Design of a CMOS Compatible CFOA and its Application in Analog Filtering

Abstract

It is presented the design of a current-feedback operational amplifier (CFOA) which is suitable for implementation with standard CMOS integratedcircuit technology of 0.35µm. The characteristics of the CFOA are derived by using biases of ±2.5V and Ibias=20µA. The usefulness of the CFOA is demonstrated by designing mixed-mode biquads which realize the low-pass, high-pass, band-pass, notch and all-pass filter functions. As a result, it is highlighted that the SPICE simulations of the biquad working in voltage-mode are in good agreement with the biquad working in current-mode.

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Design of a CMOS Compatible CFOA and its Application in
Analog Filtering
A. Peña-Pérez, E. Tlelo-Cuautle, A. Díaz-Méndez, C. Sánchez-López
INAOE - Department of Electronics
{aldopp, etlelo, ajdiaz, csanchez}@inaoep.mx
Abstract
It is presented the design of a current-feedback
operational amplifier (CFOA) which is suitable for
implementation with standard CMOS integrated-
circuit technology of 0.35μm. The characteristics of the
CFOA are derived by using biases of ±2.5V and
Ibias=20μA. The usefulness of the CFOA is
demonstrated by designing mixed-mode biquads which
realize the low-pass, high-pass, band-pass, notch and
all-pass filter functions. As a result, it is highlighted
that the SPICE simulations of the biquad working in
voltage-mode are in good agreement with the biquad
working in current-mode.
1. Introduction
Recently, the current-feedback operational amplifier
(CFOA) has received prominent attention, so that it is
called a versatile four-terminals device which offers an
alternative way to design analog building blocks whose
usefulness allows implementation of current-mode and
voltage-mode circuits [1]-[11]. Most important is that
the CFOA possesses significant advantages over the
traditional voltage-mode op-amp (VOA) [1], mainly its
behavior does not depend on the gain-bandwidth trade-
off, it exhibits excellent high speed performance, very
high slew rate and ease of realizing various functions
with the less possible number of external components.
In fact, some of these advantages have been exploited
in numerous analog applications which include the
realization of filters [3-5], chaotic non-linear circuits
[6], [10], integrators/differentiators [7], sinusoidal
oscillators [8], analog dividers [9], etc.
Although there exist commercially available
CFOAs, such as the AD844 [11], they require high
biases. On the other hand, with the analog designers
preferences for CMOS technology, which is suitable
for low-power low-voltage implementations, there
exist the need to develop novel designs. Furthermore, a
CMOS compatible CFOA is presented herein to show
its advantages in designing mixed-mode analog
filtering applications. In section 2 is described the
proposed CFOA design. Further, the application of the
CFOA to design voltage-mode biquads is given in
section 3. Afterwards, in section 4 the voltage-mode
design is modified to work in current-mode. Finally,
the conclusions are given in section 5.
2. CMOS compatible CFOA
The behavior of the four-terminals CFOA can be
described by (1), while its schematic representation can
be sketched as shown by Fig. 1, where it is appreciated
the cascade connection of a positive second-generation
current conveyor (CCII+) [2], and a voltage follower
(VF) [6].
0
0
1
0
1
0
0
0
0
0
0
1
0
0
0
0
XX
YY
ZZ
WW
V
I
I
V
I
V
V
I
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
=
(1)
Y
X
W
1
CCII+
CFOA
Z
Figure 1. Circuit description of the CFOA
For the CFOA, its inverting and non-inverting
inputs (X, Y) are characterized by a very low and very
high impedance, respectively; while its current and
voltage outputs are delivered by Z and W, respectively.
On the other hand, among all the possible realizations,
in Fig. 2 is shown a design by CMOS integrated-circuit
technology. Furthermore, from this design it can be
appreciated that the non-inverting input (Y) presents a
high impedance. A voltage signal is followed to the
low impedance inverting input terminal (X) via a
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complementary class AB source-follower stage
composed by M1-M4.
As one sees, the inverting and non-inverting inputs
are not bidirectional, as it is in the conventional op-
amp, where both inputs also presents high impedance.
The drain currents of M2 and M4 are sensed by the
cascode current-mirrors (formed by M1ZN-M4ZN and
M1ZP-M4ZP) which transfer the current from terminal
X to the high impedance terminal denoted by Z.
Afterwards, the voltage at the Z-terminal is then
buffered to the output-terminal (W) via the follower
formed by M1b-M4b, which in this design the VF is
identical to the one between terminals Y and X. Note
that the high impedance in Z-terminal is formed by the
output impedance of the cascode current-mirrors and
the input impedance of the VF between Z and W.
By using standard CMOS integrated-circuit
technology of 0.35-μm of AMS processes, the CFOA
shown in Fig. 2 was biased with ±2.5V, Ibias=20μA,
so that the size of the transistors are: L=1.2μm,
W=60μm for all P-Channel MOSFETs, and W=35μm
for all N-Channel. In this manner, the main
characteristics of the designed CFOA are listed in
Table 1.
3. Designing Voltage-Mode Biquads
A popular application of CFOAs has been in
realizing voltage-mode (VM) and current-mode (CM)
biquad filters, and numerous circuits have appeared in
the literature [3-5]. For instance, in [3] a novel
configuration has been presented, which realizes all the
five generic filter functions, namely: low-pass (LP),
band-pass (BP), high-pass (HP), notch and all-pass
(AP). Most important is that all functions are realized
from the same circuit, which also realizes the four
transfer functions, i.e.
transimpedance, and transadmittance functions. The
biquad employs four CFOAs as shown in Fig. 3.
To show the workability of the biquad, the authors
in [3] confirmed the realization of the five filter
functions by making a hardware implementation based
on the commercially available AD844, which requires
higher biases than the ones used to design CMOS
compatible CFOAs. For this reason, the goal of this
paper is to show the usefulness of the CMOS CFOA to
generate the same behavior for the biquad. From Fig.
3, the five transfer functions in voltage-mode are
shown in Table 2.
voltage, current,
CFOA
CFOA
C1
C2
CFOA
CFOA
Y
X
AP
BS
r3 r5
r7
n
r4
R2
r6
R1
r2
r1
m
VIN
VO3
VO2
VO1
VO4
z
W
Y
X
z
W
Y
X
z
W
Y
X
z
W
Figure 3. CFOA-based mixed-mode universal
biquad working in voltage-mode [3]
The four generic filter functions in voltage-mode
(LP, BP, HP and notch) are derived with the switch at
BS and by choosing r2=r3; r4=r5=r6=r7. Thus, the
circuit realizes a LP response at VO1, BP response at
VO2, a HP response at VO3 and notch in VO4. The AP
response is realized when the switch is at the AP
position. The universal biquad shown in Fig. 3 was
designed with the proposed CMOS CFOA for
QO=0.82, fO=1KHz and HO=1; the values of the
passive elements were
R2=2.64M?, r1=r2=…=r7=100k? and C1=C2=80pF.
The frequency responses are shown in Fig. 4, while the
taken as: R1=1.5M?,
YX
M1
M2
M3
M4
M4BNM5BN
M1BNM2BN
M1BPM2BP
M4BP M5BP
Ibias
M3ZN
M1ZN
M4ZN
M2ZN
M1ZP
M3ZP
M2ZP
M4ZP
W
M1b
M2b
M3b
M4b
M5BNb
M2BNb
M2BPb
M5BPb
VDD
VSS
Z
Figure 2. Schematic of the CFOA
Table 1. Simulated and measured results
of the CMOS CFOA shown in Fig. 2
Parameters
Open Loop Gain (dB)
Unity Gain Bandwith (MHz)
Phase Margin (degree)
CMRR (dB)
Impedance at X-Terminal (?)
Impedance at Y-Terminal (?)
Impedance at Z-Terminal (?)
Impedance at W-Terminal (?)
Input Noise Voltage @ 100mHz (μV/?Hz)
Measured results
122
34.6
45.7
41
1.65k?
227.16M?
125.51M?
1.65k?
7.24
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measured characteristics are given in the top part of
Table 4.
4. Designing Current-Mode Biquads
To generate the five generic filter functions in
current-mode, r1 and r7 should be deleted [3].
Therefore, the circuit has a zero input impedance and
ideally infinite output impedance. An input current Iin
is injected into terminal m, and the output current Iout is
taken out from the node n. Without r5 and r6 the
circuits realizes the LP and BP responses, without r6
and r4 the circuit realizes the HP response, with r5 and
r4 open-circuited, the notch response is realized
(considering r2=r4=r6=r0), and finally, the AP
response is generated with r2=r3=r4=r5=r6 , yielding
H0=1.
Table 4. Simulated and measured results of
the biquads
Voltage-Mode
Parameters LP BP
HO(V/V) 0.978 0.965
FO (KHz)
@ 0.7
QO -- 0.823
Current-Mode
HO (I/I) 0.962 0.953
FO (KHz)
@ 0.7
QO -- 0.822
HP
0.953
Notch
0.015
AP
0.929
0.989 0.981 0.989 0.977 --
-- 0.821 --
0.966 0.015 0.941
0.991 0.986 0.955 0.977 --
-- 0.823 --
The universal biquad working in current-mode was
designed with the proposed CMOS CFOA for
QO=0.82, fO=1KHz and HO=1, and by using the same
values for the passive elements. The frequency
Table 2. Universal biquadratic generic functions in voltage-mode
Filter Voltage-Mode
−
=
Low-Pass (LP)
( )(
1
R C
+
( )(
2
R C
+
−
+
( )(
2
s
) ( )
=
)
3
2
12 1 1 2
+
2
3
1 12 1 1 2
)
2
2
0100
2
( )
( )
( )
D s
r
r
rr R C R C
s
r
in
r R C R C
s
+
V
V
H
D s
s
ω
=
Band-Pass (BP)
3
1
1
( )
1
s
0
01 1
3
1 1 2 1 1 22
1
0
02
r
rR CQ
r
in
r R C R C
Hs
V
V
ss
ω
==
High-Pass (HP)
3
3
1 1
r
r
2 1 1 2
+
+
(
1
R C
2
2
2
030
( )
2
1
r
r
r
in
H
RC r RC R C
s
V
V
H s
D s
s
)
==
+
Notch
(
)
3
1
3
2 1 1 2
r RC R C
2
3
1 12 1 1 2
)
+
2
2
22
00
04
1
( )
r
r
in
RCr R C R C
s
s
D s
V
V
s
ω
+
==
+
All-Pass (AP)
( )
( )
D s
(
(
s
)
0
0
3
1
3
1 1
R C
2 1 1 2
r R C R C
r
r R C R C
2
3
1 1 2 1 1 22
222
1
00
04
2
r
r
r
Q
in
Hssss
V
V
s
ω
ω−+−+
=
+
Table 3. Universal biquadratic generic functions in current mode
Filter Current-Mode
ω
=
Low-Pass (LP)
( )(
+
)
r
)
3
41122
3
1112122
1
2
00
2
1
( )
( )
( )
D s
r
r R C R C
s
+
( )(
1
R C
s
( )
1
R C
s
( )(
2
1
R C
+
(
2
s
out
I
in
R C
−
+
r R C R C
IH
D s
s
=
Band-Pass (BP)
3
5
0
011
3
1112122
1
0
2
r
rR C
+
Q
out
I
r
in
r R C R C
sHs
I
s
ω
==
High-Pass (HP)
3
6
3
1112
−
122
2
2
0
( )
2
r
r
out
I
r
in
r R C R C
s
IH s
D s
(
D s
( )
( )
D s
s
)
==
++
Notch
)
3
01122
3
1121
)
122
2
22
1
00
( )
r
r R C R C
+
out
I
r
in
r R C R C
s
s
+
s
Hs
I
ss
(
ω
+
==
All-Pass (AP)
())
0
06 1 1 5
R C r
4 1 1 2
r R C R C
r
r R C R C
2
3
1 12 1 1 22
222
1
r
11
003
1
Q
out
I
in
R C
Hssrs
I
ω
ω−+−
+
+
==
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responses are shown in Fig. 5, while the measured
characteristics are given in the bottom part of Table 4.
Figure 4. SPICE simulation of the frequency
response of the voltage-mode biquad
5. Conclusions
It has been introduced the design of a CMOS
compatible CFOA implemented with integrated-circuit
technology of 0.35μm. The designed CFOA was used
to implement universal biquads working in voltage and
current-mode. Finally, form the SPICE simulation
results it was demonstrated the usefulness of designing
mixed-mode filter functions, which requires only bias
levels of ±2.5V and 20μA, which are lower than by
using the AD844. Most important is that the behavior
of the voltage and current-mode filters are in good
agreement.
6. Acknowledgments
The author is thankful to CONACYT by the support
to make this work by the scholarship No. 190605.
7. References
[1] G. Palumbo and S. Pennisi, “Current-Feedback versus
Voltage Operational Amplifiers”, IEEE Trans. on Circuits
and Systems I, Vol. 48, No. 5, May 2001, pp. 617-623.
[2] A. S. Sedra and K. C. Smith, “A Second Generation
current conveyor and its applications, IEEE Trans. on
Circuits and Systems I, Vol.17, February 1990, pp. 132-135.
[3] V. K. Singh, A. K. Singh, D. R. Bhaskar and R. Senani,
“Novel Mixed-Mode Universal Biquad Configuration”,
IEICE Electronics Express, Vol. 2, No. 22, November 2005,
pp. 548-553.
Figure 5. SPICE simulation of the frequency
response of the current-mode biquad
[4] R.K. Sharma and R. Senani, “On the Realization of
Universal Current Mode Biquads Using a Single CFOA”,
Analog Integrated Circuits and Signal Processing, 41, 2004,
pp. 65-78.
[5] M. Djebbi, A. Asii and M. Sawan, “Design of Monolithic
Tunable CMOS Band-Pass Filter Using Current Feedback
Operational Amplifiers”, Analog Integrated Circuits and
Signal Processing, 45, 2005, pp. 143-154.
[6] E. Tlelo-Cuautle, D. Torres-Munoz, L. Torres-Papaqui
and A. Gaona-Hernández, “Synthesis of CCII+s and
CFOAs by manipulation of VFs and CMs”, IEEE
BMAS, web-publication, September 2005.
[7] J.-L, Lee and S.-I Liu, “Integrator and Differentiator with
Time Constant Multifunction using Current Feedback
Amplifiers”, Electronics Letters, Vol. 37, No. 6, 2001, pp.
331-333.
[8] S. Celma, P.A. Martínez and A. Carlosena, “Current
Feedback Amplifier based Sinusoidal Oscillator”, IEEE
Trans. on Circuits and Systems I, Vol. 41, No. 12, 1994, pp.
906-908.
[9] S.-I. Liu and J.-J. Chen, “Realization of Analogue Divider
using Current Feedback Amplifiers”, IEE Proc.-Circuits
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[10] A. S. Elwakil and M. P. Kennedy, “Improved
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[11] ANALOG DEVICES “60MHz, 2000V/μs Monolithic
CFOA AD844”, Data Sheet.
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