Hardware Reference
In-Depth Information
V
satH
R
1
þ
I
satH
¼
ð
8
:
34
Þ
R
2
V
satL
R
1
þ
I
satL
¼
ð
8
:
35
Þ
R
2
The two threshold voltages can be found by assuming first that v
z
is set to the stable
state V
satH
.To change this stable state, the current i
x
must satisfy the condition
i
x
>
i
z
, which implies:
v
in
v
y
R
s
>
V
satH
R
1
þ
ð
8
:
36
Þ
R
2
Hence, higher threshold voltage (V
TH
) is given by:
R
1
R
s
V
TH
¼
R
2
V
satH
ð
8
:
37
Þ
R
1
þ
Using a similar procedure, it can be determined that the lower threshold voltage
(V
TL
) is given by:
R
1
R
s
V
TL
¼
R
2
V
satL
ð
8
:
38
Þ
R
1
þ
The transfer characteristics of this Schmitt trigger using the above derivation can be
easily deduced to be as shown in Fig.
8.24b
.
This Schmitt trigger leads to a square wave generator when a capacitor is
connected from terminal-X to ground and the resistor R
s
is eliminated and in its
place the internal parasitic resistance of the CCII+ is accounted for. This square
wave generator was proposed by Abuelma
atti and Al-Absi [
17
] and is shown in
'
Fig.
8.25
It is straight forward to determine that the frequency of square wave generated
from this circuit would be given by:
1
2
C
0
R
x
l
n
2
R
R
x
f
¼
ð
8
:
39
Þ
1
A modified two CCII+ based Schmitt trigger/pulse squaring circuit was proposed
by Srinivasulu [
19
] which is shown in Fig.
8.26
.
The operation of this circuit can be explained as follows. Assume that the input
V
in
is positive rising triangular wave changing from
V
in
to +V
in
. Also assume
during this transaction the current in the terminal-X
1
changes from
i
x1
to +i
x1
whereas the Z-port current rises from
i
z1
to +i
z1
. It is further assumed that at node
'
b
'
the current changes from +V
T
/R
3
to
V
T
/R
3
and current at port-
X
2
of CCII+
(2) is rising from +i
x2
to
i
x2
i.e. +V
x2
/R
2
to
V
x2
/R
2
and furthermore the Z-port
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