Design of Wideband Power Amplifier based on Microstrip Lines M.Ribate 1, R.Mandry 1, A.Errkik 1, M.Latrach 2 1 LMEET FST of SETTAT, University of Hassan 1 st Morocco 2 Microwave Group ESEO ANGERS, France Abstract A wideband power amplifier based on microstrip lines is illustrated in this paper. This amplifier operates in the microwave band ( GHz) within the S-Band and it is based on ATF13786 GaAs FET transistor. The design and the analysis approach is entirely Scattering Parameters dependent. The simulated results obtained by ADS (Advanced Design System) simulator show good performances. For the final circuit, the input reflection coefficient S 11 is between -12 dB and - 33 dB while the output reflection coefficient S 22 ranges between -12 dB and -41 dB. Besides, the reverse transmission coefficient S 12 attain a maximum value of -65 dB. The proposed wideband power amplifier achieves an average saturated output power of dBm and an average gain of 30 dB. Power Amplifier is one of the most basic building blocks of electronic systems. Input signals to a radio frequency reception chain coming from sensors such as antennas are generally weak, because their power decreases during emission. However, the output signals must be strong. For this reason, it is indispensable to provide power gain between input and output by power amplifiers. If it were not, the receiving information cannot be recognized [1], [2]. The feature of the power amplifier design is evaluated by realizing maximum power gain under stable operating conditions with minimum amplifier stages regardless of its linearity, which is very important for many telecommunications systems [3]. For several microwave applications, such as testing systems, radar…, the transceivers operate in a very wide frequency range. The power amplifier design based on wideband concept offers identical advantages when there is no need to tune resonant circuits, and it is possible to transmit a wide multimode signal spectrum [4]. This paper presents a new design of a wideband RF power amplifier using the microwave transistor ATF13786 which is a Field–Effect-Transistor (FET). The proposed amplifier covers the mainstream communication standards such as GSM1800, UMTS, HSPA, HSPA+, LTE and b/g [5]. In this work, a complete study of the amplifier has been done by studying the stability conditions, the scattering parameters analysis, the relationship between input versus output power, and finally the 1-dB gain compression to identify the performances and features of the proposed amplifier. Moreover, the design approach achieves a simple configuration of this circuit. A single stage microwave power amplifier can be modelled by the circuit of figure 1 [6]. At high frequencies, the input and output matching are required to provide maximum transfer to the load of the RF power available from the RF source. This operation can be done by using particular impedance matching network that can match the input and the output of a transistor, at a specific DC bias, with source and load impedance Z0 respectively [7]. Table 1 present the components values of the proposed wideband power amplifier. Introduction Wideband Power Amplifier Design Figure 1. The general RF amplifier block diagram By using the idea mentioned above, the proposed single stage wideband power amplifier is shown in figure 2. Figure 2. The proposed single stage wideband power amplifier schematic Table 1. Schematic components values Both input and output networks adopt the wideband matching topology that are designed by a combination of lumped elements and distributed elements. A. DC Analysis The first analysis that requests to be accomplished is the DC analysis. This analysis is achieved to find the right bias points for the amplifier. Figure 3 shows the DC analysis results for the ATF13786 FET transistor. Figure 3. ATF13786 Bias characteristics Figure 4. The bias network design If an amplifier needs to be designed at low frequency, a Choke (inductor) is used to block the higher frequency alternating current (AC) while passing lower frequency or direct current (DC). However, at high frequency, getting discrete inductors is so difficult. Thus, the quarter wavelength /4 at center frequency is the best choice to design the bias network [8]. B. The proposed circuit of the Wideband power amplifier In order to increase input and output matching, the power output versus input output, and the stability conditions, it was necessary to use the tree stage configuration based on the single stage power amplifier discussed above. Figure 5 shows the final circuit of the wideband power amplifier. Figure 5. Tree stage wideband power amplifier Results and discussion The design and simulation of the proposed amplifier is achieved by using ADS (Advanced Design System) in order to optimize the lengths of the distributed elements. Practically, the lumped elements values are determined by using the command “TUNE” of ADS. The simulation results are illustrated below. According to the stability analysis of a transistor, the necessary and sufficient conditions for unconditional stability are given by the equations (1) and (2) [9], [10]: As shown in figure 6 and figure 7, the power amplifier improves the stability conditions in the wideband frequency between the frequencies 1.2 GHz and 3.2 GHz. Figure 6. Curve of the stability factor K versus frequency Figure 7. Curve of the stability measure B1 versus frequency The scattering parameters of the proposed power amplifier are presented below. The feasibilities of the input and the output matching are confirmed by simulation. For the input reflection coefficient S 11 shown in the figure 8, we can clearly observe that the proposed amplifier presents a good input matching. In fact, the S 11 parameter changes between dB and dB over the operating band [1.2 GHz – 3.2 GHz]. As shown in figure 9, the output reflection coefficient S 22 varies between dB and dB in the simulation band. Therefore, the reflection of the signal from the output network to the load is very weak. Figure 9. Output reflection coefficient S 22 versus frequency Figure 8. Curve of the input reflection coefficient S 11 versus frequency Figure 10 shows the reverse transmission coefficient S 12 curve. We can remark that the maximum value that can be reached by this parameter is dB. Consequently, the proposed amplifier establish the conditions that allow to the entire electrical signal to be transmitted without any reflection. Figure 11 depicts the evolution of the power gain S 21 versus the operating band [1.2 GHz – 3.2 GHz]. As can be seen the power gain S 21 varied between 12 dB and dB with respect to the 50Ω load and source impedance. Thereby, these values present a good amplification of the proposed RF amplifier over a wideband. Figure 11. Power gain S 21 versus frequency Figure 10. Reverse transmission coefficient S 12 versus frequency Figure 12 illustrates the Input/Output characteristic of the proposed power amplifier. For the low input signal, the output power progresses linearly when the input power increases. However, it deviate from the linear area when we go beyond – 20dBm level of the input power. Figure 12. Input versus Output power Conclusion The Wideband power amplifier introduced in this paper presents higher power gain over a flat bandwidth of 2 GHz. The proposed amplifier improves the necessary and sufficient conditions for unconditional stability with a good input/output matching. Both the input reflection coefficient S 11 and the output reflection coefficient S 22 are less than -10 dB in the operating bandwidth [1.2 GHz – 3.2 GHz]. Moreover, the reverse transmission coefficient S 12 achieves a maximum value of -65 dB. Compared to conventional wideband power amplifier operating in S-Band [11], [12], the proposed amplifier present a higher power gain and a good matching under stable operating conditions. References [1] Henrik sjöland, “Highly Linear Integrated Wideband Amplifiers : Design and analysis techniques for frequency from audio to RF”, Springer Science + Business media, New York, [2] Devendra k.Misra, “Radio frequency and Microwave communication circuits, Analysis and Design”, Second Edition, John Willey & Sons, Inc, [3] Sami Mahers, Mohamed Dhieb, Hamadi Ghariani and Mounir Samet, “Design of a Wideband Power Amplifier Using Scattering Parameters”, International Journal of Computer Applications (0975 – 8887) volum 66, No.11, March [4] Andrei Grebennikov, “RF and Microwave Power Amplifier Design”, McGraw – Hill companies, Inc, [5] Duye Ye, Youngle Wu and Yuanan Liu, “A Tradeoff Design of Broadband Power Amplifier in Doherty configuration utilizing a Novel coupled – line Coupler”, Progress in Electromagnetics Research C, vol.48, 11-19, [6] David M.Pozar, “Microwave Engineering”, Third Edition, John Wiley & Sons, Inc, [7] Andrei Grebennikov, Narendra Kumarr, Binboga S.Yarman,“ Broadband RF and Microwave Amplifiers”, Taylor + Francis Group, LLC, [8] Agilent Technology, “Agilent EEsof EDA, Advanced Design System, Circuit Design Cookbook 2.0”, November [9] Narendra Kumar, Andrei Grebennikov, “ Distributed Power Amplifiers for RF and Microwave Communications”, Artech House, [10] Guillermo Gonzalez, “Microwave Transistor Amplifiers”, Second Edition, Prentice Hall, Inc, 1997, [11] Xin-Yan Gao, Wen-Kai Xie and Liang Tang, “Optimum Design for a Low Noise Amplifier in S-Band”, Journal of Electronic Science and Technology of China, Vol 5, No.3, September [12] Yingjie Xu, Jingqui Wang, and Xiaowei Zhu, “Design of a Microstrip Broadband LDMOS Class-E Power Amplifier”, from March 2010, High Frequency Electronics, Summit Technical Media LLC. Based on the DC power consumption and Gm requirement, bias points are selected as follow: VDS = 3.5V and VGS= 0V. The bias network design for the power amplifier depends on the frequency range wherein this amplifier needs to be designed. Figure 4 shows the circuit design for the bias network.