Constant-Power CMOS LC Oscillators Usin Hih-Q Active Inductors JYH-NENG YANG, 2, MING-JEUI WU 2, ZEN-CHI HU 2, TERNG-REN HSU, AND CHEN-YI LEE. Department of Electronics Enineerin and Institute of Electronics National Chiao Tun University Enineerin 4 th Buildin, 00 Ta-Hsueh Road, Hsin-Chu 300 TAIWAN 2. Department of Electronics Enineerin Min Hsin University of Science and Technoloy. Hsin-Hsin Rd., Hsin-Fon, Hsin-Chu 304 TAIWAN Abstract: - CMOS LC oscillators usin hih-q active inductors with constant-power consumption are presented. The active inductor with one feedback resistor results in a ain-boostin factor to improve the Q-value and the inductance (L) of the active inductor. Based on this hih-q active inductor, a neative resistance LC-tuned oscillator with low constant-power consumption and wide tunin-rane can be achieved. Simulation results show that the improved active inductor can achieve the maximum Q-value of E8 and dissipate 2.5mw power consumption under 2.5 V power supply. The die size included boundin pad is about 853um 43um. In the wide-tunin frequency rane oscillator, low constant power consumption at 2.5 V power supply around 0mW is observed. This oscillator also depicts a reasonable phase noise performance of -9dBc/Hz at 600 khz. The phase noise from 0.8GHz to 3GHz tunin-rane is less than 6dBc/Hz phase noise deviation. The small die size included boundin pad is about 965um 482um. Key-Words: - CMOS, Active Inductor, Q-value, LC Oscillator, Phase Noise, and Power Consumption. Introduction The quarter-micron or below complementary metal oxide semiconductor (CMOS) technoloy has found an attractive alternative to GaAs and BiCMOS technoloies for implementation of interated radio frequency (RF) transceivers due to the lower cost and the possibility for interation of RF front-end and diital circuits on the same chip. Amon the CMOS RF front-end blocks, the oscillator circuit is the major challene for implementin a fully interated transceiver. To achieve a hih-performance oscillator, low power consumption, wide tunin-rane, and strinent phase noise should be simultaneously considered in the RF circuit applications [, 2]. However, a low Q-value passive spiral inductor in fabricatin CMOS process will seriously limit the wide tunin-rane and the phase noise. The lower Q-value spiral inductor will limit the wide tunin-rane [3] and result in lower phase noise [4]. Althouh the low power consumption is one of the most considerations of the subjects in RF applications, the constant power consumption, which is independent of the frequency, is also a sinificant factor that improves the noise production of the circuit. Recently, wide tunin-rane rin oscillators in diital circuit and low phase noise LC oscillators usin passive spiral inductors have been demonstrated [5]. On the other hand, CMOS active inductors have been applied in microwave/rf amplifier and oscillator desins to achieve hih power ain, wide tunin-rane and save chip area [6-8]. The rin oscillator circuits can achieve wide tunin-rane, but the larer phase noise exits in rin oscillator circuit [9]. The LC-tuned oscillator circuits can achieve lower phase noise, but the low Q-value passive inductor limits the frequency tunin-rane [0]. In addition, the oscillators usin active inductors have wide tunin-rane and small chip area, but the lare phase noise is also encountered due to the Q-value of the active inductors is not hih enouh [-2]. Also, the power consumption of these oscillators will be sensitively influenced by the frequency tunin as well, resultin in extra circuit noise in different operatin frequency. Therefore, it is desired to desin an oscillator usin hih-q active inductor to improve wide tunin-rane, reasonable phase noise, and small chip area. Besides, constant power
consumption and low deviation of phase noise in taret wide frequency band can be achieved. In this paper, we propose a CMOS wide tunin-rane LC oscillator usin hih-q active inductors. The active inductor, simpler than the circuit of [], results in improvin both Q-value and inductance (L) of the active inductor. Based on this inductor, a neative resistance LC-tuned oscillator with wide tunin-rane, reasonable phase noise, constant power consumption, and low deviation of the phase noise has been desined. These results are carried out in Ailent-ADS simulator usin TSMC 0.25um CMOS process model at 2.5V. 2 Improvin Hih-Q Active Inductor Desin Based on the yrator theory [2], the simple rounded active inductor circuit and its equivalent circuit are shown in Fi. (a) and (b). Each MOS transistor is modeled by the equivalent device components includin m, ds, C s, and C d. Assumin m >> ds and C s, >> C d, then the equivalent input impedance (Z in ) of this circuit can be derived as below. The component values of the inductor are also expressed from Eq. (2) to Eq. (5). Vdd I M2 ds Rs (3) m m2 L C s2 (4) m 2 C C s (5) where mi, dsi, and C si are the transconductance, output conductance, and ate-source capacitance of correspondence transistors, respectively. From Eq. (2), the increasin parallel conductance loss of G will reduce the Q-value of the active inductor. Therefore, in order to improve the performance such as the Q-value and the inductance (L), we propose hih-q active inductors with a feedback resistor. The improved hih-q active inductor circuit is illustrated in Fi. 2 (a). This circuit is composed of common source transistor M, common drain transistor M 2, feedback resistor R f and two biasin current sources I and I 2. Feedback resistor R f and transistor M construct a ain network. This network produces a ain factor to reduce the parallel conductance (G) in such a way that the internal loss of the inductor will be decreased, and then the Q value is increased. Therefore, the inductance (L) is also increased due to the feedback resistor. At hih frequency, this circuit is equivalent to a lossy resonator as well, which is shown in Fi. 2 (b). The values of each component includin three parameters, C si, dsi, and mi, are derived from Eq. (6) to Eq. (9). M I2 Zin C G RS L Fi. Simple active inductor and equivalent circuit Z in ( ds2 + ) + S( Cs2 + Cd + Cd 2 ) () ( SC + + )( S( C + C ) + ) d 2 ds2 ds2 s2 d m2 G + (2) Fi. 2 Hih-Q active inductor and equivalent circuit G + (6) ds2 + R f ds
ds Rs (7) m m2 L C ( + R ) s2 f ds (8) m m2 Fiure 4 and 5 indicate that in the rane of 0.5GHz to 2.5GHz, the maximum Q-value is around E8 and the inductance chanes from 5nH to 7.5nH. The Q-value and the inductance of the active inductor with feedback resistor are hiher than that of the one without it. C C s (9) m m2 ω 0 (0) Cs Cs2( + Rf ds ) C () ( R ) m2 s2 Q + 2 f ds ds Cs From Eq. (6), the effect of the factor, (+R f ds ), is desined to be a value reater than unity. This factor will result in the equivalent conductance loss to be minimized, as well as an increase of the equivalent inductance by (+R f ds ) factor. The result of scatterin parameter (S) performance of the inductor is illustrated in Fi. 3. This fiure can be treated as the curve followin the increase of the feedback resistance R f between 0.5GHz and 2.5GHz. Fi. 4 Q-value of the proposed active inductor circuit Fi. 5 Inductance of the proposed active inductor circuit Fi. 3 Microwave performance of the proposed hih-q active inductor (S) Fis. 4, 5, and 6 show the Q-value, the inductance, and the equivalent loss comparisons between the active inductor with feedback resistor R f and the one without it. Fi. 6 Equivalent loss of the proposed active inductor circuit Fi. 6 shows the minimum equivalent loss of a proposed active inductor with feedback resistor is.2e-8ω and it is much smaller than that of the one without the feedback resistor between the frequency ranes of 0.5GHz and 2.5GHz. Consequently, the
active inductor has shown a sinificant improvement. The power consumption is only about 2.5mW under 2.5 V supply voltae, and there is lesser power consumption in this active inductor. Furthermore, in this active inductor, the external bias voltaes are used to tune the characteristics of the active inductor due to the variation in the circuit implementation. Therefore, it can be achieved the performances that independent of the process variation. The comparisons between improved and oriinal at.5ghz is shown in TABLE I. Table Comparisons between improved and oriinal @.5GHz Oriinal Improved Q-Value.4 E8 Loss (Ω) 8.2E-8 Inductance (nh) 2 5.8 Power Consumption (mw) 3 2.5 the Q-value is shown in Fi. 8. The Q-value is increased with the increasin feedback resistance. In Fi. 9, accordin to Eq. (8), the inductance of the inductor is also increased by the increasin feedback resistance R f in the rane of 0.8GHz to 3GHz. As a result, the performances of the proposed active inductor containin the Q value and the inductance may be tremendously improved with a simple loss compensation network. The layout of the proposed active inductor is showed in Fi. 0. The die size included boundin pad is 853um 43um. Because the dc current does not pass throuh the feedback resistance R f, the voltae drop of the feedback resistance is zero, and then the voltae will not be chaned when the resistance R f is varied. Consequently, power consumption of the improved active inductor can be retained constant. The movin trend of this curve inclines to the outside of the circle, indicatin that the loss is decreased, but the Q-value is increased. Fi. 7 shows the curves of the S, which the curves are move outside of the circle as the feedback resistor is increased. Fi. 8 Q value of hih-q active inductor in different resistance Fi. 7 Microwave performance of hih-q active inductor (S) in different resistance The Q-value at self-resonant frequency ω 0 (Eq. (0)) can be written as Eq. (). Therefore, the Q-value is promoted with the feedback resistance R f + R f factor and the trend variation of ds by ( ) Fi. 9 Inductance of hih-q active inductor in different resistance Therefore, the unchaned power consumption characteristic can be applied to desin a constant power consumption oscillator circuit with wide tunin-rane.
Fi. 0 Layout of the proposed active inductor 3 Oscillator Circuit Desin The chief desin considerations of the oscillator are to obtain a low constant-power consumption, wide tunin-rane and low phase noise. The circuit diaram of the proposed oscillator shown in Fi. has a cross-coupled connection of NMOS transistors M NR and M NL to form a positive feedback loop for providin neative resistance, called neative impedance converter (NIC) to compensate the loss of the active inductor in the LC tank. Two improved hih-q active inductors depicted in Fi. 2 replace the conventional inductors of the LC tank. Throuh these active inductors a superior oscillator, bein composed of M R, M L, M 2R, M 2L, M SR, M SL, M PR, M PL, R fr and R fl, can be completely desined. These active inductors are behaved as the equivalent inductance in this oscillator. Because the oscillator circuit is symmetric and the Q value of the active inductor is hih enouh, all transistors only have the same minimum dimension, where the lenth and the width of each MOSFET are 0.24um and 40um, respectively. No varactors are employed in this oscillator; the oscillator frequency modulation function will be constrained. To provide an adjustable frequency rane, the feedback resistance R f of the active inductor is added to tune the desired oscillator frequency. Thouh the capacitance is kept unchaned, the equivalent inductance values are deviated by the resistance R f. Thus, the output frequency of the oscillator will only be adjusted by R f. Assume the total equivalent capacitance from the output node of the NIC is C T, then the output oscillatin frequency can be expressed as: Fi. Proposed LC voltae controlled oscillator m2 ω 0 = (2) C + C )[ C ( + R )] ( s T s2 f ds Fi. 2 The frequency of output v.s. feedback resistance R f From Eq. (2), the frequency ω0 is the inverse proportion of the feedback resistance R f. In other words, when the feedback resistance is decreased, then the frequency will be increased, and vice versa. The output frequency shows a wide tunin frequency rane. The result between the output frequency and the feedback resistance is iven in Fi. 2. This fiure points out the frequency has wide tunin-rane, from 0.8GHz to 3GHz. Althouh the wide tunin frequency rane is achieved, the power consumption is still constant.
size included boundin pad is only 965um 482um. Fi. 3 Oscillator power consumption v.s. tunin frequency Fi. 5 Phase noise v.s. tunin frequency Fi. 4 Output amplitude v.s. tunin frequency Fi. 3 shows that the constant power consumption of 0mW is constantly maintained in this rane, 0.8GHz to 3GHz. The relationship between the output amplitude and the output frequency is appeared in Fi. 4. It indicates that the variation of the output amplitude is about 5dBm durin the wide tunin-rane. Besides, the phase noise in the wide tunin-rane is exposed in Fi. 5. It explores that the variation of the phase noise durin 0.8GHz to 3GHz is around 6dBc/Hz and the phase noise almost retains a constant value between.5ghz and 3GHz. Moreover, the oscillator has the reasonable phase noise below -9dBc even thouh the active inductors have the hiher noise than the passive counterparts. Finally, this proposed circuit exhibits wider tunin-rane, constant power consumption, and reasonable phase-noise. Fi. 6 is showed the layout of the oscillator. The small die 4 Conclusion A CMOS LC oscillator usin an improved hih Q-value active inductor is proposed. Only usin a feedback resistor R f, a tunable hih-q active inductor can be achieved. The power consumption of the oscillator can be retained as a constant value in a wide frequency tunin rane. The phase noise depicts the uninfluenced value even thouh the active inductor has larer noise than the passive inductor counterparts. These simulation results of the proposed circuit are better than those of earlier publications [0-2]. Therefore, this study shows that the proposed circuit can achieve constant power consumption, wide frequency tunin-rane and reasonable phase noise to fit RF requirements. The comparisons between this work and other works are shown in TABLE II. Fi. 6 Layout of the oscillator Table 2 Comparisons between this work and other works
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