Circuits and Systems, 2011, 2, 60-64
doi:10.4236/cs.2011.22010 Published Online April 2011 (http://www.SciRP.org/journal/cs)
Voltage/Current-Mode Multifunction Filters Using One
Current Feedback Amplifier and Grounded Capacitors
Jiun-Wei Horng, Chun-Li Hou, Wei-Shyang Huang, Dun-Yih Yang
Department of El ect roni c Engi neeri ng,C hu n g Yuan Christian Universi t y ,C hun g-Li, Taiwan, China
Received November 18, 2010; revised November 22, 2010; accepted November 23, 2010
Abstract
One configuration for realizing voltage-mode multifunction filters and another configuration for realizing
current-mode multifunction filters using current feedback amplifiers (CFAs) are presented. The proposed
voltage-mode circuit exhibit simultaneously lowpass and bandpass filters. The proposed current-mode circuit
exhibit simultaneously lowpass, bandpass and highpass filters. The proposed circuits offer the following
features: No requirements for component matching conditions; low active and passive sensitivities; employ-
ing only grounded capacitors and the ability to obtain multifunction filters from the same circuit configura-
tion.
Keywords: Current Feedback Amplifier, Active Filter, Voltage-Mode, Current-Mode
1. Introduction
The current feedback amplifier (CFA) can provide not
only constant bandwidth independent of closed-loop gain
but also high slew-rate capability. Thus, it is ben eficial to
use a current feedback amplifier as a basic building block
to realize analogue signal processing circuits [1-12].
In 1992 [7], Fabre proposed a voltage-mode bandpass
and highpass filters circuit by using two CFAs, one
grounded capacitor, one floating capacitor and three resis-
tors. In 1993 [8], Fabre proposed another voltage-
mode or current-mode biquads. The voltage-mode biquad
exhibits simultaneously bandpass and highpass filters by
using one CFA, one grounded capacitor, one floating ca-
pacitor and two resistors. The current-mode biquad exhib-
its simultaneously bandpass and highpass filters by using
one CFA, two grounded capacitors and two resistors. Sev-
eral single-CFA voltage-mode biquads were proposed in
[9-11]. However, only one filter function (lowpass, band-
pass or highpass) can be obtained in each realization,
which implies the need to change the circuit topology to
obtain other types of filter functions. Moreover, these sin-
gle-CFA voltage-mode biquads employ floating capacitors.
In 1995 [12], Liu proposed four voltage-mode biquads
with high input impedance for realization lowpass, band-
pass or highpass filters by using two CFAs, two (or three)
capacitors and three (or two) resistors. However, only one
filter function can be obtained in each realization. More-
over, two topologies of Liu’s circuits used floating ca-
pacitors. In 1996 [10], Soliman proposed many volt-
age-mode biquadratic filter circuits. The four two-CFA
biquads in [10] realize lowpass and bandpass filters si-
multaneously and using o nly groun ded capacitors.
In this paper, a new configuration is proposed to real-
ize voltage-mode lowpass and bandpass filters simulta-
neously by using on e CFA, two grounded capacitors and
three resistors. One more filtering signal can be obtained
with respect to the previous single-CFA biquads in [9-11]
and two-CFA biquads in [12]. With respect to the volt-
age-mode biquads in [7-8], the proposed circuit uses only
grounded capacitors. The use of grounded capacitors
makes the proposed circuit attractive for integrated cir-
cuit implementation [13]. With respect to the volt-
age-mode two-CFA lowpass and bandpass biquads in
[10], the proposed circuit uses one less active compo-
nents.
One new configuration is proposed to realize cur-
rent-mode lowpass, bandpass and highpass filters simul-
taneously. One more filtering sig nal can b e obtained with
respect to the previous current-mode biquad in [8]. Criti-
cal component matching conditions are not required in
the design of all proposed circuits.
2. Voltage-Mode Circuit
Using standard notation, the port relations of a CFA can

J.-W. HORNG ET AL. 61
be characterized by
x
y
v v, oz v v,
z
x
i i and 0
y
i . The proposed
voltage-mode circuit is shown in Figure 1. The output
transfer functions of Figure 1 can be expressed as

13
2122 12323
lp
in
VGG
V
s
CCsC GGGGG


(1)

21
2122 12323
bp
in
VsCG
V
s
CCsC GGGGG

(2)
Thus, the circuit realizes an inverting lowpass signal at
Vlp and a non-inverting bandpass signal at Vbp, simulta-
neously. The circuit employs two grounded capacitors,
three resistors and only one CFA. Critical component
matching conditions are not required. Because the output
impedance of the CFA (terminal vo) is very small, the
output terminal of Vlp can be directly connected to the
next stage. The various parameter values of Figure 1 are
given by:
23
12
o
GG
CC
 , 123
1
oGGG
QC
 
 and
123
123 2
1CGG
QGGG C  (3)
The gain constants are

1
2
olp
G
HG  and

1
123
obp
G
HGGG  (4)
One possible design equations for the specified o

and Q can be obtained by
2
12
123 2
9
3o
CCQ
GGG CQ





 (5)
Under the design Equation (5), the gain constants of
Figure 1 become

1
olp
H and

1
3
obp
H (6)
Figure 1. The proposed voltage-mode lowpass and band-
pass filter.
All capacitors are grounded in Figure 1. The use of
grounded capacitors is particularly attractive for inte-
grated circuit implementation [13]. Moreover, the ca-
pacitor C2 in Figure 1 is connected to the z terminals of
the CFA, this design offers another feature of a direct
incorporation of the parasitic compensation capacitance
(Cp) as a part of the main capacitance [14]. Note that,
while cascade the bandpass signal of Figure 1, other
buffering device is needed because the output impedance
of Vbp in Figure 1 is not small.
Taking into account the tracking errors of CFA, namely


vsv  
xy
,


vsv  
oz
and

isi  
zx
, where


s
 and


s
 represent the frequency transfers of
the internal current and voltage followers of the CFA,
respectively, and


s
 represents the frequency transfer
of the output voltage follower of the CFA. They can be
approximated by the first order lowpass functions [8].
Assuming the circuits are working at frequencies much
less than the corner frequencies of

s
 ,


s
 and


s
 , that is, 1 1



 and

11 1
  is the input
voltage tracking error, 2 1



 and

22 1


 is
the output voltage tracking error, 3 1

  and


33 1


 is the current tracking error of a CFA. The
resonance angular frequency o

, bandwidth oQ

and
quality factor Q of Figure 1 become
2311
12
o
GG
CC

 , 123
1
oGGG
QC
 
 and
12311
123 2
1CGG
QGGG C

 (7)
The active and passive sensitivities of this filter are
2311 12
1
2
oo
G,G, ,C,C
SS



, 111 2
1
2
QQ
C, ,C
SS

,
1
1
123
Q
G
G
SGGG
 ,2
2
123
1
2
Q
G
G
SGGG
 ,
3
3
123
1
2
Q
G
G
SGGG  .
All the active and passive sensitivities are no larger
than 1.
3. Current-Mode Circuit
The proposed current- mode circuit is shown in Figure 2.
The output transfer functions of Figure 2 can be ex-
pressed as

34
2122 12323
lp
in
IGG
I
s
CCsC GGGGG


(8)

121
2122 12323
bp
in
IsCG
I
s
CCsC GGGGG
 (9)
R
3
x
y
z
R
1
R
2
Vbp
C2
Vlp
C1
Vin CFA1

J.-W. HORNG ET AL. 62
Figure 2. The first proposed current-mode filter.

223
2122 12323
bp
in
IsCG
I
s
CCsC GGGGG

 (10)

212
2122 12323
hp
in
IsCC
I
s
CCsC GGGGG
 (11)
Thus, the circuit realizes an inverting lowpass signal at
Ilp, a non-inverting bandpass signal at Ibp1, an inverting
bandpass signal at Ibp2 and a non-inverting highpass sig-
nal at Ihp, simultaneously. The resonance angular fre-
quency, o

, bandwidth, oQ

, and quality factor, Q,
have the same values as in Equation (3). The gains of
Figure 2 are
4
2
o( lp )
G
HG  ,

1
1123 obp
G
HGGG ,

3
2123
obp
G
HGGG
  and

1
ohp
H (12)
4. Non-ideal Equivalent Circuit of CFA
The non-ideal equivalent circuit model of the CFA is
shown in Figure 3, where Rx is the x terminal input re-
sistance, Ry//(1/sCy) represents the y terminal parasitic
input impedance, Rp//(1/sCp) represents the parasitic im-
pedance at the compensation terminal z [8]. The typical
data sheet values of the various parasitics for the bipolar
CFAs (such as AD844) are: 50
x
R , 55pF
p
C.

,
3M
p
R

, 2M
y
R

 and 2pF
y
C . When
non-ideal equivalent circuit model of the CFAs are used
instead of ideal ones and assuming the circuits are work-
ing at frequencies much less than the corner frequencies
of


s
 ,


s
 and


s
 , namely, 1

 
,
the voltage transfer functions of Figure 1 become

13
lp
in
VGG'
VDs

 (13)

21 11bp p
in
VsC'GGG
VDs

 (14)
where




21221231 1p
DssCC' sC'GGG'CG








231 123p
GG' GGGG' (15)


33X1221
1R; p G'RC' CC  (16)
From Equations (13) to (16), undesirable factors are
yielded by the effects of CFA’s parasitic impedances. It
is found that such factors can be made negligible by op-
erating the filters in high frequencies. But, if the filters
are used for lower frequencies, the parasitic impedances
could not be negligible. So the characteristics will depart
from the theoretical values, especially for the bandpass
filter signal in Figure 1. Note that the influence of the
parasitic elements on the frequency response of the cur-
rent-mode filter in Figure 2 can be studies by a similar
procedure, as above.
5. Experimental Results
Experiments were carried out to demonstrate the feasi-
bility of the proposed circuits. The CFA was imple-
Figure 3. Non-ideal equivalent circuit of the CFA includes the parasitic impedances.
R
3
x
y
z
R
1
R
2
I
bp1 C2
I
hp
C1
I
in CFA1
I
bp2
R
4
I
lp
)(s

)(s

Cy
R
x
iy
R
y
Cp
ix
R
p
y
z
x
)(s

ix
o

J.-W. HORNG ET AL. 63
(a)
(b)
Figure 4. Experimental frequency responses of Figure 1 design with C1 = C2 = 100 pF and R1 = R2 = R3 = 10 k. (a) Lowpass
filter (Vlp), (b) bandpass filter (Vbp).
mented using one AD844. Figure 4 (a) and (b) represent the frequency responses for the lowpass and bandpass
10
3
10
4
10
5
10
6
–
40
–
35
–
30
–
25
–
20
–
15
–
10
–
5
0
5
0
90
180
Phase, de
g
Fre
q
uenc
y
, Hz
Gain, dB
Exp.
Phase
Gain
Theo.
-.-.-.- x x x
o o o
10
3
10
4
10
5
10
6
–
45
–
40
–
35
–
30
–
25
–
20
–
15
–
10
–
5
–
90
0
90
Phase, deg
Frequency, Hz
Gain, dB
Exp.
Phase
Gain
Theo.
-.-.-.- x x x
o o o

J.-W. HORNG ET AL. 64
filters of Figure 1, respectively, designed with C1 = C2 =
100 pF and R1 = R2 = R3 = 10 k. Experimental results
confirm the theoretical analysis.
6. Conclusions
In this paper, a configuration for realizing voltage-mode
multifunction filters and a configuration for realizing
current-mode multifunction filters using CFAs are pre-
sented. The proposed voltage-mode circuit exhibits si-
multaneously lowpass and bandpass filters by using one
CFA, two grounded capacitors and three resistors. The
proposed current-mode circuit exhibit simultaneously
lowpass, bandpass and highpass filters by using one CFA,
two grounded capacitors and four resistors. The proposed
circuits have no requirements for component matching
conditions. The active and passive sensitivities are low.
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