A 2V CMOS Capacitorless Current-Tunable All-Pass Filter using Current Mirrors

Transcription

1 Thammasat Int. J. Sc. Tech., Vl.6, N.l, January-April 2001 A 2V CMOS Capacitrless Current-Tunable All-Pass Filter using Current Mirrrs Banlue Srisuchinwng and Adisrn Leelasantitham Electrical Engineering Prgram, Sirindhrn Internatinal Institute f Technlgy, Thammasat University, P.O.Bx 22,Thammasat Rangsit Pst Office, Patumthani l2l2l,thailand, Tel : (+662) , Fax: (+662) ,9869l13, banlue@siit.tu.ac.th Abstract A 2V CMOS capacitrless current-tunable all-pass filter using current mirrrs is presented thrugh the use f the MOS internal capacitances. The frequency f where the magnitude and phase shift f the transfer functin are apprximately 0 db and -90, respectively, is tunable thrugh the bias current. The maximum useful f is in excess f 700MHz depending n the internal parameters. l. Intrductin All-pass filters are utilised in many applicatins such as in sinusidal quadrature scillatrs. Mst techniques f integrable allpass filters emply relatively large external capacitrs and therefre the perating frequencies are nt relatively high tl-4] Recently, capacitrless techniques have explited nt nly the paracitic capacitances f BJTs fr high-frequency all-pass filters [5] r integratrs [6], but als the internal capacitances f MOS fr high-frequency bandpass filters [7] r switched-current circuits [8]. In this paper, a 2V high-frequency CMOS capacitrless current-tunable all-pass filter using current mirrrs is presented thrugh the use f the MOS internal capacitances. The frequency I where the magnitude and phase shift f the transfer functin are apprximately 0 db and -90", respectively, is tunable thrugh the bias current. The maximum useful f" is in excess f 700 MHz depending n the internal parameters. 2. CircuitDescriptins Figure I shws the circuit f the CMOS capacitrless current-tunable all-pass filter. The circuit cnsists f current mirrrs frmed by five NMOS transistrs Ql t Q5 and three current surces l,2l and 4I. Ql t Q3 are identical and the aspect ratis are WL. The aspect ratis f Q4 and Q5 are separated int tw cases. In the first case, the ratis are 16W/L and 32WlL respectively. In the secnd case, the ratis are 2WlL and 4W/L, respectively. The small signal input current i,n is applied t the drain f Ql and the resulting small signal utput current i is taken frm the drain f Q5. 3. Ideal Analysis Fr a MOS transistr, the unity current gain frequency {D1 is equal t g./c where C : (Cg, + Cga + Cr6) [9] and g. : (2ly(Vcs - Vr). Cg,, Cea and Ceu ate the MOS internal capacitances. The subscripts g, s, d and b stand fr gate, surce, drain and bdy f a MOS transistr. V6s is the gate surce vltage. V1 is the usual threshld vltage and I is the bias current. Refening t Figure I, the small signal current i* yields. l ' l. st (1) where the time cnstant Tu: CJE^t, Cu: (Cr, + Cgu)r + (Cg, + Cga + Ceu)z + (Cg, + C*6 + Cgu)r and g.1 : QD l(vcs' - V1). The subscripts 1 t 3 refer t Ql t Q3. Similarly, the small signal current i, is equal t i.. : 2i., l +srb Q) 46

2 Thammasat Int. J. Sc. Tech., Vl.6, N. l, January-April 200 where the time cnstant \: CJg,nc, C5 = (Cr' + Cru)a + (Cg, + Cga + Cru)s and g'r -- QD /(Vcs- Vr). The subscripts 4 and 5 refer t Q4 and Q5. In additin, i, : i + i*. Therefre, the transfer functin T(s) = ijir" yields T(s) = (l-sr6) (l + sr )(1 + s16 ) (3) Fr sinusidal input, the magnitude M and phase shift $ fequatin (3) are fthe frm It can be seen frm (7) and (8) that < and r1 are tunable thrugh the bias current I and hence the name "current-tunable". 4. Simulatin Results The perfrmance f the circuit shwn in Figure I has been simulated using SPICE and all transistrs are in saturatin mde. The CMOS transistrs are mdeled by Alcatel Mietec 0.5 pm CMOS C05MD Technlgy frm EUROPRACTICE. The width W and length L are equal t 0.5 pm. The supply vltage Vcc: 2 V and V1 : 0.68 V. Table I shws the internal capacitances f Ql t Q5. Table 2 shws values f tu and 16 at I : 20 1t"4. M = -20lg l+ 12t2 (4) Table l.intemal capacitances f Ql t Q5 Q = tan-t (-an )- tan-t (an )- tan-t (rt ) (5) Equatin (3) can be cnsidered in tw cases. In the first case fr r"((rb, equatin (3) is reduced t a first-rder all-pass filter f the frm Ratis c". (F.) c"" (F.) c"h (F.) Ql, w/l lxl0-'' I x l0-'' 3 x l0-r6 02. C)3 Q4 2WU- 2x l0-'' 2x l0-16 6xl0' Q5 4WtL 4x l0-'' 4x I 0-r6 l2xl0' Q4 l6wl 6x l0-r5 l6x l0-48x l0' Q5 32WtL 32x 10-'' 32x l 0-96x I 0' (1-sr^) TG) =;- -i (r+srr, (6) Table2. Values f tu and t6 at I : 20 1tA. Ratis currenl (rra) Vcs (v) T^ ( I 0-' 's) X6 ( I 0-r's) In this case, the magnitude and phase shift f equatin (6) are 0 db and -90, respectively, at the frequency : cdr fr rr = l/tu : E^ql Ca: QD I [(Vcs- Vr) Cb ] (7) In the secnd case fr r,71x6, equatin (3) may be apprximated as an all-pass filter where M described in (4) appraches 0 db at { = -90 and at r : J = hrr fr (8) Q1, w/l r:20 l. t W/t- t:20 l Q5 4W/L 2t:40 l.0l Q4 l6w/l t: WlL 2t= Let the aspect ratis f Q4 and Q5 be 16W/L and 32WlL, respectively, and represent an apprximated example f the all-pass filters described in the first case where tu << t6. In this case, h:0.81 where M and $ are -0.1 db and -90, respectively. Fr such a case, Figure 2 depicts f": al(2n) and M at O : -90" versus I using dtted and slid lines fr the expected and simulated results, respectively. On the ther hand, let the aspect rati f Q4 and Q5 be 2WL and 4W/L, respectively, and represent an example f the apprximated all-pass filter described in the secnd case 4'7

3 Thammasat Int. J. Sc. Tech., Vl.6, N.1, January-April 2001 where r,< 16. In this case, h: 0.64 where M and Q are {.7 db and -90, respectively. Fr such a case, Figure 3 shws f, -- J(2n) and M at $ : -90" versus I using dtted and slid lines fr the expected and simulated results, respectively. It can be seen frm Figures 2 and 3 that the erpected and simulated results are in resnably gd fits. Figure 4 shws the magnitude (db) and phase shift (degrees) f (6) and (3) versus frequency (Hz) at bias current I:20 pa and 50 pa fr the first case (tu<<t6) and the secnd case (t"<t6), respectively. It can be seen frm Figure 4 that, the frequency (f") at phase shift f -90' fr the first case and the secnd case are apprximately 700 MHz and 2 GHz, respectively, whilst the crrespnding magnitude are apprximately -0. I db and -0.7 db, respectively. 5. Discussins It can be seen frm Figures 2 and 3 that f f the first case (tu<<t6) is lwer than f f the secnd case (tu<t5), and the maximum useful f are apprximately 700 MHz and 2 GHz, respectively. On the ther hand, M f the first case is better than M fthe secnd case but may be further amplified by a wide band amplifier fr further use. Fr applicatins at high frequencies, the secnd case (tr<t6) can be cnsidered mre attractive than the first case (tu<<ru) in terms f higher value f I if tu f the tw ' cases are the same. Under small-signal peratin and ideal current surces, the input and utput impedances f the circuit shwn in Figure I are Zin= (l/g,') / (1+st") and Z,, = rj2, respectively, where r is a finite resistance f MOS between drain and surce ccurring frm the effect f channel-length mdulatin. The input impedance is a typical value f apprximately llg^t at lw frequency. The utput impedance can be imprved thrugh the use f cnventinal cascde techniques with an expense f lwer f" due t additinal internal capacitances. The ideal current surces shwn in Figure i can be realized thrugh practical PMOS current surces but the f and magnitude (db) at -90 degrees versus I may slightly be decreased because f additinal capacitance C6, frm a PMOS transistr. Hwever. the surce is cnnected t ihe bdy and therefre Ca, = Cau where Ca5 can usually be neglected [10]. 6. Cnclusins A 2V CMOS capacitrless current-tunable all-pass filter using current mirrrs has been presented thrugh the use f the MOS internal capacitances. The frequency f where the magnitude and phase shift f the transfer functin are apprximately 0. db and -90, respectively, is tunable thrugh the bias current. The maximum useful f is in excess f 700 MHz depending n the internal parameters. 7. Acknwledgements The authrs are grateful t The Ryal Glden Jubilee PhD Preram fr the financial supprts. 8. References tl] Pkaiyaudm, S. and Samtrut, K., Current-Mirrr Phase-Shifter Oscillatr Electrn. Lett., vl. 23, n.l, January, pp. 2t-23,198'7. l2l Bult, K. and Walling, A., A CMOS Analg Cntinuus-Time Delay Line with Adaptive Delay-Time Cntrl, IEEE J. f Slid-State Circuits, vl. 23, n. 3, June, pp , t3l Higashimura, M., Current-Mde Allpass Filter Using FTFN with Grunded Capacitr, Electrn. Lett., vl.27, n. 13, June, pp. I , t4l Cmer, D.T., Cmer, D.J. and Gnzales, J.R., A High-Frequency Integrable Bandpass Filter Cnfiguratin, IEEE Transactins n Circuits and Systems-Il: Analg and Digital Signal Prcessing, vl. 44, n.l0, Octber, pp , t5l Pkaiyaudm, S. and Mahattanakul, J., A 3.3-Vlt High-Frequency Capacitrless Electrnically-Tunable Lg-Dmain Oscillatr, Prceedings f the 1995 IEEE Internatinal Sympsium n Circuits and Systems, ISCAS'95, vl.2, pp , [6] Tang, J. D. v.d. and Kasperkvitz, D., A GHz Mnlithic Quadrature Mixer Oscillatr fr Direct-Cnversin Satellite Receivers, Prceedings f the IEEE Intematinal Slid-State Circuits Cnference, pp ,

4 Thammasat Int. J. Sc. Tech., Vl.6, N.1, January-April 2001 UI t8l Nagasawa, T. Hyg, A. and Sekine, K,, A 2V-500MHz CMOS Cntinuus-Time Current-Mde Filter using the Gate-Surce Capacitance f the MSFET, ISPACS'99, pp , Tumazu, C., Hughes, J.B. and Battersby, N.C. (ed.) Switched-Currents an Analgue Technique fr Digital Technlgy, IEE Circuits and Systems Series 5, Shrt Run Press, Ltd., Exeter, UK, t9l Sedra, A.S. and Smith, K.C., Micrelectrnic Circuits, Oxfrd University Press, p. 445,1998. [10] Baker, R. J., Li, H.W. and Byce, D.E., CMOS Circuit Design, Layut and Simulatin, IEEE Press, U.S.A., p. 174, lc 4I Figure I : Schematic diagram f the CMOS capacitrless current-tunable all-pass filter. 49

5 Thammasat Int. J. Sc. Tech., Vl.6, N. l, January-April 2001 N >, <, q) t) Simulated Frcqucncy Expected Magnitude fh ao 1'l ce t! { ) v ; q. E v E G I 10.0E{ Current I (A) Figure 2 : Frequency f (Hz) and magnitude M (db) at 0 = -90" versus I where the aspect ratis f Q4 and Q5 are, fr the first case (r, << 16), l6wl and 32WtL, respectively..:- N I I Simulatcd Frcquency Expected Magnitude!'l + E gi h0 ) vtt! d - l. E r tr61 bl) Figure 3 : Frequency f (Hz) and magnitude M (db) at 0 = -90" versus I where the aspect ratis f Q4 and Q5 are, fr the secnd case (r" < 16), 2W1L and 4WlL, respectively. 50

6 Thammasat Int. J. Sc. Tech., Vl.6, N.1, January-April 2001 IO bn a c-15 h bi ra -_- --- H I80 le+00? le+008 Frequency (If:) f i/ i;" Figure 4 : Magnitude (db) and phase shift (degrees) f (6) and (3) versus frequency (Hz) at = 20 ua and 50 ua fr the first case (r, << t5) and the secnd case (ra < rb), respectively. le+ lcl 5l