New Simple CMOS Realization of Voltage Differencing Transconductance Amplifier and Its RF Filter Application

Transcription

1 63 A. YESIL, F. KACAR, H. KUNTMAN, NEM SIMPLE CMOS REALIZATION OF OLTAGE DIFFERENCG... New Simple CMOS Realization of oltae Differencin Transconductance Amplifier and Its RF Filter Application Abdullah YEŞİL 1, Fırat KAÇAR 1, Hakan KUNTMAN 1 Dept. of Electrical and Electronics Enineerin, University of Istanbul, Istanbul, Turkey Dept. of Electronics and Communication Enineerin, Istanbul Technical University, Istanbul, Turkey yesilabdullah@hotmail.com, fkacar@istanbul.edu.tr, kuntman@itu.edu.tr Abstract. The voltae differencin transconductance amplifier (DTA) is a recently introduced active element for analo sinal processin. However, the realization of DTA is not iven by any author yet. In this work, a new and simple CMOS realization of DTA is presented. The proposed block has two voltae inputs and two kinds of current output, so it is functional for voltae- and transconductance-mode operation. Furthermore, DTA exhibits two different values of transconductance so that there is no need to external resistors for DTA based applications which seems to be a ood advantae for analo circuit desiner. A CMOS implementation of DTA and a voltaemode DTA based filter are proposed and simulated. An application example of fourth order flat-band band-pass amplifier is iven and the performance of the circuit is demonstrated by comparin the theory and simulation. Keywords oltae differencin transconductance amplifier, voltae mode filter, CMOS interated circuit. 1. Introduction Active elements are widely used in analo sinal proressin, such as filters, oscillators and inductance simulators etc. Many active elements were and are bein proposed [1] [13] because there are important features relative to each other, such as input and/or output terminal can be different/same features, any parameter can be controlled by external voltae or current etc. New desins are provided for researchers with these new proposed active elements. In [14] the present-day active elements are reviewed and several new elements are introduced. However, the realizations of the newly introduced active elements are not iven. One of these active elements is DTA, which can be compared with CDTA, a previously introduced active element [9]. In CDTA, differential input current (I p, I n ) flows over the Z terminal. The voltae drop at the terminal Z is transferred to current at the terminal X by a transconductance ain. Several CDTA applications are iven in the literature [15] []. In DTA, differential input voltae ( P, N ) is transferred to current at the terminal Z by first transconductance ain and the voltae drop at the terminal Z is transferred to current at the terminals X+ and X- (neative of X+) by second transconductance ain. Both transconductance are electronically controllable by external bias currents. Compared to other active blocks [1] [8], the advantaeous feature of DTA is that this new element exhibits two different values of transconductances so that several applications such as biquad filters, oscillator, inductance and FDNR (frequency dependent neative resistor) simulator can be realized with a sinle active block employin one or two capacitors. Another important feature, this block can be used easily at transconductance mode applications owin to input terminals is voltae and output terminals is current. In this paper, a new CMOS implementation of DTA is iven. The performance of the proposed circuit is tested with an application example of voltae-mode filter. By selectin input terminal voltaes, this construction can enerate the standard filter functions for voltae-mode. The proposed circuit employin minimum number of passive and active components uses no external resistors. Furthermore, no parameter matchin condition is required.. Circuit Description The circuit symbol of the proposed active element, DTA, is shown in Fi. 1, where P and N are input terminals and Z, X+ and X- are output terminals. All terminals exhibit hih impedance values. Usin standard notation, the terminals relationship of an ideal DTA can be characterized by: I. (1) Z m1 m1 P I X m N I X m Z

2 RADIOENGEERG, OL., NO. 3, SEPTEMBER Fi. 1. The circuit symbol of the DTA. Consequently, the above describin-equations, the input stae and output stae can be simply implemented by floatin current sources. Accordin to input terminals, an output current at Z terminal is enerated. The intermediate voltae of Z terminal is converted to output currents. The new CMOS realization of the DTA is shown in Fi.. The introduced circuit employs two Arbel-Goldminz transconductances [1]. Input and output transconductance parameters of DTA element in the circuit are determined by the transconductance of outputs transistors. It can be approximated as m1 3 4 or m 5 8, (a) (b) m 6 7 where i is the transconductance value of i th transistor defined by W i IBi. i. COX L, i µ i is (i = n, p) the mobility of the carrier for NMOS (n) and PMOS (p) transistors, C OX is the ate-oxide capacitance per unit area, W is the effective channel width, L is the effective channel lenth and I Bi is bias current of i th transistor. The realization of voltae-mode filter with dual-inputs dual-outputs is shown in Fi. 3. This proposed circuit employin a sinle DTA and only two capacitors realizes LP, BP and HP filter. Circuit analysis yields voltae-mode biquad transfer functions shown in (3) (6). If 1 = and =, then OUT1 sc1m1 BP, (3) sc 1 T 1 m m1 m LP If = and 1 =, then HP BP OUT m1 m 1 T sc1m m1m OUT1 1 1 T sc1m m1m sc OUT m 1 T sc1m m1m. (4), (5). (6) The natural frequency ω and quality factor Q can be iven as follows; m1 m, (7) CC 1 T Q C T m1. (8) C1 m The passive sensitivities of natural frequency and quality factor for the proposed circuit of Fi. 3 are iven as; 1 S S S S, (9) m 1 m C1 C T Q Q Q Q 1 S S S S. (1) m 1 m C T C1 Fi.. CMOS implementation of DTA. 3. Application Examples The DTA is very flexible in different mode filter realizations. The advantaes obtained are that the values of transconductances are adjusted by bias currents to realize these circuits without any requirement of external resistors. Fi. 3. Application of proposed DTA. The non-ideal effects of the CMOS DTA include input parasitic capacitances and output parasitic conductances. Parasitic capacitances (C P ) appear parallel at P, N and Z terminals, while parasitic conductances (G P ) appear parallel at Z, X+ and X- terminals. The parasitic impedances belon to DTA based filter are indicated in Fi. 4.

3 634 A. YESIL, F. KACAR, H. KUNTMAN, NEM SIMPLE CMOS REALIZATION OF OLTAGE DIFFERENCG... DTA is a m based active element. Therefore, temperature dependence of DTA is related to temperature dependence of m W m n COX GS T. (18) L Dependin upon the absolute manitude of T, the approximate absolute chane in T is.4 m/ o C. The temperature dependence of mobility is Fi. 4. Parasitic impedances of DTA affectin filter operation. Considerin the above parasitic impedances, the transfer functions analyses lead to the followin result: If 1 = and =, then sm1 C1 C OUT1 P1 m1gp1 BP, (11) s If = and 1 =, then HP LP s OUT1 OUT m1 m s C C1 CP1 scgp1 s. (1), (13) OUT scm BP (14) s where Δs is the characteristic equation and is iven by 1 P1 P P P1 1 P1 m P s s C C C C s C C G C C G G G m1 m P1 m P (15) where C P1 = C PN, C P = C PZ, G P1 = G PX+ and G P = G PZ + G PX- are consisted of parasitic impedances. The parameter natural frequency ω and quality factor Q of the filter with effects of parasitic impedances are calculated as m1 m P1 m P C C C C = G G 1 P1 P, (16) T ( T) ( T ) T 1.5. (19) The threshold voltae decreases with the temperature in order.4 %, but the mobility decreases with the temperature in order 1.5 %. Obviously, mobility decreasin is dominant on the m and m decreases with the increasin temperature. 4. Simulation Results We perform the simulations by usin LTSPICE proram with TSMC CMOS.18 μm process parameters. The aspect ratios of the transistors are iven in Tab. 1. Supply voltaes are taken as DD = SS =.9 and I B1 = I B = I B3 = I B4 = 15 µa biasin currents are used. Simulation results show that this choice yields transconductance values of DTA as m1 = m = µa/ and the parasitic capacitance at the Z terminal is specified as C p =.15 pf. The DC transfer characteristic of I X+ and I X- aainst Z for output stae of proposed DTA is shown in Fi. 5. The DC transfer characteristic of input stae of DTA is the same as I X+ of output stae. Transistors W(µm) L(µm) M 1, M, M 5, M M 3, M 4, M 7, M Tab. 1. Transistors aspect ratios for the DTA. Q= C1 CP1C CP m1m GP1 m GP. (17) C C G C C G P P1 1 P1 m P It is observed from equations (16) and (17) that parasitic capacitances and conductances increase effective capacitances and conductances respectively so values of ω and Q decrease. It should be mentioned that some parameters such as output conductances at Z, X+ and X- terminals of the DTA can be improved by usin improved FCS stae [] in its CMOS structure instead of simple FCS stae. In this way, output parasitic conductances can be easily inored. Fi. 5. The DC transfer characteristic of the DTA.

4 RADIOENGEERG, OL., NO. 3, SEPTEMBER The proposed circuit in Fi. 3 is simulated with the followin passive element values C 1 = 1.13 pf and C =.863 pf, a total capacitance of C T = 1.13pF at Z terminal which results in quality factor of Q = 1 and 1 MHz center frequency. Fi. 6 illustrates the simulated results of LP, BP and HP of voltae-mode ain-frequency responses by selectin input voltae terminals. Gain (db) Band-pass filter for in=1 Hih-pass filter Low-pass filter Band-pass filter for in= Ideal filters Hz 1 8 Hz 1 9 Hz Frequency Fi. 6. Gain-frequency responses of voltae-mode LP, BP and HP sinals. Baseband active filters should be low-pass networks for zero IF and band-pass networks for IF or superheterodyne applications. Fi. 7 shows the block diaram of a typical multi-step superheterodyne receiver with a diital back-end. As the fiure shows, a sequence of filter operations is used to convert the desired sinal from radio frequency (RF), typically in the HF (3 MHz to 3 MHz) rane, down to one or more intermediate frequencies (IFs) and finally down to baseband, where the sinal is diitized by an ADC [3]. For the above typical superheterodyne receiver, a band-pass filter is realized with double tuned amplifiers [4]. Bandwidth B = / of this band-pass filter is equal to the diameter of the pole circle. The transfer function of the example application illustrated in Fi. 8 is iven by OUT p1 p s s Qp 1 Qp K s s s s p1 p p1 p Qp 1 Qp () where K is the ain factor, p1, p, Q p1 and Q p are the pole frequencies and the quality factors of the proposed second order band-pass filter iven by (7) (8), respectively. The pole frequencies are determined to slihtly different frequencies, which can be approximated by p 1 B fp 1 f sin45, (1) f B f. () p p sin 45 To obtain a filter with a center frequency 1 MHz and bandwidth of 8 MHz, the pole frequencies and quality factors of the proposed circuits are chosen as f p1 = MHz, f p = MHz and Q p1 = Q p = 1, respectively. The transconductance and capacitance values providin these properties are determined as m1,,3,4 = µa/ and C 1 =.789 pf, C =.639 pf, C 3 = 1.41 pf, C 4 = 1.6 pf. The frequency responses of the amplifier obtained from theory and from LTSPICE simulation are shown in Fi. 9. The center frequency and the bandwidth of the desined amplifier are determined with LTSPICE simulation as f = MHz and B = 8.54 MHz respectively. Dependence of the center frequency of the double tuned amplifier on biasin currents is iven in Fi. 1 by chanin the biasin currents I B1,,3,4 between 5 and 36 µa. Capacitors with the above values of their capacitances were used. -1 Fi. 7. A typical superheterodyne receiver. - Gain (db) order BPF First stae BPF Second stae BPF Ideal BPFs Fi. 8. Double tuned amplifier s realization usin proposed circuit. the 1 7 Hz 1 8 Hz 1 9 Hz Frequency Fi. 9. Ideal and simulation frequency responses of the desined double tuned amplifier.

5 636 A. YESIL, F. KACAR, H. KUNTMAN, NEM SIMPLE CMOS REALIZATION OF OLTAGE DIFFERENCG... To test the input dynamic rane of the double tuned amplifier, a sinusoidal input sinal of f = 1 MHz is applied to the input of the band-pass filter and the output waveform is obtained by simulations. Fi. 11 shows that the input dynamic rane of the filter response extends up to amplitude of m (peak to peak) without sinificant distortion. The THD results for the double tuned amplifier are iven in Fi. 1, which clearly shows that for an input sinal lower than 5 m (p-p), the THD remains in acceptable limits i.e. 5 % thus confirmin the practical utility of the proposed circuit. oltae Fi. 1. Dependence of center frequency on biasin currents (I Bs ). 1m 5m m -5m -1 m out in ns ns 4ns 6ns 8ns 1ns Time Fi. 11. Time domain response of the circuit in Fi. 3 for m (p-p) 1 MHz sine wave for voltae-mode band-pass filter confiuration. THD(%) 6 4 m m 4m 6m (peak to peak) Fi. 1. Dependence of output voltae harmonic distortion on input voltae amplitude of the double toned amplifier. 5. Conclusion A new voltae differencin transconductance amplifier, DTA is presented. A new and simple CMOS realization of this element is iven. In contrary to simple realization topoloy employin a few numbers of transistors the proposed circuit exhibits ood performance. Due to these properties, frequency response of this topoloy for the DTA is suitable for hih frequency applications. Besides, since supply voltaes are ±.9, the circuit is suitable for low voltae applications. An application example of a voltae-mode filter employin the proposed CMOS DTA realization has been presented. In this application of DTA, disadvantae caused by external resistors was avoided, since the proposed filter is constructed employin only a sinle active element, DTA, and two capacitors, where the value of transconductances can be adjusted by biasin currents. Simulation results that are simulated usin LTSPICE confirm the theoretical results. Acknowledments This work was supported by Istanbul University Research Fund with the project code The authors would like to thank Istanbul University Research Fund for this financial support. References [1] SEDRA, A. S., SMITH, K. C. A second eneration current conveyor and its application. IEEE Transactions on Circuit Theory,197, vol. 17, no. 1, p [] FABRE, A. Third eneration current conveyor: A new helpful active element. Electronics Letters, 1995, vol. 31, no. 5, p [3] CHIU, W., LIU, S. I., TSAO, H. W., CHEN, J. J. CMOS differential difference current conveyors and their applications. IEE Proceedins Circuits, Devices and Systems, 1996, vol. 143, no., p [4] ELWAN, H. O., SOLIMAN, A. M. Novel CMOS differential voltae current conveyor and its applications. IEE Proceedins Circuits, Devices and Systems, 1997, vol. 144, no. 3, p [5] KAEWDANG, K, KUMWACHARA, K., SURAKAM- PONTORN, W. Electronically tunable floatin CMOS resistor

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Seventh-order elliptic video filter with.1 db pass band ripple employin CMOS CDTAs. AEU: International Journal of Electronics and Communications, 7, vol. 61, no. 5, p [1] ARBEL, A. F., GOLDMZ, L. Output stae for current-mode feedback amplifiers, theory and applications. Analo Interated Circuits and Sinal Procesin, 199, vol., no. 3, p [] ALTUN, M., KUNTMAN, H. Desin of a fully differential current mode operational amplifier with improved input-output impedances and its filter applications. AEU: International Journal of Electronics and Communications, 8, vol. 6, no. 3, p [3] STEYEART, M., ROERMUND, A., HUIJSG, J. Analo Circuit Desin. Berlin: Spriner, 6. [4] OZCAN, S., KUNTMAN, H., CICEKOGLU, O. Realization of inductorless RF bandpass amplifiers usin immittance simulators employin CCIIs. In Proceedins of the 1 th International Conference on Microelectronics ICM'98. Monastir (Tunisia), 1998, p About Authors... Abdullah YEŞİL was born in Istanbul, Turkey, in He is M.Sc. student. He received the B.Sc. deree from Istanbul University in Electrical and Electronic Enineerin in 9. His main research interests are active network synthesis and electronic circuits for computer-aided desin. Fırat KAÇAR received his B.Sc., M.Sc. and Ph.D. derees from Istanbul University, all in Electrical and Electronics Enineerin in 1998, 1 and 5 respectively. He is currently an Assistant Professor at the Electrical and Electronics Enineerin Department of Istanbul University. His current research interests include analo circuits, active filters, synthetic inductors, CMOS based circuits electronic device modelin and hot-carrier effect on MOS transistor. He is the author or co-author of about 5 papers published in scientific journals or conference proceedins. Hakan KUNTMAN received his B.Sc., M.Sc. and Ph.D. derees from Istanbul Technical University in 1974, 1977 and 198, respectively. In 1974, he joined the Electronics and Communication Enineerin Department of Istanbul Technical University. Since 1993, he is a professor of electronics in the same department. His research interests include desin of electronic circuits, modelin of electron devices and electronic systems, active filters, desin of analo IC topoloies. Dr. Kuntman has authored many publications on modelin and simulation of electron devices and electronic circuits for computer-aided desin, analo LSI desin and active circuit desin. He is the author or the coauthor of 93 journal papers published or accepted for publishin in international journals, 149 conference papers presented or accepted for presentation in international conferences, 148 Turkish conference papers presented in national conferences and 1 books related to the above mentioned areas. Furthermore, he advised and completed the work of 9 Ph.D. students and 38 M.Sc. students. Currently, he acts as the advisor of 5 Ph.D. students. Dr. Kuntman is a member of the Chamber of Turkish Electrical Enineers (EMO).