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A 30 W 800 MHz complimentary LDMOS power amplifier for wireless application
Accepted Manuscript A 30 W 800 MHz Complimentary LDMOS Power Amplifier for Wireless Application Sagar Mukherjee, Swarnil Roy, Chandan Kumar Sarkar PII: DOI: Reference:
S0749-6036(14)00145-1 http://dx.doi.org/10.1016/j.spmi.2014.05.001 YSPMI 3254
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
3 December 2013 30 April 2014 3 May 2014
Please cite this article as: S. Mukherjee, S. Roy, C.K. Sarkar, A 30 W 800 MHz Complimentary LDMOS Power Amplifier for Wireless Application, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi. 2014.05.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A 30 W 800 MHz Complimentary LDMOS Power Amplifier for Wireless Application Sagar Mukherjee,Swarnil Roy,Chandan Kumar Sarkar Department of Electronics and Telecommunication Engineering, Jadavpur University, Kolkata, West Bengal, India [email protected]
ABSTRACT This work presents the design of an optimized power amplifier using Complementary LDMOS (CLDMOS) with 5µm Gate Length. Final results show that the high performance CLDMOS amplifier has been achieved using the optimization of various device parameters and the circuit bias conditions. Optimization of the drift length and the drift area doping in device parameters have been done for both P type LDMOS (PLDMOS) and N type LDMOS (NLDMOS) for various Analog/RF applications. And these optimizations yields CLDMOS Power Amplifier with 36 dB gain, 800 MHz bandwidth and 46.4% efficiency. Comparing with existing structure a good improvement of gain and bandwidth has been observed. Index Terms— Complementary LDMOS (CLDMOS), Power Amplifier, Optimization, Device Parameter, Gain, Bandwidth, Drain Efficiency, Power Added Efficiency (PAE).
INTRODUCTION A rapid development of wireless and mobile technology demands a rising need of high performance power amplifier with low cost for the frequency range of 900 MHz to 2.7 GHz, which is the frequency band, used by GSM, CDMA and other wireless systems [1]. Now a day this bandwidth is also useful for NFC and Cordless telephony application which lies in the frequency band known as ISM Band[2]. For these applications, wide band gap compound GaN type semiconductor HEMT are popular due to their high mobility, high gain and power handling capability [3].But high production cost of these devices make them unsuitable for large scale production [4]. Conventional MOSFET does not have problems related to cost. However, the conventional MOSFET is having issues related to the mobility and power handling capability which make it incapable as power amplifier in the high frequency domain [5]. Conventional MOSFETs show some reliability issues with parameters like on-resistance, breakdown voltage and other important parameters required for power devices. From the aspect of the cost, the gain, the power handling capability, the breakdown voltage and the on-resistance LDMOS is the best choice over HEMT and also MOSFET [6].
For a weak signal(-10 to -70 dBm) in wireless network [7], a high gain amplifier is required which makes LDMOS a suitable choice as it is having a high breakdown potential, low onresistance and high gain due to its structural uniqueness. Due to its structural advantage it can handle high power which makes it very useful for the application as Power Amplifier. A significant development is observed using LDMOS transistor in last few years, in terms of output power, Gain bandwidth, linearity, efficiency [6][8]. Different stages of amplifier for driver application is designed using high efficiency LDMOS which were introduced for the need of high bandwidth of the present day mobile communication systems [9]. On the other hand, LDMOS can be fabricated using a low cost reliable fabrication method [10] unlike HEMT, which proves the significance of LDMOS technology as an innate choice for wireless systems over other existing technology in the market. Amplifiers designed using complementary MOS structure has an inbuilt high gain property [11]. Using the same property of complementary structure a CLDMOS power amplifier is designed in which various device and circuit parameters optimizations are done. Through the proper impedance matching and proper biasing and also at the same time optimization of the drift region doping & the drift region length is done to achieve 800 MHz bandwidth, 30W power handling capacity, 36 dB Gain and 46.4% efficiency. DEVICE STRUCTURE Unlike conventional MOSFETS, LDMOS has a drift region which helps to achieve the high current handling capability, high on-resistance and large breakdown voltage[12]. Increasing the drift region length increases the breakdown voltage and on-resistance. But increasing the length also reduces the overall gain of the circuit. Thus careful optimization of drift length is required for optimized gain performance.
Fig. 1: Device Structure of NLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=0.05 µm
Fig. 2: Device structure of PLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=0.05 µm
For the design of single stage complimentary push pull power amplifier, NLDMOS and PLDMOS are used. Fig.1 & 2 shows the proposed structure of NLDMOS and PLDMOS respectively. According to the basic structure of PLDMOS and NLDMOS [13], it is seen that the region between the gate and the drain can be divided into two regions which are named as the channel region and the drift region. Low doping and long drift region increase the breakdown voltage. The channel region behaves like a normal channel of conventional MOS. The gate voltage has a negligible effect on the drift region. Fig. 3-5 provides the current voltage characteristics of these devices. All the simulations are performed using a 2-D device simulator [14].For the purpose of device simulation, parallel field mobility model is used which considers Caughey and Thomas expressions to model the mobility [15]. And both for NLDMOS and PLDMOS this model can be used [14]. Also, the Shockley–Read–Hall recombination is incorporated for the modeling of recombination. Impact ionization is modeled with the introduction of Selberherr’s impact ionization model. PLDMOS is also having the same structure like NLDMOS and it also has same models mentioned above for NLDMOS. In this paper NLDMOS & PLDMOS with breakdown voltage of 24V and 22V respectively, are considered. As expected NLDMOS has higher breakdown voltage[16].
Fig. 3: Drain current Vs. drain voltage of NLDMOS for gate to source voltage of 10V
Fig. 4: Drain current Vs. drain voltage of PLDMOS for gate to source voltage of -10V
For this proposed structure the channel & gate lengths are 2µm & 5µm respectively, the oxide thickness is 0.05 µm. In order to achieve an optimization in the performance of the power amplifier using the LDMOS, these two device parameter are varied. Using this PLDMOS and NLDMOS, we can design a CLDMOS which can behave as a push pull power amplifier. In the next few sections, the design and optimization of these devices as well as the power amplifier are described.
Fig. 5: Drain current Vs. gate voltage of PLDMOS & NLDMOS for drain to source voltage of 2V and 10V
DESIGN AND OPTIMIZATION OF POWER AMPLIFIER Just like a conventional CMOS amplifier PLDMOS and NLDMOS are connected in complimentary configuration to design a power amplifier. As the two LDMOS are connected in complimentary configuration, it is named as CLDMOS. Here a single stage power amplifier is designed and simulated; but same is also applicable for two stage configuration. For the optimization of the performance of the Power amplifier, the variation of main device parameters like the drift region length and the doping of drift region are done. For the parametric analysis, a drift region length sweep of 1.5µm to 4µm is used and doping density sweep is 1016 cm-3 to 1018 cm-3. For each sweep ac analysis of the amplifier is done and from that the gain and the bandwidth are derived. From the results, the optimum value of drift length and the doping is obtained which is used to design the NLDMOS and PLDMOS. After the device optimization, the circuit optimization is done to find a suitable Q point. And to achieve proper Q-point for the amplifier parametric analysis of dc bias voltage is done. Then the proper impedance matching is achieved through S parameter analysis. Ideally the source impedance
should be the complex conjugate of input impedance of the
amplifier for proper matching. Through S-parameter analysis of the circuit the matching circuit is designed as shown in Fig. 6. It consist a series combination of resistance and capacitance. analysis.
This optimized CLDMOS is used in the next section for simulation and
Fig. 6: Schematic diagram of CLDMOS power amplifier with matching network at the input
RESULT AND DISCUSSION Performance optimization of CLDMOS power amplifier is obtained through several AC and DC analysis of the circuit. At the time of simulation, the parametric variation of drift length and doping concentration is done. Drift length of 1.5 µm, 2 µm, 2.5 µm, 3 µm, 3.5 µm, 4 µm and drift region doping concentration of 1016, 3016, 5016, 7016, 1017, 5017, 1018 cm-3 are used. Gain bandwidth performance of power amplifier is shown in the following Fig. 8 & 9. From these two graphs, it is evident that the effect of changing the doping concentration on the frequency response of the power amplifier is more prominent than that of changing the drift length. Low on-resistance and high on current are the two main device criteria for a LDMOS based Power Amplifier. Low doping concentration in the drift region reduces the number of immobile charges. Thus the scattering effect of mobile carrier is also reduced, increasing the overall current and as a result a high on current is observed. This high current flow delivers the high power at output introducing a high gain. Likewise small drift length reduces the transit time, hence the chance of recombination. That reduces the channel resistance which decreases the on-resistance of the driver LDMOS. At the time of reducing the channel resistance during optimization, the breakdown voltage is another parameter of concern. Optimization of drift length is required between the on-resistance and the breakdown voltage because a large reduction of drift length will not give the desired breakdown voltage.
Optimized values of the drift length and doping concentration are achieved from the simulated Gain frequency performance of the power amplifier. The drift length of 1.5 µm and
doping concentration of 1016 cm-3 are the optimized value for the proposed Power Amplifier structure.
Fig. 7: Small signal equivalent circuit of proposed CLDMOS
Fig. 8: Gain bandwidth performance of the CLDMOS power amplifier for different values of doping concentration of drift region
Around 20 to 36 dB gain is observed using only a single stage amplifier for wireless receiver with an input power range of -57.5 dB to -30 dB. Fig. 10 shows that within -40dBm to10dBm input power range it provides necessary gain which is in between 20 to 36 dB. This input power range is taken as input range for ISM band[7]. In this range, with an increase in input power the variation of dc bias point increases which shifts the operating region from its optimum value which decreases the gain of the circuit. Fig. 11 shows the Gain response of the power amplifier for different biasing voltages. 20 to 21 dB gain is achieved by a variation of DC bias voltage range 5.8 V to 6.2 V. For other biasing voltages gain is less than that. The DC point 5.85 V has been taken as the optimized value to set the Q point properly. For faithful amplification, the amplitude range of 0 to 1.5V is used (Fig. 12). Using all the above
mentioned optimization, for a single stage amplifier, 20 to 36 dB gain, 94.5-03 W PEP with only 15V of DC bias is obtained. Proper impedance matching is also required to achieve optimized performance of the amplifier. From the maximum power transfer theorem it is evident that for the maximum power transfer from the source to load, the output impedance of source and input impedance of load should be the complex conjugate of each other[17]. Sparameter analysis is done for the proper input impedance matching and from that result the designed circuit shown in Fig. 6 is obtained. Proper matching is required to achieve high isolation and low insertion loss. Fig. 7 shows the small signal equivalent circuit of the proposed CLDMOS amplifier which indicates the value of CIN. The value is CIN=(CGDN+CGDP)+(CGSP+CGSN +CGBP+CGBN) which clearly proves that it is greater than the input capacitance (CGB) of a single transistor. Therefore it can be concluded that CLDMOS provide higher isolation because of higher CIN. Also from the expression of insertion loss we see that it is inverse of the gain for an amplifier, as CLDMOS provides higher gain its insertion loss is very low. Low insertion loss results in high power transmission by the amplifier and high isolation provide a good separation between the output and the input[18].
Fig. 9: Gain bandwidth performance of the CLDMOS power amplifier for different values of length of drift region
Fig. 16 shows the Insertion loss performance and Isolation of the designed CLDMOS power Amplifier. The result shows a high value of Isolation and low value of Insertion loss which makes the circuit ideal for RF application.
Fig. 10: Study of gain vs. input power of power amplifier for the desired range of input power.
1 dB compression point is around -41dBm with a sufficiently high Power Added Efficiency (PAE) of 28.5% and drain efficiency of 46.4% is achieved [19]. Fig. 13 shows that in the range of -55 dB to -41 dB the amplifier performance is linear as the 1 dB compression point is located at -41dB. This High value of 1dB compression point makes the circuit performance linear for a large range. On the other hand, another parameter of linearity IIP3 [20] is having a high value of 0 dBm which makes the amplifier performance linear also beyond the target range of -10 to -40 dBm. The result of IIP3 is shown in Fig. 14. Fig. 15 shows the two types of efficiency studies for the amplifier. Here it is observed that the PAE is less than the drain efficiency as expected. From Fig.15 it is seen that the peak value of PAE is 28.5% and it is achieved in the desired range of the amplifier. Fig 15 also shows the drain efficiency study of the proposed circuit. It shows a constant efficiency throughout the output range with a peak value of 46.4%.
Fig. 11: Study of gain variation with shift in DC bias point for the CLDMOS power amplifier
Fig. 12: Study of gain saturation of CLDMOS power amplifier with increase in input amplitude
Fig. 13: Study of input power vs output power and measurment of 1dB compression point of CLDMOS power amplifier
Fig. 14: Measurment of IIP3 as the meausre of linearity for the power amplifier
Fig. 15: Output power vs. PAE & efficiency for the power amplifier
Fig. 16: Frequency vs. insertion loss and isolation for the power amplifier
CONCLUSION: In this work, a CLDMOS based power amplifier is designed and optimized to achieve a 20 to 36 dB gain, 94.5-03 W PEP for 15V supply voltage. The drain efficiency of the power amplifier is 46.4% and PAE is 28.5%. The amplifier is having a bandwidth of 800MHz and a gain of 36dB.An improvement of 20.4dB gain and 680 MHz bandwidth with 15V supply is observed over the existing structure which provides 15.6 dB gain and 120MHz bandwidth for 28V supply[21].The performance of the amplifier is linear for the input range of -10 dBm to 40 dBm. In this range the linearity of the circuit is observed from 0dBm IIP3 point and -41dB 1dB compression point. Average Isolation of the circuit is 22 dB and average Insertion Loss is 5dB which makes the circuit ideal for the application of Wireless Network.
ACKNOWLEDGEMENT The authors want to thank the All India Council of Technical Education (AICTE), Govt. of India for funding the TCAD tools.
REFERENCES [1] S. C. Cripps, “RF Power Amplifier for Wireless Communications”, Second Edition, Norwood, Artech House 1999. [2] J. Leong Tham , C.Hull , A. Ali, F. Carr, R. Chu, J. Walley, and I. Koullias “A directconversion transceiver chip set for 900 MHz (ISM band) spread-spectrum digital cordless telephone,” IEEE International Symposium on Circuits and Systems, pp.85-88 vol.4, 12-15 May 1996 . [3] S. A. Smerzi, G. Girlando, T. Copani, and G. Palmisano, “A Ku-band monolithic receiver for DVB-S applications” IEEE Communications Magazine, vol.42, no.8, pp.132,139, Aug. 2004. [4] B. J. Baliga, “Fundamentals Of Power Semiconductor Devices”, First Edition New York, Springer, 2008. [5] F. Larsen, M. Ismail, "The design of high performance low cost BiCMOS op-amps in a predominantly CMOS technology", IEEE International Symposium on Circuits and Systems, 1995, vol.3, no., pp.1720-1723 ,30 Apr-3 May, 1995. [6] N. Dye, and H. Granberg, “Radio Frequency Transistors, Principles and Practical Applications”,Second Edition, Newnes ,2001. [7]Datasheet of cisco Aironet 1400 series Wireless bridge. [8] S. J C H. Theeuwen, J.H. Qureshi, "LDMOS Technology for RF Power Amplifiers," IEEE Transactions on Microwave Theory and Techniques, vol.60, no.6, pp.1755-1763, June 2012 [9] M. P van der Heijden, H.C. de Graaff, L. C N De Vreede, J.R. Gajadharsing, and J.N. Burghartz, , "Theory and design of an ultra-linear square-law approximated LDMOS power amplifier in class-AB operation," IEEE Transactions on Microwave Theory and Techniques, vol.50, no.9, pp.2176-2184, Sep 2002. [10] T. Efland, T. Keller, S. Keller, and J. Rodriguez, "Optimized complementary 40 V power LDMOS-FETs use existing fabrication steps in submicron CMOS technology", IEDM '94. Technical Digest, pp.399-402, 11-14 Dec. 1994.
[11] B. Razavi, “Design of Analog CMOS Integrated Circuits”, First Edition New Delhi, Tata McGraw-Hill 2002. [12]S. C.Sun, J.D.Plummer, , "Modeling of the on-resistance of LDMOS, VDMOS, and VMOS power transistors," , IEEE Transactions on Electron Devices , vol.27, no.2, pp.356367, Feb 1980. [13] C.W Tang,and K.Y Tong, “A compact large signal model of LDMOS”, Solid-State Electronics, Volume 46, Issue 12, December 2002, Pages 2111-2115. [14] Atlas Manuals, Silvaco Inc., Santa Clara, CA, 2012. [15] D.M. Caughey, R.E. Thomas, , "Carrier mobilities in silicon empirically related to doping and field," Proceedings of the IEEE , vol.55, no.12, pp.2192-2193, Dec. 1967 [16] S. Malobabic, J.A. Salcedo, J.-J. Hajjar, and J.J. Liou, "Analysis of Safe Operating Area of NLDMOS and PLDMOS Transistors Subject to Transient Stresses," IEEE Transactions on Electron Devices, vol.57, no.10, pp.2655-2663, Oct. 2010. [17] D. M. Pozar “Microwave Engineering”, Third Edition, Delhi, Wiley India,2008. [18] H. Chih-Min, H. Chung-Yu,C. Chun-Hsue, C. Da-Chiang, H. Chih-Fang,G. Jeng, C. Ching-Yu, "Design of an RF Transmit/Receive Switch Using LDMOSFETs With High Power Capability and Low Insertion Loss," IEEE Transactions on Electron Devices, vol.58, no.6, pp.1722-1727, June 2011. [19]R. Bagger, C.D. Shih, Y. Yu, J.Sjöström, and P. Andersson, "RF LDMOS Power Amplifier Integrated Circuit for W-CDMA/TD-SCDMA applications," 36th European Microwave Conference, 10-15 Sept. 2006, pp.1343-1346, [20]T. H. Lee,” The Design of CMOS Radio-Frequency Integrated Circuits”, Second Edition, Chembridge, Chembridge University Press, 2004 [21] F.J. Ortega-Gonzalez, "High Power Wideband Class-E Power Amplifier," Microwave and Wireless Components Letters, IEEE , vol.20, no.10, pp.569-571, Oct. 2010
List Of Figures Figure 1. Device structure of NLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=.05 µm Figure 2. Device structure of PLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=.05 µm Figure 3. Drain current Vs. drain voltage of NLDMOS for gate to source voltage of 10V
Figure 4. Drain current Vs. drain voltage of PLDMOS for gate to source voltage of -10V Figure 5. Drain current Vs. gate voltage of PLDMOS & NLDMOS for drain to source voltage of 2V and 10V Figure 6. Schematic diagram of CLDMOS power amplifier with matching network at the Input Figure 7. Small signal equivalent circuit of proposed CLDMOS Figure 8. Gain bandwidth Performance of the CLDMOS power amplifier for different values of doping concentration of drift region Figure 9. Gain bandwidth performance of the CLDMOS power amplifier for different values of length of drift region Figure 10. Study of gain vs. input power of power amplifier for the desired range of input power. Figure 11. Study of gain variation with shift in DC bias point for the CLDMOS power amplifier Figure 12. Study of gain saturation of CLDMOS power Amplifier with increase in input amplitude Figure 13. Study of input power vs output power and measurment of 1dB compression point of CLDMOS power amplifier Figure 14. Measurment of IIP3 as the meausre of Linearity for the power amplifier Figure 15. Output power vs. PAE & efficiency for the power amplifier Figure 16. Frequency vs. insertion loss and isolation for the power amplifier
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Highlight for Review 1. 2. 3. 4.
Optimization of LDMOS Complementary LDMOS Power Amplifier CLDMOS Power Amplifier for ISM Band CLDMOS Power Amplifier with High Gain and High Linearity
S0749-6036(14)00145-1 http://dx.doi.org/10.1016/j.spmi.2014.05.001 YSPMI 3254
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
3 December 2013 30 April 2014 3 May 2014
Please cite this article as: S. Mukherjee, S. Roy, C.K. Sarkar, A 30 W 800 MHz Complimentary LDMOS Power Amplifier for Wireless Application, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi. 2014.05.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A 30 W 800 MHz Complimentary LDMOS Power Amplifier for Wireless Application Sagar Mukherjee,Swarnil Roy,Chandan Kumar Sarkar Department of Electronics and Telecommunication Engineering, Jadavpur University, Kolkata, West Bengal, India [email protected]
ABSTRACT This work presents the design of an optimized power amplifier using Complementary LDMOS (CLDMOS) with 5µm Gate Length. Final results show that the high performance CLDMOS amplifier has been achieved using the optimization of various device parameters and the circuit bias conditions. Optimization of the drift length and the drift area doping in device parameters have been done for both P type LDMOS (PLDMOS) and N type LDMOS (NLDMOS) for various Analog/RF applications. And these optimizations yields CLDMOS Power Amplifier with 36 dB gain, 800 MHz bandwidth and 46.4% efficiency. Comparing with existing structure a good improvement of gain and bandwidth has been observed. Index Terms— Complementary LDMOS (CLDMOS), Power Amplifier, Optimization, Device Parameter, Gain, Bandwidth, Drain Efficiency, Power Added Efficiency (PAE).
INTRODUCTION A rapid development of wireless and mobile technology demands a rising need of high performance power amplifier with low cost for the frequency range of 900 MHz to 2.7 GHz, which is the frequency band, used by GSM, CDMA and other wireless systems [1]. Now a day this bandwidth is also useful for NFC and Cordless telephony application which lies in the frequency band known as ISM Band[2]. For these applications, wide band gap compound GaN type semiconductor HEMT are popular due to their high mobility, high gain and power handling capability [3].But high production cost of these devices make them unsuitable for large scale production [4]. Conventional MOSFET does not have problems related to cost. However, the conventional MOSFET is having issues related to the mobility and power handling capability which make it incapable as power amplifier in the high frequency domain [5]. Conventional MOSFETs show some reliability issues with parameters like on-resistance, breakdown voltage and other important parameters required for power devices. From the aspect of the cost, the gain, the power handling capability, the breakdown voltage and the on-resistance LDMOS is the best choice over HEMT and also MOSFET [6].
For a weak signal(-10 to -70 dBm) in wireless network [7], a high gain amplifier is required which makes LDMOS a suitable choice as it is having a high breakdown potential, low onresistance and high gain due to its structural uniqueness. Due to its structural advantage it can handle high power which makes it very useful for the application as Power Amplifier. A significant development is observed using LDMOS transistor in last few years, in terms of output power, Gain bandwidth, linearity, efficiency [6][8]. Different stages of amplifier for driver application is designed using high efficiency LDMOS which were introduced for the need of high bandwidth of the present day mobile communication systems [9]. On the other hand, LDMOS can be fabricated using a low cost reliable fabrication method [10] unlike HEMT, which proves the significance of LDMOS technology as an innate choice for wireless systems over other existing technology in the market. Amplifiers designed using complementary MOS structure has an inbuilt high gain property [11]. Using the same property of complementary structure a CLDMOS power amplifier is designed in which various device and circuit parameters optimizations are done. Through the proper impedance matching and proper biasing and also at the same time optimization of the drift region doping & the drift region length is done to achieve 800 MHz bandwidth, 30W power handling capacity, 36 dB Gain and 46.4% efficiency. DEVICE STRUCTURE Unlike conventional MOSFETS, LDMOS has a drift region which helps to achieve the high current handling capability, high on-resistance and large breakdown voltage[12]. Increasing the drift region length increases the breakdown voltage and on-resistance. But increasing the length also reduces the overall gain of the circuit. Thus careful optimization of drift length is required for optimized gain performance.
Fig. 1: Device Structure of NLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=0.05 µm
Fig. 2: Device structure of PLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=0.05 µm
For the design of single stage complimentary push pull power amplifier, NLDMOS and PLDMOS are used. Fig.1 & 2 shows the proposed structure of NLDMOS and PLDMOS respectively. According to the basic structure of PLDMOS and NLDMOS [13], it is seen that the region between the gate and the drain can be divided into two regions which are named as the channel region and the drift region. Low doping and long drift region increase the breakdown voltage. The channel region behaves like a normal channel of conventional MOS. The gate voltage has a negligible effect on the drift region. Fig. 3-5 provides the current voltage characteristics of these devices. All the simulations are performed using a 2-D device simulator [14].For the purpose of device simulation, parallel field mobility model is used which considers Caughey and Thomas expressions to model the mobility [15]. And both for NLDMOS and PLDMOS this model can be used [14]. Also, the Shockley–Read–Hall recombination is incorporated for the modeling of recombination. Impact ionization is modeled with the introduction of Selberherr’s impact ionization model. PLDMOS is also having the same structure like NLDMOS and it also has same models mentioned above for NLDMOS. In this paper NLDMOS & PLDMOS with breakdown voltage of 24V and 22V respectively, are considered. As expected NLDMOS has higher breakdown voltage[16].
Fig. 3: Drain current Vs. drain voltage of NLDMOS for gate to source voltage of 10V
Fig. 4: Drain current Vs. drain voltage of PLDMOS for gate to source voltage of -10V
For this proposed structure the channel & gate lengths are 2µm & 5µm respectively, the oxide thickness is 0.05 µm. In order to achieve an optimization in the performance of the power amplifier using the LDMOS, these two device parameter are varied. Using this PLDMOS and NLDMOS, we can design a CLDMOS which can behave as a push pull power amplifier. In the next few sections, the design and optimization of these devices as well as the power amplifier are described.
Fig. 5: Drain current Vs. gate voltage of PLDMOS & NLDMOS for drain to source voltage of 2V and 10V
DESIGN AND OPTIMIZATION OF POWER AMPLIFIER Just like a conventional CMOS amplifier PLDMOS and NLDMOS are connected in complimentary configuration to design a power amplifier. As the two LDMOS are connected in complimentary configuration, it is named as CLDMOS. Here a single stage power amplifier is designed and simulated; but same is also applicable for two stage configuration. For the optimization of the performance of the Power amplifier, the variation of main device parameters like the drift region length and the doping of drift region are done. For the parametric analysis, a drift region length sweep of 1.5µm to 4µm is used and doping density sweep is 1016 cm-3 to 1018 cm-3. For each sweep ac analysis of the amplifier is done and from that the gain and the bandwidth are derived. From the results, the optimum value of drift length and the doping is obtained which is used to design the NLDMOS and PLDMOS. After the device optimization, the circuit optimization is done to find a suitable Q point. And to achieve proper Q-point for the amplifier parametric analysis of dc bias voltage is done. Then the proper impedance matching is achieved through S parameter analysis. Ideally the source impedance
should be the complex conjugate of input impedance of the
amplifier for proper matching. Through S-parameter analysis of the circuit the matching circuit is designed as shown in Fig. 6. It consist a series combination of resistance and capacitance. analysis.
This optimized CLDMOS is used in the next section for simulation and
Fig. 6: Schematic diagram of CLDMOS power amplifier with matching network at the input
RESULT AND DISCUSSION Performance optimization of CLDMOS power amplifier is obtained through several AC and DC analysis of the circuit. At the time of simulation, the parametric variation of drift length and doping concentration is done. Drift length of 1.5 µm, 2 µm, 2.5 µm, 3 µm, 3.5 µm, 4 µm and drift region doping concentration of 1016, 3016, 5016, 7016, 1017, 5017, 1018 cm-3 are used. Gain bandwidth performance of power amplifier is shown in the following Fig. 8 & 9. From these two graphs, it is evident that the effect of changing the doping concentration on the frequency response of the power amplifier is more prominent than that of changing the drift length. Low on-resistance and high on current are the two main device criteria for a LDMOS based Power Amplifier. Low doping concentration in the drift region reduces the number of immobile charges. Thus the scattering effect of mobile carrier is also reduced, increasing the overall current and as a result a high on current is observed. This high current flow delivers the high power at output introducing a high gain. Likewise small drift length reduces the transit time, hence the chance of recombination. That reduces the channel resistance which decreases the on-resistance of the driver LDMOS. At the time of reducing the channel resistance during optimization, the breakdown voltage is another parameter of concern. Optimization of drift length is required between the on-resistance and the breakdown voltage because a large reduction of drift length will not give the desired breakdown voltage.
Optimized values of the drift length and doping concentration are achieved from the simulated Gain frequency performance of the power amplifier. The drift length of 1.5 µm and
doping concentration of 1016 cm-3 are the optimized value for the proposed Power Amplifier structure.
Fig. 7: Small signal equivalent circuit of proposed CLDMOS
Fig. 8: Gain bandwidth performance of the CLDMOS power amplifier for different values of doping concentration of drift region
Around 20 to 36 dB gain is observed using only a single stage amplifier for wireless receiver with an input power range of -57.5 dB to -30 dB. Fig. 10 shows that within -40dBm to10dBm input power range it provides necessary gain which is in between 20 to 36 dB. This input power range is taken as input range for ISM band[7]. In this range, with an increase in input power the variation of dc bias point increases which shifts the operating region from its optimum value which decreases the gain of the circuit. Fig. 11 shows the Gain response of the power amplifier for different biasing voltages. 20 to 21 dB gain is achieved by a variation of DC bias voltage range 5.8 V to 6.2 V. For other biasing voltages gain is less than that. The DC point 5.85 V has been taken as the optimized value to set the Q point properly. For faithful amplification, the amplitude range of 0 to 1.5V is used (Fig. 12). Using all the above
mentioned optimization, for a single stage amplifier, 20 to 36 dB gain, 94.5-03 W PEP with only 15V of DC bias is obtained. Proper impedance matching is also required to achieve optimized performance of the amplifier. From the maximum power transfer theorem it is evident that for the maximum power transfer from the source to load, the output impedance of source and input impedance of load should be the complex conjugate of each other[17]. Sparameter analysis is done for the proper input impedance matching and from that result the designed circuit shown in Fig. 6 is obtained. Proper matching is required to achieve high isolation and low insertion loss. Fig. 7 shows the small signal equivalent circuit of the proposed CLDMOS amplifier which indicates the value of CIN. The value is CIN=(CGDN+CGDP)+(CGSP+CGSN +CGBP+CGBN) which clearly proves that it is greater than the input capacitance (CGB) of a single transistor. Therefore it can be concluded that CLDMOS provide higher isolation because of higher CIN. Also from the expression of insertion loss we see that it is inverse of the gain for an amplifier, as CLDMOS provides higher gain its insertion loss is very low. Low insertion loss results in high power transmission by the amplifier and high isolation provide a good separation between the output and the input[18].
Fig. 9: Gain bandwidth performance of the CLDMOS power amplifier for different values of length of drift region
Fig. 16 shows the Insertion loss performance and Isolation of the designed CLDMOS power Amplifier. The result shows a high value of Isolation and low value of Insertion loss which makes the circuit ideal for RF application.
Fig. 10: Study of gain vs. input power of power amplifier for the desired range of input power.
1 dB compression point is around -41dBm with a sufficiently high Power Added Efficiency (PAE) of 28.5% and drain efficiency of 46.4% is achieved [19]. Fig. 13 shows that in the range of -55 dB to -41 dB the amplifier performance is linear as the 1 dB compression point is located at -41dB. This High value of 1dB compression point makes the circuit performance linear for a large range. On the other hand, another parameter of linearity IIP3 [20] is having a high value of 0 dBm which makes the amplifier performance linear also beyond the target range of -10 to -40 dBm. The result of IIP3 is shown in Fig. 14. Fig. 15 shows the two types of efficiency studies for the amplifier. Here it is observed that the PAE is less than the drain efficiency as expected. From Fig.15 it is seen that the peak value of PAE is 28.5% and it is achieved in the desired range of the amplifier. Fig 15 also shows the drain efficiency study of the proposed circuit. It shows a constant efficiency throughout the output range with a peak value of 46.4%.
Fig. 11: Study of gain variation with shift in DC bias point for the CLDMOS power amplifier
Fig. 12: Study of gain saturation of CLDMOS power amplifier with increase in input amplitude
Fig. 13: Study of input power vs output power and measurment of 1dB compression point of CLDMOS power amplifier
Fig. 14: Measurment of IIP3 as the meausre of linearity for the power amplifier
Fig. 15: Output power vs. PAE & efficiency for the power amplifier
Fig. 16: Frequency vs. insertion loss and isolation for the power amplifier
CONCLUSION: In this work, a CLDMOS based power amplifier is designed and optimized to achieve a 20 to 36 dB gain, 94.5-03 W PEP for 15V supply voltage. The drain efficiency of the power amplifier is 46.4% and PAE is 28.5%. The amplifier is having a bandwidth of 800MHz and a gain of 36dB.An improvement of 20.4dB gain and 680 MHz bandwidth with 15V supply is observed over the existing structure which provides 15.6 dB gain and 120MHz bandwidth for 28V supply[21].The performance of the amplifier is linear for the input range of -10 dBm to 40 dBm. In this range the linearity of the circuit is observed from 0dBm IIP3 point and -41dB 1dB compression point. Average Isolation of the circuit is 22 dB and average Insertion Loss is 5dB which makes the circuit ideal for the application of Wireless Network.
ACKNOWLEDGEMENT The authors want to thank the All India Council of Technical Education (AICTE), Govt. of India for funding the TCAD tools.
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[11] B. Razavi, “Design of Analog CMOS Integrated Circuits”, First Edition New Delhi, Tata McGraw-Hill 2002. [12]S. C.Sun, J.D.Plummer, , "Modeling of the on-resistance of LDMOS, VDMOS, and VMOS power transistors," , IEEE Transactions on Electron Devices , vol.27, no.2, pp.356367, Feb 1980. [13] C.W Tang,and K.Y Tong, “A compact large signal model of LDMOS”, Solid-State Electronics, Volume 46, Issue 12, December 2002, Pages 2111-2115. [14] Atlas Manuals, Silvaco Inc., Santa Clara, CA, 2012. [15] D.M. Caughey, R.E. Thomas, , "Carrier mobilities in silicon empirically related to doping and field," Proceedings of the IEEE , vol.55, no.12, pp.2192-2193, Dec. 1967 [16] S. Malobabic, J.A. Salcedo, J.-J. Hajjar, and J.J. Liou, "Analysis of Safe Operating Area of NLDMOS and PLDMOS Transistors Subject to Transient Stresses," IEEE Transactions on Electron Devices, vol.57, no.10, pp.2655-2663, Oct. 2010. [17] D. M. Pozar “Microwave Engineering”, Third Edition, Delhi, Wiley India,2008. [18] H. Chih-Min, H. Chung-Yu,C. Chun-Hsue, C. Da-Chiang, H. Chih-Fang,G. Jeng, C. Ching-Yu, "Design of an RF Transmit/Receive Switch Using LDMOSFETs With High Power Capability and Low Insertion Loss," IEEE Transactions on Electron Devices, vol.58, no.6, pp.1722-1727, June 2011. [19]R. Bagger, C.D. Shih, Y. Yu, J.Sjöström, and P. Andersson, "RF LDMOS Power Amplifier Integrated Circuit for W-CDMA/TD-SCDMA applications," 36th European Microwave Conference, 10-15 Sept. 2006, pp.1343-1346, [20]T. H. Lee,” The Design of CMOS Radio-Frequency Integrated Circuits”, Second Edition, Chembridge, Chembridge University Press, 2004 [21] F.J. Ortega-Gonzalez, "High Power Wideband Class-E Power Amplifier," Microwave and Wireless Components Letters, IEEE , vol.20, no.10, pp.569-571, Oct. 2010
List Of Figures Figure 1. Device structure of NLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=.05 µm Figure 2. Device structure of PLDMOS of channel length=2 µm , gate length =5 µm, oxide thickness=.05 µm Figure 3. Drain current Vs. drain voltage of NLDMOS for gate to source voltage of 10V
Figure 4. Drain current Vs. drain voltage of PLDMOS for gate to source voltage of -10V Figure 5. Drain current Vs. gate voltage of PLDMOS & NLDMOS for drain to source voltage of 2V and 10V Figure 6. Schematic diagram of CLDMOS power amplifier with matching network at the Input Figure 7. Small signal equivalent circuit of proposed CLDMOS Figure 8. Gain bandwidth Performance of the CLDMOS power amplifier for different values of doping concentration of drift region Figure 9. Gain bandwidth performance of the CLDMOS power amplifier for different values of length of drift region Figure 10. Study of gain vs. input power of power amplifier for the desired range of input power. Figure 11. Study of gain variation with shift in DC bias point for the CLDMOS power amplifier Figure 12. Study of gain saturation of CLDMOS power Amplifier with increase in input amplitude Figure 13. Study of input power vs output power and measurment of 1dB compression point of CLDMOS power amplifier Figure 14. Measurment of IIP3 as the meausre of Linearity for the power amplifier Figure 15. Output power vs. PAE & efficiency for the power amplifier Figure 16. Frequency vs. insertion loss and isolation for the power amplifier
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Highlight for Review 1. 2. 3. 4.
Optimization of LDMOS Complementary LDMOS Power Amplifier CLDMOS Power Amplifier for ISM Band CLDMOS Power Amplifier with High Gain and High Linearity