Hardware Reference
In-Depth Information
reducing the number of building blocks considerably by incorporating a circuit
technique in which one of the inputs is added to the output node directly.
The summation of one of the input current signal (say I 3 ) on the output node
itself raises an important question and implies the (hidden) requirement of one more
active element to make the circuit
which is generally not been recog-
nized by the concerned authors who have reported earlier several biquad filter
circuits employing this kind of summing of an input at the output node.
Consider now the following:
'
practical
'
(i) Looking into the node from where the output I out is being taken, the output
impedance should be very high (ideally infinite); since input I 3 is also applied
at the same node, the input impedance looking into the same node (as seen by
I 3 ) should be low (ideally zero). Obviously the same node cannot exhibit these
two absolutely contradictory requirements of impedance levels.
(ii) The above mentioned anomalous situation can be corrected if input I 3 is added
to the output node through a current follower (CF) exhibiting low input
impedance and high output impedance. By doing this, I 3 will see a low input
impedance (that of this additional CF)) and at the output terminal now, one has
the effective output impedance equal to the parallel combination of the output
impedance of the active building block employed and the output impedance of
this additional CF which can be still considered to be high if the employed
building block and CF both are designed to have very high output impedances.
(iii) The addition of the
input current
from a current source and the
output
'
'
'
current
of the building block considered, as employed in a number of circuits
earlier for instance see [ 80 , 93 , 99 , 101 , 107 , 116 , 120 , 123 , 131 ], might work
in SPICE simulations but the circuits of the quoted type has the flaw and
anomalous situation pointed out in (i) above. In practice, such an addition
should require one more active element (a CF) thereby increasing the total
count of active building blocks employed by
'
'
one
'
.
addition of an input current at the
output node ' , in the circuits quoted above and reported earlier
In the opinion of the present authors, the type
'
' misleadingly '
conceals this fact.
Such mixed-mode circuits have been developed around CCCIIs also and several
varieties have so far been developed for instance, see [ 30 , 102 , 117 , 132 ]. In this
section, we include one such mixed-mode universal biquad configuration which
uses no more than four CCCIIs and can realize various filtering functions in VM as
well as in CM (Fig. 9.33 ).
From straight forward analysis of this circuit it is found that the various current
and voltage outputs of this circuit are given by the following equations:
N 1 s
ðÞ
Ds
N 2 s
ðÞ
Ds
V in 1
V out 2
sC 2 R x 2 þ
1
sC 2 I in 2
V out 1 ¼
, V out 2 ¼
, V out 3 ¼
ð
9
:
46
Þ
ðÞ
ðÞ
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