Download Multivibrator

Output
(No input
signal)
Astabl
e
multivibrat
(a)
or
Bistabl
e
o-----1~_m_u_lt_iv_ib
~_r_at_o_r
T
1
T2
(c)
Output
~-.-~
-Vout
Monostabl
e
multivibrat
or (b)
-v-l
=r
..............
···········
·~
out
..
..
• A MULTIVIBRATOR is an electroniccircuit
that switches rapidly by means of positive
feedback between two or more states.
• Its basically a two amplifier circuit.
• A multivibrator generates square, pulse,
triangular waveforms.
• Also called as nonlinear oscillators or
function generators.
CLASSIFICATION
MULTIVIBRATOR
ASTABLE
BISTABLE
MONOSTABLE
Astable
Multivibrators
) The astable
has no
stat
circuitn extern stable
e.
Wit
signal
h
o al
transist
switch from th
applied,
e
alternat
ors
cutoff to
at
a frequency
ely
saturati
bythe
the
RC timedetermined
constants of
on
coupling circuits.
) Astable multivibrator circuit
consist of two
cross coupled RC ampIifiers.
vc
c
R
1
Consists of two amplifying devices by
and
cross-coupled
Typically,
R2 = R3, R1 = R4, C1 = C2resistors capacitors.
and R2 >> R1.
= = The circuit has two states
< State 1: Vc1 LOW, Vc2 HIGH, 01 ON
(saturation) and 02 OFF.
< State 2: Vc1 HIGH, Vcz LOW, 01 OFF and 02
ON
J::: (saturation).
It continuously oscillates from one
state to the other.
R
4
\
Astable Multivibrators
) When the circuit is first powered up, neither
transistor is ON.
)
V8 an V8 rise via base resistor R3 and R2 respectively.
Both 1 dwill2 conduct
Any onefaster
of thethan other due to some circuit
transistor
imbalance. We can
not say which transistor will turn on first so for analysis
purpose we assume 01 conducts first and 02 off {C1 is fully
charged).
) Sinc 0 conducts
of hence Vc1 = OV and Vc2
e
1 and 02
f =Vee· - state1
vc
c
r ,r1
v., v,,
,:
v3 ~-t
·1
-----
i
V.
~
Vx
I
t
-------ii
.---...--.
I
.....1..
1:
._.l._
... :---t
~
V J7
.V
.. t-------#V
~a.;
• t
Astabl Multivibrato
e
rs
) Since 01 conducts and 02 off hence Vc1 = OV and Vc2 = Vcc· Due to higher
voltage at Vc2•
capacitor C2 will be charged via R4 (low resistance path because R4 <R2). C1
(which was charged earlier, and can not hold the charge for indefinite period)
starts discharging via R2 (high resistance path because R2>R1). Time taken to
discharge C1(T1 = R2C1) s tirne taken to charge C2 (T2 =R4C2)
) When C2 is fully charged then left plate of C2 will be at -Vcc which switch off
the 01 . When C1
is fully discharged then left plate of C1 will be at +Vcc which switch on the 02. State 2V82 reaches V00, the circuit enters in state 1 again, and the process
When
repeats. s
Rl
R
-ive
from
Capacitor C2
R2
R
R
C
l
R
l
R2
C
2
Discharging
-ive
from
Capacitor Cl
Collector - Coupled Astable
Multivibrator
Ve
e
SW1
RL1
R1
C2
RL2
C1
Q1
Figure (a)
• It consists of two common emitter amplifying stages.
• Each stage provides a feedback through a capacitor at
the input of the other .
• Since the amplifying stage introduces a 180 degrees
phase shift and another 180 degrees phase shift is
introduced by a capacitor
• The feedback signal and the circuit works as an
oscillator.
• In other words because of capacitive coupling none of
the transistor can remain permanently out-off or
saturated.
• Instead circuit has two quasi-stable states (ON and OFF)
and it makes periodic transition between these two
states.
• The output of an astable multivibrator is available at the
collector terminal of the either transistors.
• The two outputs are 180degrees out of phase with each
other. Therefore one of the output is said to be the
complement of the other.
Emitter - Coupled Astable Multivibrator
Vi
n
VC
N2
VBR
1
t
VE
R1
RE
1
c
t
RE
2
VER
2
+
• In a collector coupled symmetrical astable
multivibrator if it is desired to vary the frequency.,
then it is necessary
 to vary both the timing capacitor simultaneously,
 to vary both the timing resistor subject to the
conduction that the values are enough to keep
the transistors in saturation,
 to vary VBB which also cannot be varied over a
long range.
• Thus it is difficult to achieve frequency control in
collector coupled astable multivibrator,
In order to explain the operation of the circuit, it is
necessary that the following conditions must be satisfied.
i. In D.C. conduction i.e. with timing capacitor C removed
bias should be so adjusted that both the transistors are
in active region.
ii. Under D.C. condition, the D.C. loop gain should be less
than unity to void bistable operation.
iii. In the active region, the loop gain must be greater than
unity at some non-zero frequency.
iv. Bias conditions ar3e so adjusted that with the capacitor
C concerned, during normal operation, transistor C1
operates between cut-off and saturation while transistor
C2 operates at the same time between its active region
and the off region. This transistor Q1 operates in
saturated mode and transistor Q1 operates in saturated
mode and transistor Q2 operates in unsaturated mode.
Switching time & Frequencyfor Astable
Multivibrators
» Time period of wave depends only
upon the discharge of
of C2: VB2 =Vee -ic1R2
»Consider C1 and discharge
capacitors
C2.
V92during
C1 charged up to Vcc, the initial discharge
» Since the
current will be
capacitor
Current decays exponentially with a time
constant of R2C1
R
.
_Vee
+Vee 2
Ze1 _
-
VB2
-
-
V
cc
(
=l 1
2Vc Re2c, )
c
V 82 = (actuall
Transistor will switch
0.7V
compare
whenfor Si which is smallOV
y
to V eel
where T2 is the off time for
transistor 02
6
Switching time & Frequencyfor Astable
Multivibrators
for transistor Q1 can be
»
Similarly off
time
» Total
obtained.
perio T:
d
I =T,+T = [R C + R C ]ln(2) =
0.694(R C
= =
Ir =
» r=
=
time
3
2
3
2
2
1
+
)
2
If R2
R3 c1 c2 c then
R,
» Frequency of1.4RC
oscillation is
given by
I
R2C1
I
f
=_!_=_Q
2
T
RC
7
MONOSTABLE MULTIVIBRATOR
+Vee
R
1
Rr
12ok
n
lOkn
Cr
lOuF
+
R
2
lOk
n
lOk
n
Vo
ut
R
3
TR
1
TR
2
"='
'{o
Trigg
er
Inp
ut
c1
l--
100nF
ioko
1N4
148
Tim
e
Consta
nt
.
.
.
.
Outp
ut
(unstable
state)
MonostableMultivibrat
ors
vc
c
»
One of the state is stable but
V
8 remov
the
other is
R
ed.
1
not. For that capacitive path
condu 1
» In stable
between
Vcastate
and any
cts
one transistor
and other is off.
» Application of external trigger Vc
R
2
R
3
V
C2
i
change the
state.
to Vcc signal
./
8 charges
»VWhen
theupexternal
R2 time T, V82=V0N,
./After
a certain
2 through
goes
high
02 turns on
./ Vc2 pulled to OV, 01 turns off.
-
» Enters
state 1 and
remains
»Wh V82there
is momentarily
en
pulled to
grou
by antoexternal signal
./ Vca rises
nd
Vcc
./ 01 turns
on
./ Vc1 pulled
to OV
MONOSTABLE
I
I I_.
- MUl TIVERATOR J....k.)..A.
l
T1 T2
LJ L
T3
n
.....J
T
1
.n n n
LJ LJ
T2
T
3
8
• Monostable Multivibrators can produce a very short
pulse or a much longer rectangular shaped waveform.
• Leading edge rises in time with the externally applied
trigger pulse.
• Trailing edge is dependent upon the RC time constant of
the feedback components used.
• This RC time constant may be varied with time to
produce a series of pulses which have a controlled fixed
time delay in relation to the original trigger pulse as
shown below.
Monostable Multivibrator Waveforms
time -
negative
trigge puls
r
e
-Vin - _ _·.,_
Vout - ,_ ~---
short
time
constan
t
time
t
Vout-
,_-J.-----------,
t
longer
time
constan
timet
-
• The time
Multivibrators
the values of
RT or both.
• Monostable
used
constant of
Monostable
can be changed by varying
the capacitor CT, the resistor,
multivibrators
are generally
to increase the width of a pulse or
to produce a time delay within a circuit
• Since the frequency of the output signal is
always the same as that for the trigger
pulse input, the only difference is the pulse
width.
BISTABLE MULTIVIBRATOR
+Vee
R4
R1
R2
R3
Output Q
Output Q
TR·
A
8
TR1
"OFF
TR
2
"OF
F
SPOT Switch
•
O
v
• The Bistable Multivibrator is another type of two
state
device
similar
to
the Monostable
Multivibrator
• The difference is that BOTH states are stable.
• Bistable Multivibrators have TWO stablestates.
(hence the name: “Bi” meaning two)
• Maintains in given output state indefinitely
unless an external trigger is applied forcing it to
change state.
• As bistable multivibrators have two stable states
they are more commonly known as Latches and
Flip-flops for use in sequential type circuits.
• The bistable multivibrator can be switched over
from one stable state to the other by the
application of an external trigger pulse.
• Thus, it requires two external trigger pulses
before it returns back to its original state.
• In each of the two states, one of the transistors
is cut-off while the other transistor is in
saturation.
Bistable
Multivibrators
} Both capacitors removed
./ Stable for either state 1
or 2
./ Can be forced to either
>:If Set
is low,
state
by Set
or
./
Q1
turns
off
Reset signals
./ Vc1 (Vout) and VB2 rises
towards Vcc
./ Q2 turns on
./ Vce pulled to OV
./ VB1 is latched to OV
./ Circuit remains in state 2
until
Reset is low
low
>:- If Reset
./ Similar operation
./ Circuit remains in state 1
until Set is low
Behave as an RS flip-flop
(memory element)
>:-
vc
c
R
1
lJ
R
2
:
set
lJ
res
et
• The bistable multivibrators output is
dependent upon the application of two
individual trigger pulses, switch position
“A” or position “B”.
• So Bistable Multivibrators can produce a
very short output pulse or a much longer
rectangular shaped output.
• Leading edge rises in time with the
externally applied trigger pulse.
• Trailing edge is dependent upon a
second trigger pulse.
Bistable Multivibrator Waveform
SNitch
Positio A
n
v
~ -
A
Switch
Positio B
n
B
...
..
...
-
Q ------------
-
T
Tirne
1~-----~~----------1................................~
Constant
Schmitt trigger
• Schmitt trigger belongs to a class of bistable
multivibrator circuits.
• In a bistable, there exist two D.C. couplings from
each output to input of the other.
• But in Schmitt trigger circuit, there exists only
one coupling.
• If in the emitter coupled bistable the feedback
network from the collector of transistor Q2 to the
base of transistor Q1 is removed , it becomes a
Schmitt trigger circuit.
• The Schmitt trigger is used for wave shaping
circuits.
• It can be used for generation of a square wave
from a sine wave input.
• Basically, the circuit has two opposite operating
states like in all other multivibrator circuits.
• The trigger signal is not, typically, a pulse
waveform but a slowly varying A.C. Voltage.
• The Schmitt trigger is level sensitive and
switches the output state at two distinct trigger
levels.
• One of the triggering levels is called a lower
trigger level and the other as upper trigger level
CIRCUIT DIAGRAM
•
----+------.......--+v
RC
1
R
1
i
n
R
C2
!-----{
R
E
,
ou
t
R
81
R
2
The circuit contains of 2
identical transistors Q1
and Q2 coupled through
an emitter RE.
• The resistor R1 and R2
form a voltage divider
across the VCC supply
and ground.
• These resistors provide a
small forward bias on the
base of transistor Q2.
Working
Let us suppose that initially there is no signal at the input.
• Then as soon as the power supply VCC is switched on,
the transistor Q2 starts conducting.
• The flow of its current through resistor RE produces a
voltage drop across it.
• This voltage drop acts as a reverse bias across the
emitter junction of transistor Q1 due to which it cuts-off.
• As a result of this, the voltage at its collector rises to VCC.
• This rising voltage is coupled to the base of transistor Q2
through the resistor R1.
• It increases the forward bias at the base of transistor Q2
and therefore drives it into saturation and holds it there.
• At this instant, the collector voltage, level are VC1 =VCC
andVC2 =VCE(sat)
Upper trigger point (UTP)
t
Lower trigger
point (LTP)
..,
..
_
Upper trigger level (UTI
----
Low er trigger Level (LTI
i
Suppose an A.C. signal is applied at the input of the Schmitt trigger
(i.e. at the base of the transistor Q1).
 As the input voltage increases above zero, nothing will happen till it
crosses the upper trigger level (U.L.T).
 As the input voltage increases, above the upper trigger level, the
transistor Q1 conducts.
 The point, at which it starts conducting, is known as upper
triggerpoint(U.T.P).
 As the transistor Q1 conducts, its collector voltage falls below VCC.
 This fall is coupled through resistor R1 to the base of transistor Q2
which reduces its forward bias.
 This in turn reduces the current of transistor Q2 and hence the
voltage drop across the resistor RE.
 As a result of this, the reverse bias of transistor Q1 is reduced and it
conducts more.
 As the transistor Q1 conducts more heavily, its collector further
reduces due to which the transistor Q1 conducts near cut-off.
 This process continues till the transistor Q1 is driven into saturation
and Q2 into cut-off.
 At this instant, the collector voltage levels are VC1 =VCE(sat)andVC2
=VCC
 The transistor Q1 will continue to conduct till the input
voltage falls below the lower trigger level (L.T.L).
 When the input voltage becomes equal to the lower
trigger level, the emitter base junction of transistor Q1
becomes reverse biased.
 As a result of this, its collector voltage starts rising
toward VCC.
 This rising voltage increases the forward bias across
transistor Q2 due to which it conducts.
 The point, at which transistor Q2 starts conducting, is
called lowertriggerpoint(L.T.P).
 Soon the transistor Q2 is driven into saturation and Q1 to
cur-off.
 This completes one cycle.
 The collector voltage levels at this instant are VC1 =VCC
and VC2 = VCE(sat).
 No change in state will occur during the negative half
cycle of the input voltage.
The output of a Schmitt trigger is a
positive going pulse width depends
upon the time during which transistor
Q1 is conducting.
The conduction time is set by the
upper and lower trigger levels.
Bistable Multivibrator Triggering
To change the stable state of the binary
it is necessary to apply an appropriate
pulse in the circuit, which will try to bring
both the transistors to active region and
the resulting regenerative feedback will
result on the change of state.
Triggering may be of two following types:
• Asymmetrical triggering
• Symmetrical triggering
Asymmetrical triggering
In asymmetrical triggering, there are two trigger
inputs for the transistors Q1 and Q2.
Each trigger input is derived from a separate
triggering source.
To induce transition among the stable states,
initially the trigger is applied to the bistable.
For the next transition, now the identical trigger
must appear at the transistor Q2.
Thus it can be said that in asymmetrical
triggering, trigger pulses derived from two
separate source and connected to the two
transistors Q1 and Q2 individually, sequentially
change the state of the bistable.
• AsymmetricaltriggeringofBistableMultivibrator
+Vt
c
R
1
--
R
2
I
U
R
4
--
Symmetrical Triggering
There are various symmetrical triggering
methods called
• symmetrical collector triggering,
• symmetrical base triggering and
• symmetrical hybrid triggering.
Symmetrical base triggering (positive pulse)
+Vee
RC
1
R
3
R1
01
RC2
02
R2
R4
C1
~1-------
_fUL_
R
x
R
E
Diodes D1 and D2 are steering diodes.
m Here the positive pulses, try to turn ON and OFF
transistor.
Thus when transistor Q1 is OFF and transistor Q2 is ON,
the respective base voltages and VB1N, OFF and VB2N, ON.
m It will be seen that VB1N, OFF > VB1N, ON.
m Thus diode D2 is more reverse-biased compared to
diode D1.
When the positive differentiated pulse amplitude is
greater than (VB1N, OFF + Vɣ),
the diode D1 gets forward biased,
transistor Q1 enters the active region,
with subsequent regenerative feedback Q1 gets ON,
transistor Q2 becomes OFF.
On the arrival of the next trigger pulse now the diode D2
will be forward biased and ultimately with regenerative
feedback it will be in the ON state.
m
Some
Importantterms
DutyCy duty cycle is defined as the ratio
cle
of pulse duration
I
I
I
I
I
to pulse
period.
I
I
.
c,
E
<
-
0
0
I
I
I
t
T
T+
t
I
2
Tim T
e
I
2T+
t
I
3
T
I
3T+
t
The pulse duration is r; this is how long the pulse
remains high (amplitude= 1 in the figure).The pulse
period is T; this is the duration of one complete
cycle, and is just the inverse of the frequency in Hz
(f = 1/T).
D=r!
T