Intermodulation Distortion of Integrated Power Amplifier ... - IEEE Xplore

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Intermodulation Distortion of Integrated Power Amplifier and Filter using Single Stub Tuners for Green Communication M. F. M. Fadhli #1, Z. Zakaria#2, A. R. Othman#3, A. Salleh#4, W. Y. Sam#5 #

Center for Telecommunication Research and Innovation (CeTRI), Faculty of Electronics and Computer Engineering, Universiti Teknikal Malaysia Melaka (UTeM), Durian Tunggal, Melaka, Malaysia 1

2

3

4

[email protected], [email protected], [email protected], [email protected], 5 [email protected]

Abstract— Integrated Power Amplifier (PA) and filter which is part of important component for Radio Frequency (RF) transmitter is designed using Adavance Design Simulation (ADS). The main aims of the design is to provide linear amplifier and filter at the center frequency of 2 GHz and biased at the conditions of class AB. GaAs transistor is used as an active device due to ability in operating at high frequencies and can generate signals with lesser noise. Linearity of the integragted design was simulated with two tone test and the intermodulation products (OIP3) value resulted to be 50 dBm. ACPR of -50.86 dBC with 10MHz frequency spacing is achieved using the Butterworth Bandpass Filter. On the other side, high linearity will lead to more efficient transmission of the signals.

II. POWER AMPLIFIER AND FILTER CHARACTERISTIC Amplitude nonlinearity and amplitude to phase conversion leads to distortion in amplifying signal. In telecommunications, measurement for a weak nonlinear system and devices is called third-order intercept point. [5] On the other hand, the intermodulation products also represented as seen in the figure 1 where the harmonics of fundamental frequencies were followed by the 2nd, 3rd and 5th order products.

Keywords— Integrated Power Amplifier, Active Bias Network, Single Stub Tuners, Filter, Intermodulation Products.

I. INTRODUCTION In the global communication industry, development and innovations in wireless technology are highly required. [1] RF power amplifier and filter is an important component of a wireless communication system. Its performance has a strong influence on the entire system reputation. The subject of green communications is compounded by the unbelievable growth of wireless communications in the springing up world which uses wireless as a medium to vault past traditional wire-line technologies. Modern communications systems which running at high data rates such as in quadrature-amplitude modulation (QAM) schemes, information is transmitted in the contour of the phase and amplitude of the envelope signal. Therefore, the instantaneous accuracy of the signal is vital to effectively extract the queried data upon demodulation. [2] RF power amplifiers deals with complicated active circuits while filter is not involving active component. The design process needs consideration about power gain, linearity, noise, efficiency and stability that leads to the challenges for power amplifier designers. [3] Wideband PA’s deal with two to three bands which almost narrow bandwidths per band and it usually operated in parallel. [4]

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Figure 1: Intermodulation products and related frequency Output power is the most important design aspect of a PA. In one sense, if the PA generates low output power, it loses its identity, making it hard to define. As for the input power PA when the signal increases, the PA becomes more nonlinear and produces high signal distortion, while its efficiency increases. [6] Filters can be used to separate, passing signals and attenuating the unwanted frequencies. Most of the design cases of Bandpass ¿lters have a narrow upper stopband. [7] In this paper, the factor of intermodulation distortion had been studied and designed within class AB with the usage of Single Stub Tuners. The integrated design used the Bandpass Filter at the input and output matching to avoid the extreme wide lines. [8]



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III. DESIGN The proposed Class AB Power Amplifier was design using Advance Design System (ADS). Figure 2 shows the block diagram of the basic PA. In order to define the class of operation, the linearity of power amplifier must be considered. Class A, AB and B is an ideal choice but the factor of efficiency need to be considered as well. Class AB operation is most suitable for single ended power amplifier as compared to class B eventhough class B has higher efficiency. To obtain high efficiency as high as class B, the gate bias voltage will be selected nearer to the threshold voltage. The bias point selection is obtained from the IDS – VGS graph provide from the transistor.

C. Stability Test Stability consideration is one of the important parameter when designing the power amplifier. The reason is to avoid the signals of an amplifier from oscillating. Oscillation is possible if either the input or the output port impedance has the negative real part. The stability of the transistor can be studied by determining the stable and unstable region in the smith chart using the S-parameter of the desired frequency. D. Input Output Matching The input and output matching technique that being used for the design of the power amplifier is single stub matching. The input and output matching design using the Smith Chart to get the value of length and distance of the stub that will be implemented in the matching network. From this matching technique, the distance and length of stub are acquired which short circuit shunt stub is used to find the length of stub. E. Biasing

Figure 2: Block Diagram of basic PA A. Selection of the transistor The wideband PA are designed using transistor Avago Technologies’ ATF-501P8. The type of transistor is built by GaAs Enhancement-mode pHEMT process. ATF-501P8 is a high linearity PA with good OIP3 performance. In addition, it achieved better PAE at 1dB gain compression point, through the use of proprietary process of 0.25um GaAs Enhancementmode pHEMT. B. Obtained Optimal Load Impedance The first step in designing the power amplifier is determining the optimum bias point for the transistor. Most circuits required biasing condition to operate properly. By knowing the active region of transistor will establish the DC voltage of the transistor. At this simulation, based on the datasheet ATF-501P8 the transistor should be biased for VGS = 4.5V and IDS = 280mA. Figure 3 shows the graph of IV characteristic of the transistor. The optimal condition of the bias point can be shown from figure 3 and the selection of the bias point is obtained at the safe operating area. This to ensure that the device operates without self-damage due to thermal heating.

The ATF-501P8 used an active biasing due to high DC power dissipation. [7] The main advantage of an active biasing scheme is the ability to maintain the drain to source current consistantly over a variaty of temperature and device. A very inexpensive method of performing this is to use two PNP bipolar transistors align in a current mirror configuration, as shown in Figure 4. This circuit is not functioning as a true current mirror due to resistors R1 and R3, except if the voltage drop across R1 and R3 is kept identical. For example, transistor Q1 was arranged with its base and collector tied together. This acts as a basic PN junction, which helps temperature indemnify the emitter-base junction of Q2. The following parameters must be calculated first in order to obtain the valued of R1, R2, R3 and R4: Ids IR Vdd Vds Vg Vbe1

= device drain to source current; = reference current for active bias; = power supply voltage available; = device drain to source voltage; = typical gate bias; = typical base-emitter turn on voltage for Q1 and Q2;

The resistor R3, is calculated based on: ͵ ൌ

†† െ †• ᇱ †• ൅ …ʹ

IC2 is chosen for stability to be 10 times the typical gate current and also equal to the reference current IR. Vds and Vds’ is used to prevent from the additional voltage drop if R6 is used.

Figure 3: Graph of Device IV Curves



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Figure 4: PA with Active Bias Circuit (without RF matching) Figure 5: Simulation circuit of Integrated PA with Filter

With a current of IR, the voltage drop over R1 must be set same as the voltage drop across R3. ͳ ൌ

Stability of the transistor must be reliable for any condition for better performance. At the operating frequency, the Rollett factor (K) must be greater than unity in order to avoid becoming oscillator and thus make sure that it is falls into the unconditionally stable region. As for better competencies, most of amplifiers are operating into this region. The amplifier is unconditionally stable at the operating frequency of 2 GHz with the value of 1.22 same as marked on Figure 6. On the frequency of 1.2GHz and below, the stability factor goes down below 1.

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R2 sets the bias current through Q1. ʹ ൌ

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R4 sets the gate voltage for transistor. Ͷ ൌ

‰ …ʹ

This circuit now regulates the drain current similar to a current mirror by changing the emitter voltage (VE) of transistor Q1 equal to Vds’ and the collector-base junction of Q2 is kept under reversed biased condition.

Figure 6: Stability Factor From figure 7, the amplifier can give about 10.3 dB at the centre frequency. Started from 1.6 GHz towards 0 GHz the gain was consistently drop to -24 dB and this same situation for 2.6 GHz onwards. The lowest return loss was at the centre frequency which reach -7 dB. 20

12

0

10

-20

8

-40

6

-60

4

-80

nf(2)

IV. SIMULATION RESULT The design of amplifier was firstly constructed as figure 5 using the ADS Simulation followed by the integration with Butterworth and Chebychev Bandpass Filter. The starting simulation was to determine the class of operation of the amplifier. This is done by selecting the Q point at the IV Curve in Figure 3 which it is selected within the range of lesser than IDS(A) until before reached VGS = 0.

dB(S(2,2)) dB(S(1,2)) dB(S(1,1)) dB(S(2,1))

F. Integration of PA and Filter The integration design was tested with two types of filter which is Butterworth Bandpass filter and Chebychev Bandpass filter at the centre frequency of 2GHz. In same filter specification, the number of poles and order in Butterworth filter is higher compared to that of the Chebyshev. This will give impact on the reduction of component required to construct a filter. Besides, the selection of the filter is the width of the transition band of filter.

2 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

freq, GHz

Figure 7: Result of S-Parameter and Noise Figure.



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Result of output and input matching can be determined from the value of S(1,1) and S(2,2) at Smith Chart as depicted in figure 8. The input matching was 20.25 + j5.68 and the output matching was 46.55 – j28.4 which been matched to 50Ÿ.

As for the OIP3 simulation result can be referred from figure 8 where it shows the intercept between the fundamental frequency and the 3rd order products. The intercept point achieved 46.2 dBm as referred to the figure 10 and table 1.

Table 1: Comparison result of the integrated design. V. CONCLUSION A high linearity integrated power amplifier with bandpass filter has been presented and designed using Avago’s ATF501P8 which it delivered the result showed in table 1. This design includes Single Stub Tuner matching network with filter and DC Bias circuit that been used to minimize the intermodulation products. This leads to more efficient signals trasmitter and compact design. Further investigation in designing power amplifier can be done to provides minimize voltage-current at the DC Bias circuit so that the effect of thermal heating heating is reduced.

Figure 8: Matching Network at Input and Output Smith Chart.

ACKNOWLEDGMENT The authors would like to thank UTeM for sponsoring this work under the research grant, UTeM, PJP/2013/FKEKK(2B)/S1131 and PJP/2013/FKEKK(41B)S01258. Figure 9: 1st order and 3rd order intermodulation product. Intermodulation distortion is a factor that determines the linearity of one PA. The intermodulation for the third order is measured with the two tone harmonic measurement in ADS. From the figure 9, m6 and m8 is the representation of the third order intermodulation products. When input power (RF_pwr) was set to 10 dBm, the simulation resulted ACPR of 43.12 dBc.

REFERENCES [1] A.R Othman, K. Pongot, Z. Zakaria, M. K. Suaidi, A. H. Hamidon, “Low Noise Figure and High Gain Single Stage Cascoded LNA Amplifierwith Optimized Inductive Drain Feedback for WiMAX Application,” International Journal of Engineering and Technology, vol. 5, no. 3, pp. 2601-2608, 2013. [2] A. Amanna, “Green Communications,” Annotated Literature Review and Research Vision, 2010. [3] G. Monprasert, P. Suebsombut, T. Pongthavornkamol, S. Chalermwisutkul, “2 . 45 GHz GaN HEMT Class-AB RF Power Amplifier Design for Wireless Communication Systems”. [4] Z. Zakaria, M. F. M. Fadzil, A. R. Othman, A. Salleh, A. A. M. Isa, N. Z. Haron, “Development of Wideband Power Amplifier for RF / Microwave Front-End Subsystem,” Jurnal Teknologi, vol. 3, pp. 105-112, 2014. [5] Yang, Guo-min, R. Jin, C. Vittoria, V.G. Harris, N.X Sun, “Small Ultra Wideband Bandpass Filter With Notched Band,” vol. 18, no. 3, pp. 176178, 2008. [6] Microwave, Maury, “Theory of Intermodulation Distortion Measurement ( IMD ),” 1999. [7] J. Jingon, H. Chin Keong, S. Sumei, “Green Wireless Communications: A Power Amplifier Perspective,” no. Cci, 2000.

Figure 10: Intercept of fundamental and 3rd order products (IP3).

[8] L. Yuan Chun, W. King Cheung, X. Quan, “Power Amplifier Integrated With Bandpass Filter for Long Term Evolution Application,” IEEE Microwave and Wireless Component Letters, vol. 23, no. 8, pp. 424-426, 2013.



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