J. of Active and Passive Electronic Devices, Vol. 1, pp. 289-294
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All Pass / Notch Filters Using Operational
Amplifier and Current Conveyors
K. PAL*
Department of Earthquake Engineering, Indian Institute of Technology,
Roorkee-247 667, India
In this paper two circuits have been reported which realise all-pass and
notch filters. The first circuit uses one operational amplifier and one CFA
used as current conveyor. The second circuit employs three current feedback amplifiers. Both the circuits use grounded capacitors and are suitable for IC implementation.
Keywords: Active filters; All-pass filters; Notch filters; Current conveyor
1. INTRODUCTION
The use of current conveyors in the realisation of various filter function
has been given wide attention in the literature [1-6]. There are number of
active elements termed as current conveyors viz, first generation current
conveyor (CCI) [7], the second generation current conveyor CCII [8] and
differential voltage current conveyor DVCCII [9]. Out of these elements the
CCII has proved its superiority over all other elements in realisation of
filters, oscillators and inductance simulation. The current feedback
amplified (CFA) is obtained from CCII just by adding a unity gain buffer to
its ‘z’ terminal. The CFA is now available in IC form as AD 844 [10]. In this
*Corresponding Author: E-mail: kiratfeq@iitr.ernet.in
289
290
PAL
paper two circuits have been proposed which use above mentioned active
elements and realises all-pass / notch filters using grounded capacitors
suitable for integration [11].
2. CIRCUITS DESCRIPTION
Consider the circuit shown in fig.1 Its voltage transfer function is given by
 Z (a + 1) 
V0
= a 1 - 2

Vi
a 
 Z1
(1)
R1
Vi
Z1
aR 1
y
CFA w
z
x
Z2
−
+
Vo
FIGURE 1
The general configuration.
The circuit realises a notch filter for a = 1,
Z1 = R +
1
sC
&
Z2 =
R
1 + sCR
and the voltage transfer function is given by
(i,e RIIC)
ALL PASS / NOTCH FILTERS USING OPERATIONAL AMPLIFIER
V0
1 + s 2R 2C2
=
Vi 1 + s 2 R 2 C 2 + 2sCR
291
(2)
The actual circuit is shown in fig.2
R1
Vi
R
y
CFA w
z
x
C
C R
aR 1
−
+
Vo
FIGURE 2
The circuit realizing an all pass / notch filter.
If a is taken as 1/3 the eqn. (1) becomes
V0 1 1 + s 2 R 2 C 2 − 2sCR
= .
Vi
3 1 + s 2 R 2 C 2 + 2sCR
(3)
The eqn (3) represents an all-pass function but with reduced gain. The
circuit is suitable where the system has controllable gain (amplifier) so that
attenuation due to all-pass filter is nullified.
The second circuit is shown in fig.3 Its voltage transfer function is given by
Vo 1 + s 2 R 2 C 2 + sCR (2 − 0.5a )
=
Vi
1 + s 2 R 2 C 2 + 2sCR
(4)
292
PAL
y
CFA w
z
x
Vi
V0
aR 1
y
CFA w
z
x
R
C
R
y
CFA
z
x
C
w
R1
FIGURE 3
Another Notch Filter / All pass configuration.
if a =4
V0
1 + s 2R 2C2
=
Vi 1 + s 2 R 2 C 2 + 2sCR
(5)
which represents a notch filter.
If a =8
V0 1 + s 2 R 2 C 2 − 2sCR
=
Vi 1 + s 2 R 2 C 2 + 2sCR
(6)
which represents an all-pass function.
The above proposed circuits have the following advantages:
(i) high input impedance (for circuit fig.3 it can be kept high by choosing
high value of R1).
(ii) low output impedance
ALL PASS / NOTCH FILTERS USING OPERATIONAL AMPLIFIER
293
(iii) easy tuning through resistance R’s as both resistances are of equal
values.
(iv) suitability for IC implementation as they use grounded capacitors.
As compared to recent circuits [12], the circuit of fig.3 use IC AD 844
and hence works in high frequency range and retains all the advantages of
the circuits reported in reference [12].
3. EXPERIMENTAL RESULTS
The circuit shown in fig.2 was tested for following component values
R = 10 KΩ, C = 0.01 µf, R1 = 100 KΩ, a = 1 for notch filter and a = 1/3
for all-pass filter. The experimental results were in close agreement with
theoretical one’s. Another circuit shown in fig.3 was also tested for
following component values:
R = 1.0 KΩ,
C = 0.001 µf,
R1 = 1.0 KΩ,
a = 4 for notch filter
a = 8 for all-pass filter
The results were very satisfactory in the frequency range of 5KHz to 500
KHz. In the case of notch filter, notch was obtained at 160 KHz.
In both the experiments AD-844 was used as CCII and IC-3140 was used
as the operational amplifier.
4. CONCLUSIONS
Two circuits of all-pass/notch filters have been reported. Both uses
grounded capacitors and are suitable for integration. The use of AD 844 can
realise the circuit which can operate in high frequency range.
REFERENCES
[1]
Soliman, M.A.(1996), New inverting non-inverting band pass and low pass biquad circuit
using current conveyors. International journal of Electronics, 81, 577-583.
294
PAL
[2]
Alami, M. and Fabre , A. (1991), Insensitive current-mode band pass filter implemented
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[3]
Pal, K. (1981). Realisation of current conveyor all-pass networks. International Journal of
Electronics,50,165-168.
[4]
Pal, K. and Singh, R. (1982). Inductorless current conveyor all-pass filter using grounded
capacitors. Electronics Letters, 18 (1),47.
[5]
Pal,K. (1991). All-pass networks using current conveyors. Microelectronics Journal, 22
(4),53-56.
[6]
Higashimura, M. (1991). Realisation of voltage-mode biquads using CCII+ Electronics
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[7]
Sedra, A., and Smith, K.C.,(1968),The current conveyor: a new circuit building block
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[8]
Sedra, A., and Smith, K.C., (1970), A second generation current conveyor and its
applications, I EEE transactions on circuit theory, 17,132-134.
[9]
Pal, K. (1989). Modified current conveyors and their applications Microelectronics
Journal, 20 (4), pp37-40.
[10] Svoboda, J.A. McGORY, L. and Webb, S. (1991). Applications of a commercially
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Electronics Letters 3, 148-149.
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