifier without feedback (the open-loop amplifier); that exposes these impedances to variations in the open loop gain, for example, due to parameter variations or due to nonlinearity of the open-loop gain. From Figure 1, the closed-loop gain, A V

?????1

·¨¨¨¨¨?

§

++?

=OL

S OL F V A G )A (G A 1111

where G F , G S , A OL is conductance of feedback resistance, conductance of source resistance and open-loop gain respectively. The closed-loop gain will be equal to an open-loop gain if G F approaches zero. In this case, the open loop gain is referring to transistor gain without feedback topology. An amplifier with the resistor feedback can achieve self matching [5].

F igure 2 shows the schematic drawing of the whole amplifier including the feedback topology, input matching network, interstage network, and the output matching network. The inductors L

G and L S are chosen to provide the desired input resistance. The inductor L D and L O is a current source for the MPA and is used for output power matching. The L G , L S , L D and L O are all implemented with on-chip spiral inductors. The capacitor C C at the output plays a role for both DC block and output matching. The capacitor C O is used for network matching. The outputs are matched for high compression point, P1dB. Based on these essential matching networks, optimizations and simulations were performed to achieve the required circuit performance.

Results and Discussions

F eedback is used to control gain and reduce distortion, as well as provide other important functions in modern electronic designs. In this work, we have designed a 0.5μm GaAs PHEMT 2-stage MPA using a simple RC feedback amplifier to operate at 5.8 GHz. The RC feedback in amplifiers is one of the common uses of feedback which is used to improve the circuit stability and also linearity. The stability of the circuit is increased by feeding a small fraction of the output to the input so the phase will change and the gain of amplifier decreases.

Figure 3 shows the stability factor, K as a function of frequency for the 2-stage MPA with the bias condition is 5 V of drain voltage, V DD . This 2-stage MPA has achieved a stability factor, K of 1.821 for simulation and 13.603 for measurement at 5.8 GHz, respectively. It shows that the circuit is unconditionally stable over the frequency because the stability factor of this amplifier is more than 1. If the stability factor is less than 1, RC feedback or lossy matching must be employed in order to maintain an unconditionally stable design. The stability factor of this circuit is depending on the RC feedback that was employed in the circuit as shown in Figure 2.

The performances of 2-stage MPA were evaluated using ADS simulator [6]. The bias condition for this design is 5 V of drain voltage, V DD and 0 V of gate voltage, V G . The simulated drain current, I DS is 106.25 mA which includes approximately 29 mA for the first stage and 77.25 mA for the output stage. Whereas the measured drain current, I DS is achieved

(1)

200.2 mA including of approximately 52.7 mA for the first stage and 147.5 mA for the output stage.

Figure 4 illustrates the small-signal performance of the 2-stage MPA over 50 MHz to 10 GHz frequencies. The linear gain (S21) of this amplifier is 16.39 dB for simulation and 8.33 dB for measurement was achieved at 5.8 GHz. The input return loss of 15.25 dB for simulation and 15.30 dB for measurement; and output return loss of 12.16 dB for simulation and 16.66 dB for measurement were obtained at the operating frequency. These result shows that the measurement data is better than simulation data. In addition, this MPA is internally matched because the return losses are more than 10 dB.

The single tone Harmonic Balance (HB) simulation was done to evaluate the power gain, output power, and power-added efficiency. F igure 7 shows the power-added efficiency, output power and power gain as a function of input power for 5.8 GHz at the same bias condition. The output power at the 1dB compression point (P1dB) is 20.18dBm was obtained at 5dBm input power. F rom -15dBm to 4dBm, this amplifier showed the linear Pin-Pout characteristic and no gain compression until Pin reached 5dBm and the maximum P1dB was estimated over 22dBm. Besides, a power gain of 15.18 dB was obtained at 5dBm input power. In addition, this 2-stage MPA exhibits a maximum PAE of 25.29% for input power of 10.5dBm. At P1dB, this amplifier has a PAE around 18.13%.

Conclusion

In this paper, 0.5 μm GaAs PHEMT 2-stage MPA was designed and fabricated using a simple RC feedback amplifier has been presented for operating at 5.8 GHz. The RC feedback amplifier is utilized to linearize the stages as well to improve the circuit stability. It has been demonstrated that at a supply voltage of 5.0V, a linear gain (S21) of 16.39 dB for simulation and 8.33 dB were achieved at 5.8 GHz. In addition, the P1dB of 20.18dBm, power gain of 15.18 dB and the PAE of 25.29 % is attained at input power of 5 dBm. This proposed 2-stage MPA is suitable for wireless LAN applications.

Acknowledgements: The authors would like to thank Telekom Malaysia Berhad for sponsoring this work under project number RDTC/080699

References

[1] A. Raghavan. H. Deukhyoun, M. Moonkyun, A. Sutono, L. Kyutae, and J. Laskar, “A

2.2-V Operation, 2.4-GHz Single-Chip GaAs MMIC Transceiver for Wireless

Applications”, 2002 IEEE MTT-S International Microwave Symposium Digest, Vol. 2, pp. 1019 –1022, 2002.

[2] Gary Breed, “Feedback Fundamentals: Basic Concepts and Circuit Topologies”, High

Frequency Electronics, pp. 46-50, July 2006.

[3] Dong Min Kang, Jin Hee Lee, Hyung Sup Yoon, Sung Jin Kim, Jae Yeob Shim, and

Kyung Ho Lee, “Wideband 36- to 44-GHz MMIC Power Amplifier Using a 0.2-μm PHEMT Process”, Journal of the Korean Physical Society, Vol. 41, No. 4, pp. 524-527, 2002.

[4] Yun-Jia Tian and David G Haigh, “Investigation into RF Feedback for Improving the

Efficiency-Linearity Trade-Off in Power Amplifiers,” Proceed ngs of As a-Pac f c Microwave Conference, 2006.

Figure 1: Feedback topology.

Figure 5: Small-signal characteristic of

the 2-stage MPA.

Figure 4: Return losses and gain versus frequency for 2-stage MPA Figure 3: Stability factor, K versus frequency

of the 2-stage MPA.

Figure 5: PAE, output power and power gain

versus input power of the 2-stage MPA.

[5] A. Marzuki et.al, “A Broadband RF Feedback Amplifier Design with Simple Feedback

Network,” RF and Microwave Conference , pp. 1-4, Oct 2004. [6] Agilent Technologies, ‘Agilent ADS 2005A: Momentum’,2005.

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d B (S (1,1))

dB(S(2,1))d B (S (2,2))d B (S (3,3))dB(S(4,3))d B (S (4,4))S 11 m eas

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Input match network

Interstage match network

Output match network

Q 1

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C S2

R S2

R F2

C F2

Figure 2: Schematic of 2-stage MPA using

0.5 μm GaAs PHEMT.