Hardware Reference
In-Depth Information
1
T
1
¼
V
C
2
R
3
C
1
f
0
¼
ð
:
Þ
15
36
ð
ð
R
1
=
R
2
Þ
Þ
15.6.2 Switch-Controllable Bi-stable Multivibrator
Chien in [
13
] demonstrated that a simple switch-controllable bistable multivibrator
with dual hysteresis modes can be made from only a single DVCC and two external
resistors along with a double-pole double-throw (DPDT) switch. This circuit is
shown in Fig.
15.9a
. The operation of this circuit can be explained as follows. It is
assumed that this circuit will be operated with an external voltage signal V
in
which
can be an existing square/triangular wave generator. The shorting of the terminals
Y
1
and Z along with the resistor R
2
provides a positive feedback in the circuit. As a
consequence, the DVCC is saturated and the output V
0
would be either at positive
saturation level
V
0
or negative saturation level
V
0
. The transfer characteristics of
the circuit are shown in Fig.
15.9b
. By proper connection of the DPDT switch, this
circuit exhibits hysteresis characteristics of two different kinds which have been
termed as clockwise (CW) mode or counter-CW (CCW) mode which are shown
respectively in Fig.
15.9b and c
.
For CW mode, the DPDT switch is made to connect node 1 to node 3 and node
2 to node 4. It is easy to see that the terminal currents I
X
and I
Z
are respectively
given by:
V
0
V
in
V
0
R
2
I
X
¼
and I
Z
¼
ð
15
:
37
Þ
;
R
1
The hysteresis characteristics for CW mode can be explained as follows. Assuming
that V
0
is at positive saturation level
V
0
and also let initial input V
in
¼
0. Further, let
R
2
>
R
1
, then from equation (
15.37
) it follows that I
x
would be larger than I
z
. Thus,
it is ensured that V
0
will be in the level
V
0
initially. If a positive V
in
is applied now,
the current I
x
will be decreasing gradually. Now because I
z
is larger than I
x
,V
0
switches from the state
V
0
to
V
0
. This explains the path 1 in Fig.
15.9b
. Similarly,
in path 2 the output is initially at the level
V
0
and hence, current I
z
is greater than
current I
x
. Now if input signal V
in
decreases from zero and is negative the current I
x
is increases until it becomes larger than I
z
, due to which V
0
switches from
V
0
to
V
0
,
this explains path 2 of the transfer characteristics of Fig.
15.7b
. The upper and lower
threshold voltages V
TH
and V
TL
can be determined from the instant of time when I
x
and I
z
are equal. The mathematical expression from V
TH
and V
TL
in CW mode are
given by
V
0
and
V
TL
¼
V
0
R
1
R
2
R
1
R
2
V
TH
¼
1
1
ð
15
:
38
Þ
For CCW mode, the DPDT switch enables in connections of terminals 1 to 4 and
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