which is independent of the closed loop gain and its high slew rate, which is typically around 2000 Vm S- 1. Recently, MOS-C ®lters using the CFOA have been ...
INT. J. ELECTRONICS,
1998,
VOL.
84,
NO.
5, 479± 485
Novel MOS -C balanced-input balanced-output ® lter using the current feedback operational ampli® er SOLIMAN A. MAHMOUD² and AHMED M. SOLIMAN² A new bandpass-lowpass ® lter using a current feedback op.-amp. (CFOA) and MOS transistors operating in the non-saturation region is presented. The ® lter employs six CFOAs and realizes balanced-input balanced-output bandpasslowpass responses with independent control of Q. PSice simulation results for the proposed balanced-input balanced-output MOS-C ® lter are also given.
1.
Introduction
The current feedback operational ampli® er (CFOA) is a very versatile building block, and is now commercially available from several manufacturers (Analog Devices 1990, Evans 1988). The CFOA has the advantages of constant bandwidth, which is independent of the closed loop gain and its high slew rate, which is typically around 2000 V m S- 1 . Recently, MOS-C ® lters using the CFOA have been introduced in the literature (Chen et al. 1995). It is well known that the balanced-input, balanced-output MOS-C ® lters using conventional op.-amps su er from the ® nite gain bandwidth of the op.-amps (Ismail 1985, Ismail et al. 1989, Sakurai et al. 1992, Banu and Tsividis 1983 and Tsividis et al. 1986). In this paper, a novel MOS-C balanced-input balanced-output ® lter using the CFOA is introduced. The proposed ® lter structure is based on the MOS-C-CFOA integrator (Soliman 1998) and realizes a bandpass-lowpass response with independent control of Q. Simulation results using PSpice of the MOS-C-CFOA ® lter circuit are given in section 3. The CFOA used in the simulations is realized by the cascaded connection of a MOS second generation current conveyor introduced by Elwan and Soliman (1996) and a MOS voltage bu er (Manetakis and Toumazou 1996).
2.
The M OS -C-CFO A bandpa ss-lowpass ® lter
Figure 1(a) shows the basic MOS-CFOA lossless integrator whose output voltage is given by Vo =
1 Vi SCR1
( 1)
where R1 is the equivalent resistance of the MOS transistor M1, which is operating in the non-saturation region. The current through M1 is linearized by applying V i and - V i to its drain and its source. The magnitude of the resistor R1 is given by R1 =
1 2K1 ( V G1 - V T )
( 2)
Received 14 March 1997; accepted 8 August 1997. Electronics and Communications Engineering Department, Faculty of Engineering, Cairo University, Egypt. ²
0020± 7217/98 $12.00
Ñ
1998 Taylor & Francis Ltd.
480
S. A. Mahmoud and A. M. Soliman
(a)
(b) Figure 1. (a) The MOS-CFOA lossless integrator; (b) the MOS-CFOA lossy integrator.
where K1 is the transconductance parameter of the transistor M1 and is given by K1 = m Cox
( ) W1 L1
where W 1 / L 1 is the transistor aspect ratio; Cox is the gate oxide capacitance per unit area; m is the electron mobility; and V T is the threshold voltage (assumed equal for all MOS transistors).
Novel MOS-C balanced-input balanced-output ® lter
481
By adding M2 to the circuit, a lossy integrator can be obtained as shown in ® gure 1(b) with an output voltage given by 1 R1 C ( 3) Vo = V 1 i S+ R2 C The magnitude of R2 is given by R2 =
1 2K2 ( V G2 - V T )
( 4)
Since the integrator is a basic building block in realizing continuous time ® lters, the MOS-CFOA integrator is used to implement a continuous time bandpass-lowpass ® lter, with balanced input and balanced output, as shown in ® gure 2. The ® lter circuit is a cascaded connection of a lossy integrator, a lossless integrator and a voltage-to-current converter. The transfer functions of the ® lter are given by S V BP R1 C1 = Vi D( s) 1 V LP R3 R4 C1 C2 = Vi D( s)
( 5)
( 6)
where 2
D( s) = S +
1 1 S+ R2 C1 R3 R4 C1 C2
Figure 2. The MOS-C-CFOA bandpass-lowpass ® lter.
( 7)
482
S. A. Mahmoud and A. M. Soliman
.MODEL NMOS NMOS LEVEL = 2 LD = 0.225112U TOX = 405.000001E-10 NSUB = 2.256421E + 15 VTO = 0.77227 KP = 4.954000E-05 GAMMA = 1.0151 PHI = 0.6 UO = 581 UEXP = 0.217142 UCRIT = 115146 DELTA = 1.360440 VMAX = 68535.3 XJ = 0.250000U NFS = 2.85E + 12 NEFF = 1 NSS = 1.000000E + 10 TPG = 1.000000 RSH = 27.020000 CGDO = 2.873845E-10 CGSO = 2.880845E-10 CGBO= 3.840832E-10 CJ = 4.100000E-04 MJ= 0.4650 CJSW = 4.803300E-10 MJSW= 0.351 PB = 0.800000 .MODEL PMOS PMOS LEVEL = 2 LD = 0.177432U TOX = 405.000001E-10 NSUB = 3.956006E + 15 VTO= -0.74058 KP= 2.526000E-05 GAMMA = 0.4251 PHI = 0.6 UO = 299.253 UEXP = 0.1933 UCRIT = 5462.67 DELTA = 0.91285 VMAX = 29720.9 XJ = 0.250000U NFS = 1.00E + 11 NEFF = 1 NSS = 1.000000E + 10 TPG = - 1.000000 RSH = 107.40000 CGDO = 2.262940E-10 CGSO = 2.268940E-10 CGBO = 3.471103E-10 CJ= 1.898000E-04 MJ= 0.439556 CJSW = 2.267600E-10 MJSW = 0.207266 PB = 0.700000 Table 1. Model parameters set for 2 m m CMOS technology (obtained through MOSIS)
Figure 3.
x
o
=
and Ri =
The CMOS CFOA circuit.
1
( R3 R4 C1 C2
, ) 1 /2
1 Ki ( V Gi
-
V T)
Q = R2
(
C1 R3 R4 C2
( 8)
f or ( i = 1, 2, 3, 4)
To simplify the design, let R1 = R3 = R4 = R, as a result x Q=
)
1 /2
( )
R2 C1 R C2
1 /2
( 9) o
ê ê ê ê 1ê ê C ê ê ê 2ê ê and = 1 / RÏ C
Therefore, Q can be controlled independently without a ecting x R2 through the gate control voltage V G2 .
o
by programming
Novel MOS-C balanced-input balanced-output ® lter
483
(a)
(b) Figure 4.
(a) The magnitude responses of the balanced output lowpass ® lter; (b) the phase responses of the balanced output lowpass ® lter.
484
S. A. Mahmoud and A. M. Soliman Aspect ratio ( W m m / L m m)
MOS transistor M1 M2 M3 M4
Lowpass Fig. 4
Bandpass Fig. 5
2/3 2/3 2/3 2/3
2/3 2/30 2/3 2/3
Table 2.
(a)
(b) Figure 5.
(a) The magnitude responses of the balanced output bandpass ® lter; (b) the phase responses of the balanced output bandpass ® lter.
Novel MOS-C balanced-input balanced-output ® lter 3.
485
S imulation results
The PSpice simulation results of the ® lter in ® gure 2 using the model parameters listed in table 1 and CMOS CFOA shown in ® gure 3 which is realized by the cascaded connection of a second generation current conveyor introduced by Elwan and Soliman (1996) followed by a voltage bu er (Manetakis and Toumazou 1996). Figures 4(a) and 4(b) represent the magnitude and phase characteristics of the lowpass ® lter designed to realize a maximally ¯ at magnitude response by taking R1 = R2 = R3 = R4 = 1. 125 kV through the aspect ratios in table 2 and C2 = 2C1 = 0. 2 nF. The simulation results of the bandpass characteristics with C2 = 2C1 = 0. 2 nF, R1 = R3 = R4 = R = 1. 125 kV and R2 = 10R = 11. 25 kV to provide Q = 5Ï ê2ê ê are shown in ® gures 5(a) and 5(b) indicating both the magnitude and phase of the balanced output bandpass ® lter. R eferences A nalog Devices, 1990, L inear Products Data Book (Norwood, MA: Analog Devices). Banu, M., and Tsividis, Y., 1983, Fully integrated active RC ® lter in MOS technology. IEEE Journal of Solid-state Circuits, 18 , 664± 671. Chen, J. J.,Tsao, H.W., Liu, S. I., and Chui, W., 1995, Parastic-capacitance-insensitive current mode ® lter using operational transresistance ampli® ers. IEE Proceedings on Circuits Devices and Systems, 142 , 186± 192. Elwan, H. O., and Soliman, A. M., 1996, A novel CMOS current conveyor realization with an electronically tunable current mode ® lter suitable for VLSI. IEEE Transactions on Circuits and Systems-II, 43 , 663± 670. Evans, S., 1988, Current Feedback Op-amp Applications Circuit Guide, (Fort Collins, CO: Complinear Corporation), 11.20± 11.26. Manetakis, K., and Toumaz ou, C., 1996, Current-feedback opamp suitable for CMOS VLSI technology. Electronics L etters, 32, 1090± 1092. Isma il, M., 1985, A new MOSFET capacitor integrator. IEEE Transactions on Circuits and Systems, 32 , 1194± 1196. Isma il, M., Smith, S.V., and Beale, R. G., 1989, A new MOSFET-C universal ® lter structure for VLSI. IEEE Journal of Solid State Circuits, 23 , 183± 194. Sakurai, S., Ismail, M., Michel, J. M., Sanchez -Sinencio, E., and Brannen, R., 1992, A MOSFET-C ® lter variable equalizer circuit with an chip automatic tuning. IEEE Journal of Solid-state Circuits, 27 , 927± 934. Soliman, A. M., 1998, A new MOS-C integrator using current feedback operational ampli® er. Electronic Engineering, submitted for publication. Tsividis, Y., Banu, M., and Khoury, J., 1986, Continuous time MOSFET ® lters in VLSI. IEEE Journal of Solid-state Circuits, 21 , 15± 31.
