A 0.6V HIGH REVERSE-ISOLATION HIGH GAIN SINGLE-STAGE NON-CASCODE DIFFERENTIAL CMOS LNA T.T.N. Tran, C.C. Boon, M.A. Do, and K.S. Yeo Department of Electrical and Electronic Engineering Nanyang Technological University, Singapore ABSTRACT II. LOW VOLTAGE CCC-LNA In many designs, low supply voltage allows for efficient use of power. However, it often leads to low reverse isolation which is critical to the RF front-end component and particularly to the low-noise amplifier (LNA). In this paper, a fully integrated differential low-voltage CMOS LNA with high reverse isolation is presented. This LNA makes use of the single-stage non-cascode structure and the capacitive cross coupling (CCC) technique. The CCC technique has been utilized in many LNA designs. However, in all of the reported works using CCC, the CCC technique was mainly used to improve the noise figure (NF), not the reverse isolation. The poor reverse isolation problem in single-stage non-cascode structure has never been analyzed in the CCCLNA. This work shows a novel analysis on the feedback self-cancellation mechanism to improve the reverse isolation. Other analysis on input matching, gain and NF was also performed to show the feasibility of employing CCC technique for low-voltage LNA as well as the advantages of CCCLNA over the conventional common-source (CS) and common-gate (CG) LNA. The LNA consumes only 0.5 mW from 0.6V supply voltage. It achieves a gain of 14dB and a NF of 3.5 dB. The schematic of the CCCLNA is shown in Fig. 1(a). Two large capacitors Cc are used to cross couple the input devices M1 and M2. The design in [5] also used the CCC technique to realize a low-voltage LNA. However, the problem with poor reverse isolation was left untouched and there were many offchip components required in the design. In [5-10], the CCC technique is only used to improve the LNA’s NF. This paper presents a novel analysis on the feedback self-cancellation using CCC technique and emphasizes on analyzing the feasibility of using CCC technique for low supply voltage LNA as well as the advantages of using this technique in LNA design when compared to the conventional CS and CG topology. I. INTRODUCTION In response to the ultra low power requirement of wireless transceiver, maximum supply voltages have been steadily decreasing. A supply voltage of 0.5-0.7V is expected for low power devices [1]. LNAs are key building blocks in many modern receiver systems [2-4]. In order to operate at low supply voltage without degrading the linearity and dynamic range, the number of transistors stacked in series must be reduced. In addition, to lower the supply voltage and reduce the power dissipation simultaneously, a circuit without folded structure is preferred. In this paper, a fully integrated ultralow power LNA operating at 0.6V for 2.4GHz ISM band is presented. It has a differential single-stage non-cascode structure. The poor reverse isolation problem in this structure is improved by employing the capacitive cross-coupling (CCC) across the two sides of a differential input stage. The paper is organized as follows. Details of the design and circuit implementations are presented in Section II and III respectively. Design’s performance and comparisons with state-of-the-art low-power low-voltage LNAs are discussed in Section IV. Finally, the paper is concluded in Section V. A. Reverse isolation Fig. 2 shows the CMOS transistor small signal model with its parasitic capacitances , , , and . A single-stage non-cascade LNA has very poor reverse isolation due to the direct feedback paths from output to input through the parasitic capacitors as shown in Fig.3. Since there are two feedback paths in the CSLNA and the feedback is normally dominated by the Cgd path, the CSLNA usually has worse reverse isolation than the CGLNA. In the CCCLNA, there exists feedback self-cancellation. The simplified half circuits as shown in Fig.4 will be used to explain this mechanism. Since capacitor is much larger than the parasitic capacitors, it is considered as short circuit in this analysis. Fig.1 (a) CCCLNA (b) CCCLNA with added capacitors for extra feedback cancellation voltages from Vout+ and Vout- are 180o out of phase which results in a feedback self-cancellation at the input and an improvement in the reverse isolation. The same cancellation mechanism can be applied for Vin-. To analyze the feedback from the outputs, let us define the equivalent drain-source, , drain-gate and gate-source parasitic impedances as and respectively. The total impedance at each input node is defined as which is equal to /2 at matching condition. , and are much larger than . S12 (dB) Fig. 2 CMOS transistor small signal model Fig.5 Fig.3 (a) Output – input feedback paths in common source (CS) LNA (b) Output – input feedback path in common gate (CG) LNA of different topologies The feedback factor from Vout- to Vin+ is: . (1) The feedback factor from Vout+ to Vin+ is: . Fig.4 Feedback paths from (a) Vout- to Vin+ (b) Vout+ to Vin+ At each input node, there are feedback voltages from both Vout+ and Vout- . Vout- is fed back to Vin+ through paths 1 and 2 as shown in Fig. 4(a). These two paths are the same as the feedback paths in the CSLNA. Vout+ is fed back to Vin+ through paths 3 and 4 as shown in Fig. 4(b). The feedback (2) To maximize the feedback cancellation, the feedback factors from Vout+ and Vout- to Vin+ should be matched. Equalizing equation (1) with (2) results in a condition . Impedance is mainly contributed by the of . Impedance is formed by the parasitic capacitance output resistance ro and parasitic capacitances and in most of the cases. Therefore, to which are smaller than maximize the feedback cancellation, the designer can add an additional capacitor, Ca, across the drain-source terminal as shown in Fig. 1(b) or an inductor across the drain-gate terminal. The former method is preferred since on-chip capacitor normally occupies less area than on-chip inductor. is as closed to The size of Ca should be designed so that as possible. To verify the theory, we simulated the circuits in Fig. 1(a), 1(b), 3(a) and 3(b). The CSLNA and CGLNA are single sides of the CCCLNA without the cross coupling capacitor . The four LNAs draw the same amount of drain current. The transistor sizes used in the four LNAs are also the same. As shown in Fig.5, the CSLNA has poor reverse isolation due to the large parasitic capacitor . At 2.4GHz, the CCCLNA shows an improvement of 8dB in term of reverse isolation when compared to the CSLNA. With the added capacitor, Ca, the improvement increases to 12dB. The value of capacitor Ca is varied from 8fF to 17fF. As Ca approaches which results in increases, the value of a better reverse isolation as seen in Fig.5. When Ca increases to 17fF, the reverse isolation starts to drop. The optimum value of Ca is 15fF in our simulation. The presented CCCLNA has superior reverse isolation than the CSLNA, especially at high frequency. At 6GHz, the CCCLNA shows a reverse isolation improvement of 10dB. With an added capacitor Ca equals to 15fF, the CCCLNA introduces an extra 26dB of reverse isolation compared to the CSLNA. Practically, it is hard to achieve a complete cancelation since matching of parasitic components is a difficult task. Luckily, in most of the applications, the reverse isolation requirement is not very strict. A value of better than -30 dB is parameter. Therefore, a complete feedback acceptable for cancellation is not required. Over-designing the reverse isolation of the LNA may result in trade-offs with other performance parameters such as gain, NF and chip area. In non-cascode CS structure, high reverse isolation is hardly achievable due to the direct feedback paths from output to input through the parasitic capacitors. By reducing the transistor’s size of the CSLNA, the parasitic capacitors become smaller, resulting in a better but smaller gain and higher NF. In the CCCLNA, the feedback self-cancellation at the input makes the -30 dB target more achievable. In our simulation, the CCCLNA achieves a reverse isolation better than -30 dB even without the added capacitors Ca. The high reverse isolation in the CCCLNA is achieved by the feedback self-cancellation mechanism. C. Gain and input matching The voltage gain is: , (5) 2 , is the load impedance. While drawing the same where current consumption, the CCCLNA provides double gain when compared to the conventional CGLNA. The input impedance of the CCCLNA can be found using the small signal analysis as shown in Fig.6. Capacitors are very large so they were considered as short circuit in this analysis. The single input impedance is found to be: 2 4 1/ (7) 1/ 2 The resonance frequency is: , 1/√4 It has been proven in [5] that the transconductance of the CCCLNA is effectively boosted to: (3) 2 where is the transconductance of the input device. The NF of the CCCLNA therefore can be expressed as: 1 1 (4) 2 2 When the input is matched, the CCCLNA’s NF is 1 / 2 [5]. This NF is lower than of the CGLNA which is 1 / . In addition, to achieve the same matching condition with the CCCLNA, the CGLNA requires double amount of power consumption. 2 At resonance frequency, , simply becomes , 1/ 2 . Therefore, the matching condition for the CCCLNA is: B. Noise Factor , (6) 1 , Fig. 6 CCCLNA’s small signal input impedance analysis (8) value or the same power of input matching. At the same consumption, the CCCLNA achieves a much better match to the 50Ω source impedance than the CGLNA. II. CIRCUIT IMPLEMENTATION A. Output buffer The main purpose of an output buffer is to match the output port to the 50Ω load of the measuring equipment. To avoid the need of de-embedding the LNA’s linearity, a highly linear buffer is favored. The conventional source follower as shown in Fig. 9(a) is commonly used due to its high linearity. Its output impedance is: Fig.7 NF of CG and CCCLNA 1/ , (11) // is the transconductance of the transistor . where For 50Ω output matching, is normally chosen to be very is designed to be 50 Ω. large and 1/ Vin Vin Ms Vout Fig.8 Input impedance of CG and CCCLNA In the case of the CGLNA, the matching condition and the resonance frequency are, respectively: RL2 Rout,sf2 Rout,sf1 RL RL1 (9) 1/ and (a) , Ms 1/√ (10) (b) Fig. 9 (a) Conventional source follower output buffer (b) Improved output buffer As we can see from equations (5) and (7)-(10), the or double current CGLNA requires double value of consumption and four times larger source inductor, , to achieve the same matching condition and same voltage gain with the CCCLNA. Plotting of NFs and input impedances of the CCCLNA , and CGLNA versus the input device’s transconductance, at the resonance frequency are shown in Fig. 7 and Fig. 8. The dotted lines represent the simulation data and the solid lines represent the analysis data. The simulation results agree well with the analysis. As shown in Fig. 7, the NF of the CCCLNA is much lower than of the CGLNA. Even when we increase the drain current of the CGLNA to make the CG’s , the NF of the CGLNA is still g equal to the CCC’s , higher than of the CCCLNA. The CCCLNA shows a superior performance in term of NF over that of the CGLNA. Fig. 8 illustrates the advantage of CCCLNA over CGLNA in term Fig. 10 Output matching of conventional source follower , Fig. 11 Output matching of improved buffer and the low-voltage CCCLNA (with output buffer) When the source follower’s input is connected to our LNA, its output matching is greatly degraded. Fig. 10 demonstrates this problem. When the source follower in Fig. , is better than 9(a) is simulated alone, its output matching, -20dB. However, when it is integrated with our LNA, the output matching of the whole circuit is degraded, especially near the resonance frequency. In this simulation, the resonance frequency is around 2.65GHz. The degradation of the output matching can be explained as follows. If two twoport networks A and B are connected in series, the scattering matrix of the equivalent circuit is: (12) 1 1 1 // 1/ When is much larger than , can be , approximated to and the effect of 1/ on total output impedance is neglected. The output is matched when / R equals to 50Ω. The buffer’s gain equals to which is much smaller than one. This helps to lower the effect of the second term in the equation to the total value. The total will be dominated by the of the buffer. Fig. 11 shows the output matching of the improved buffer and of the low-voltage CCCLNA integrated with this buffer. As expected, the output matching in the two cases are not very different. The simulation results agree well with the analysis. B. Full circuit implementation Fig. 12 shows a low-voltage CCCLNA designed to operate at 2.4GHz in a 0.18µm RF CMOS technology with 0.6V act as output impedance supply voltage. On-chip inductors and have quality factor of 8.3. Capacitors are used to cross couple and . Capacitors and are bypass capacitors. On-chip inductors are used to tune out the total capacitances at the source nodes including pad capacitances. To facilitate accurate measurement, two separate differential output pads are implemented. Output 1 is placed before the buffer and Output 2 is placed after the buffer. Voltage gain, output and input matching will be measured from Output 2. The other parameters like NF, reverse isolation and IIP3 will be measured from Output 1. When one output is measured, the other is open circuit. The gain of the LNA equals to the gain measured from Output 2 subtracting the loss from the output buffer. 1 and are the S-parameters of A and B where respectively . In our case, network A represents the LNA and network B represents the output buffer. Ideally, we want to be dominated by . It can be realized by designing and to be much smaller than one so that the contribution of to the total / 1 is neglected. However, the source follower’s gain normally equals to one and at resonance frequency, the output impedance of the LNA is very large which results in a value of approximately to one. Therefore, the equals to: / 1 (13) contributes a is small, the term / 1 As significant amount to the total . The value can’t be approximated to and the total output matching is therefore degraded. To solve this problem, a buffer as shown in Fig. 9(b) was designed. The output impedance of the improved buffer is: (14) Fig. 12 The low-voltage CCCLNA with improved output buffer 1.2 mm Output 2 NF (dB) Input Output 1 Fig.13 Chip micrograph of the presented low-voltage CCCLNA S11 and S22 (dB) Voltage gain (dB) Fig.16 Simulated and measured NF of the presented low-voltage CCCLNA Fig. 14 Simulated and measured voltage gain of the presented low-voltage CCCLNA Fig.17 Simulated and measured S11 and S22 of the presented low-voltage CCCLNA 25.0 Fundamental tone 0 IM3 tone Pout (dBm) -25.0 -50.0 -75.0 -100.0 IIP3 = -6.8dBm @2.4 GHz Freq (GHz) -125.0 -40 -30 Fig.15 Simulated IIP3 at 2.4GHz -20 Pin (dBm) -10 0 Fig.18 Simulated and measured S12 of the presented low-voltage CCCLNA TABLE I. PERFORMANCE COMPARISONS Tech (nm) Vdd (V) Power (mW) Freq (GHz) Gain (dB) NF (dB) S11 (dB) IIP3 (dBm) Differen tial? This work 180 0.6 0.5 2.4 14 3.5 <-22 -6.8 Yes [11]* 90 0.6 3 2.4 15 3 <-10 -7 No [12]* 90 0.75 3.75 1.95 23 1.6 <-37 0 Yes 0.9 0.5 0.7 0.6 3.9 1.0 7.76 0.4 2.44 4.7 2-2.5 3 15.3 16.8 16.7 9.1 3.34 2.7 3.53 4.7 <-16 <-18 <-15 <-20 -10 -18.5 -0.5 -11 No No No No [13] 0.13 [14] 90 [15]* 180 [16] 130 *: simulation results IV. MEASUREMENT RESULTS The prototype of the CCCLNA is fabricated in a six-metal 0.18 µm CMOS technology for the 2.4 GHz ISM band. The die micrograph is shown in Fig. 13. The total die area including the output buffer and pads is 1.2x1.0mm2.Inductors and are on chip. Inductors are used to tune out the total capacitances at the source nodes including pad act as output impedance. The LNA capacitances. Inductors was purposely designed with a positive frequency offset in order to account for un-modeled parasitic capacitances and inductances. The simulated and measured voltage gain is plotted in Fig.14. The LNA’s voltage gain is 14dB at 2.4GHz. Fig. 15 plots an IIP3 of -6.8 dBm at 2.4GHz. The simulated and measured NF is shown in Fig. 16. Several dies were measured and mean value of the NF is plotted. The difference between the measured and simulated NF is owed to the process variations. The LNA achieves a NF of 3.5dB at 2.4GHz. The comparison between the measured and simulated and is plotted in Fig. 17. The LNA has good value is better than -22dB and input/output matching. The value is better than -15dB at 2.4GHz. The simulated the is shown in Fig.18. The LNA has good and measured reverse isolation of -37dB. The core LNA draws 0.83mA from a 0.6V voltage supply. Total power consumption is 0.5mW. The comparisons of this LNA with published literatures are summarized in Table II. The LNAs in [11]-[16] operate at supply voltages of 0.5-0.9V. Among the designs, the presented LNA consumes the least power. The power consumption of [16] is quite comparable to the presented LNA, but the NF is 4.7dB which is quite high. In the presented LNA, there is a tradeoff between NF and power. For lower NF, higher power is required. However, LNA’s NF is not a critical performance parameter for the ISM band standard [18]. A NF of 3.5 dB is acceptable for such low power consumption. The design in [12] and [15] have good linearity but more power is consumed when compared to the rest. The figure of merits (FOM1 and FOM2) defined in [19] are used to compare the performance of the LNAs. Topology Single-stage non-cascode CCC Inductive source degeneration with forward body biasing Inductive source degeneration with off-chip gate inductors Inductive source degeneration Transformer – folded cascode Folded cascode Sub-threshold CS FOM1 FOM2 12.88 6.08 6.55 -0.4 12.17 12.17 4.98 15.76 2.45 10.4 -5 -2.74 2 -0.6 The FOM1 is a function of the operating frequency, gain, noise factor, and power consumption. It is given by: 1 (15) . 10 1 . FOM2 includes IIP3 and is given by: 2 10 . 1 . . 3 (16) Based on the FOM calculated in Table I, the presented work shows comparable performances to the other designs. V. CONCLUSION An ultra-low voltage single-stage non-cascode LNA was presented. With the same structure, the conventional CSLNA normally has better noise figure but lower reverse isolation. The basic CGLNA on the other hand has better reverse isolation but much higher NF. The CCC technique introduces the self-cancellation of output-to-input feedback, therefore solves the reverse isolation problem in CSLNA. The LNA using this technique can also achieve NF much lower than of the CGLNA. Novel analysis on the reverse isolation and input matching of the low-voltage CCCLNA was performed. Other analysis on gain and NF was also presented to show the advantages over the conventional CS and CGLNA. The design is very suitable for low supply voltage such as 0.6V. 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Mohammadi, "A 3GHz subthreshold CMOS Low noise amplifier", in IEEE RFIC Symposium, San Franciso, CA, USA, pp.494497, Jun. 2006. A.V. Do, C.C. Boon, M.A. Do, and K.S. Yeo, "A subthreshold lownoise amplifier optimized for ultra-low-power applications in the ISM band", IEEE Transactions on Microwave Theory and Techniques, vol. 56, no.02, pp. 286-292, 2008. S. Joo, T. Choi, and B. Jung, "A 2.4GHz Resistive Feedback LNA in 0.13um CMOS", IEEE Journal of Solid-State Circuits, vol.44, pp.30193029, Nov.2009. Thi Thu Nga Tran received the B.E. degree (Hons.) in electronics from Nanyang Technological University (NTU), Singapore, in 2008. Currently, she is working toward the Ph.D. degree in NTU, Singapore. Her research interests include ultra-low power RF front-end IC design and mixed-signal circuits for high speed applications. Chirn Chye Boon received the B.E. (Hons) (Elect) in 2000 and Ph.D. (Elect. Eng.) in 2004 from the Nanyang Technological University, Singapore. In 2005, he joined NTU as a Research Fellow and became an Assistant Professor in the same year. Before that, he was with Advanced RFIC, where he worked as a Senior Engineer. He specializes in the RFIC design for bio-medical and communication applications. Professor Boon is a reviewer for IEEE Transactions of Circuits and Systems–I, IEEE Microwave and Wireless Components Letters and IEEE Transactions on Microwave Theory and Techniques. He provides consultation to multinational companies and serves as a program committee member in several international conferences. Manh Anh Do obtained his B.E. (Hons) (Elect) in 1973, and Ph.D. (Elect. Eng.) in 1977 both from University of Canterbury, New Zealand. Between 1977 and 1989, he held various positions including: R & D engineer and production manager at Radio Engineering Ltd., research scientist at Fisheries Research Centre, New Zealand, and senior lecturer at National University of Singapore. He joined the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Singapore as a senior lecturer in 1989, and obtained the Associate Professorship in 1996 and the Professorship in 2001. He has been a consultant for many projects in the Singapore electronic industry, and was the principal consultant for the design, testing and implementation of the $200 million Electronic Road Pricing (ERP) island-wide project in Singapore, from 1990 to 2001. His current research is on digital and mobile communications, RF IC design, mixed-signal circuits and intelligent transport systems. Before that, he specialsed in sonar designing, biomedical engineering and signal processing. Since 1995, he has been Head of Division of Circuits and Systems, School of EEE, NTU. He is a Fellow of IEE, UK, a Chartered Engineer (UK) and a Professional Engineer (Singapore). Prof. Do is a Chartered Engineer. He is a Fellow of the Institution of Engineering Technology (IET). He was a Council Member of IET, U.K., from 2001to 2004 and an Associate Editor for the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES in 2005 and 2006. Kiat Seng Yeo received his B.E. (Hons) (Elect) in 1993, and Ph.D. (Elect. Eng.) in 1996 both from Nanyang Technological University, Singapore. He is currently the Head of the Division of Circuitsand Systems and a Professor of electrical and electronic engineering with NTU. He is a recognized expert in CMOS technology and low-power CMOS IC design. He gives consulting to multinational corporations. He is the holder of 15 patents. He has published four books (International editions) and more than 250 papers in his areas of expertise. His research interests include device modeling, low-power circuit design, and RFIC design. Prof. Yeo serves in the organizing and program committee of several international conferences as the General Chair, a Cochair, the Technical Chair, etc.