#### New Universal Filter Using Two Current-Feedback Amplifiers

International Journal of
NEW UNIVERSAL FILTER USING TWO CURRENT-FEEDBACK AMPLIFIERS
MUHAMMAD TAHER ABUELMA' ATTI 0
HUSAIN ABDULLAH AL-ZAHER 0
0 King Fahd University of Petroleum and Minerals , Box 203 Dhahran 31261
A new universal voltage-mode second-order filter circuit is presented. The circuit has three inputs and one low-impedance output and can realize all the standard filter functions; lowpass, highpass, bandpass, notch, and allpass, without changing the passive elements. The proposed circuit uses only five passive components and enjoys independent control of the natural frequency and the bandwidth, orthogonal control of the natural frequency and the quality factor, as well as low active and passive sensitivities.
INTRODUCTION
At present there is a growing interest in designing active filters using
current-feedback amplifiers (CFAs) [1-7]. This is attributed to the
extended operating bandwidths and relatively large values of slew rates
compared to the conventional realizations using the voltage-feedback
amplifiers. In [2], a voltage-mode highpass and bandpass filter using two CFAs is
proposed and in [3] a voltage-mode bandpass and highpass/lowpass filters
using one CFA are proposed. Both circuits are with one input and two
outputs. In [4], a voltage-mode highpass/bandpass/lowpass filter using one
CFA, but with different passive components, is proposed. In
mode notch, lowpass, and bandpass filter using three CFAs and five
passive components; namely, three floating resistors and two grounded
ca*Corresponding author.
pacitors, is proposed. The circuit has one input and three low-impedance
outputs. In [5] and [7], two universal filters using two CFAs and six
passive elements each; namely four resistors and two capacitors, are
presented. Each circuit has three inputs and one low impedance output and
uses the same passive components for all realizations. In [6], a high input
impedance filter is presented. The circuit can be configured to realize
either a bandpass or a lowpass or a highpass filter function. The circuit
uses five passive elements; namely, three (or two) capacitors and two (or
three) resistors depending on the required filter function. The three filter
functions cannot be realized without changing the locations of the passive
components used.
The major intention of this paper is to present a new universal filter
using CFAs. The proposed circuit enjoys the following advantages:
a. it uses two CFAs.
b. it uses five passive elements only; namely, two capacitors and three
resistors.
c. it can realize all the filter functions; namely, lowpass, highpass,
bandpass, notch, and allpass, without changing the topology or the number
of resistors and capacitors used.
d. it enjoys independent tuning of the natural frequency, too, and the
bandwidth, Oo/Qo.
e. it enjoys orthogonal tuning of the natural frequency, oo, and the
quality factor, Qo.
f. it has one low-impedance output and can, therefore, be easily
cascaded to produce higher order filters.
PROPOSED CIRCUIT
The proposed circuit is shown in Fig. 1. Assuming ideal current-feedback
amplifiers with x, iy O, v vy, and v v routine analysis shows
that the transfer function of the circuit of Fig. can be expressed as
Vo
FIGURE
Proposed universal filter.
V3
V3
V2
0
From (
1
), one can see that:
(i) a second-order lowpass filter can be obtained with VoFV1 if V2
0 (grounded)
(ii) a second-order bandpass filter can be obtained with Vo/V2 if V
0 (grounded)
(iii) a second-order highpass filter can be obtained with Vo/V if V
0 (grounded)
(iv) a second-order notch filter can be obtained with Vo/Vi,p.t if V
(grounded) and VI V
(v) a second-order allpass filter can be obtained with VjVi,p.t if V
V2 Vi,p.t and R4
The proposed circuit is attractive for realizing the lowpass filter
function, because the two capacitors are grounded. The circuit requires the
minimum number of active and passive components with no requirement
for match components except for the allpass filter realization. Moreover,
the impedance of the single output is very small, so the proposed circuit
can be easily cascaded to produce higher order filter realizations. From
(
1
), the parameters (D2o and (Do/Qo can be expressed as
(D2
1/C2C3R1R5
(
2
)
and
Qo
1/C3R4
From (
2
) and (
3
), one can see that the parameter coo can be adjusted by
tuning C2 and/or R and/or R5 without disturbing the parameter coo/Qo.
Also, the parameter co/Qo can be adjusted by tuning R4 without disturbing
the parameter co(/Qo. Thus, the parameters coo and Coo/Qo are
independently adjustable. From (
2
) and (
3
), Qo can be expressed as
Qo
R4(C3/C2R1R5) 1/2
From (
2
) and (
4
), one can easily see that the parameters coo and Qo can be
orthogonally adjusted.
Taking into consideration the nonidealities of current-feedback
amplifiers, namely, otix, ot q, Il 1, denotes the input voltage
tracking error, vx [3Vy, [3 2, Iq121 denotes the input voltage
tracking error, and v /v, / q3, ]q31 denotes the output
voltage tracking error, the transfer function (
1
) becomes
(
3
)
(
4
)
(
5
)
(
6
)
(
7
)
Vo
(otlot2"Yl)G1GsV
(ot2"Yl)sC2GsV2 / 02"2s2C2C3V3
sC2C3 + c2/zsC2G4 + (lO213//2)GG
From (
5
), the parameters CO2o and co/Qo can be expressed as
2
COo- tlt2l]l12/C2C3R1R5
(!)
Qo
ot2]2/C3R4
It is easy to show that the passive and active sensitivities of the parameters
co and co(/Qo can be expressed as
and
and
1/2, qCt3%,/QR,,
_ot2,]
So3‘’
So3/Q
0
Thus, all the active and passive sensitivities are no more than unity.
The proposed circuit was tested experimentally and the results obtained
from lowpass, highpass, bandpass, and notch filter realizations using the
AD844 current-feedback amplifier are shown in Fig. 2. The experimental
results are in good agreement with the theory presented.
_1
1_
IK
100K 1N
Frequency, Hz
b highpass characteristic obtained with R R 2K, R 1K, C2
FIGURE 2 Measured characteristics of the universal filter of Fig.
,
4M
c bandpass characteristic obtained with R
1K, R
10K, R.s 2K, C
C
1M
Frequency, Hz
2M
4M
d Notch characteristic obtained with R
1K, R4
4K, R
2K, C
C
A new minimum component universal filter using two current-feedback
amplifiers, two capacitors, and three resistors has been presented. The new
circuit uses less number of passive components and enjoys all the
following attractive features: independent control of the natural frequency and
the bandwidth, orthogonal control of the natural frequency and the quality
factor, low active and passive sensitivities, low output impedance, and the
same passive components are used to realize all the standard filter
functions; lowpass, highpass, bandpass, notch, and allpass with no component
matching requirement except for the allpass realization.
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